Proceedings on the Seventh Workshop on

STANFORD GEOTHERMAL PROGRAM STANFORD UNIVERSITY STANFORD, CALIFORNIA 94305

SGP-TR- 55

PROCEEDINGS O F THE SEVENTH WORKSHOP ON GEOTHERMAL RESERVOIR ENGINEERING

Editors

Paul K r u g e r H e n r y J . R a m e y , Jr.

Frank G. M i l l e r R o l a n d N. H o r n e W i l l i a m E. B r i g h a m T a n G. D o n a l d s o n

Jon S . G u d m u n d s s o n

D e c e m b e r 15- 17, 1981

SPONSORED BY THE GEOTHERMAL AND HYDROPOWER TECHNOLOGIES D I V I S I O N O F THE DEPARTMENT OF ENERGY STANFORD-DOE CONTRACT NO, DE-ATO3-8OSF11459

TABLE OF CONTENTS

Page Preface

......................................

-V-

Developments i n Geothermal R e s e r v o i r Engineering S t a n f o r d Geothermal Workshops:

Overview

-

The F i r s t S i x Years

-

I. G. Donaldson and P. Kruger

-

U n i v e r s i t y Research i n Geothermai R e s e r v o i r Engineering

R. N. Horne

...

The Role o f t h e N a t i o n a l L a b o r a t o r i e s i n Geothermal R e s e r v o i r Engineering - P. A. Witherspoon and C. F. Tsang

.......................

................................

Geothermal R e s e r v o i r Engineering Development through I n t e r n a t i o n a l Cooperation - F. G. Miller and H. J. Ramey, Jr.

-

....................... ..........................

C. L. R i t z

Geothermal R e s e r v o i r Engineering

5 7

The Role of t h e U. S. Geological Survey

Geothermal R e s e r v o i r Engineering: - W. A. D u f f i e l d

Resource Development

1

-

U t i l i t y Industry Perspective

-

V. W. Roberts

...

11 13 19 21

F i e l d Development A p p l i c a t i o n of a Lumped Parameter Model t o t h e Cerro P r i e t o Geothermal F i e l d - J . D. Westwood and L. M. C a s t a n i e r

...................... - M.

Well Log A n a l y s i s Applied t o Cerro P r i e t o Geothermal F i e l d V. A r e l l a n o and R. L. McCoy

Castaneda, A. A b r i l ,

...........................

Recent R e s u l t s of t h e Well D r i l l i n g Program at Cerro P r i e t o M. J. Lippmann and F. Bermejo M.

-

23 29

B. Dominguez A. and

........................

35

Design of a T r a c e r T e s t i n t h e Geothermal F i e l d of Los Azufres, Michoacan, Mexico: P r o g r e s s Report - E. R. I g l e s i a s and G. H i r i a r t

41

Analysis of Flow Data from S e v e r a l Baca Wells

-

T.

................. D. Riney and S. K. Garg . . . . . .

F r a c t u r e S t i m u l a t i o n Experiments a t t h e Baca P r o j e c t A r e a M. J . Bunyak

-

C. W. Morris and

..................................

Subsurface E x p l o r a t i o n and Well Discharge The Reykjanes Geothermal F i e l d i n Iceland: C h a r a c t e r i s t i c s - J. S. Gudmundsson, T. Hauksson and J. Tomasson

........

A n a l y s i s of Well Data from t h e K r a f l a Geothermal F i e l d i n I c e l a n d S. Benson, 0. Sigurdsson, G. K. H a l l d o r s s o n and V. S t e f a n s s o n F i r s t R e s u l t s of a R e i n j e c t i o n Experiment a t L a r d e r e l l o G. C a p p e t t i and R. C e l a t i

-

47

- G.

53

61

S. Bodvarsson,

..........

71

A. Giovannoni, G. A l l e g r i n i ,

............................

77

Use of Environmental I s o t o p e s as N a t u r a l Tracers i n a R e i n j e c t i o n Experiment a t L a r d e r e l l o - S. N u t i , C. Calore and P. Noto

85

Temporal Evolution of t h e Composition of t h e F l u i d from Serrazzano Zone ( L a r d e r e l l o ) - F. D'Amore, C. C a l o r e and R. C e l a t i

91

...................

......................

-iii-

Page General Summary of Hot-Dry-Rock Geothermal Reservoir Testing 1978 t o 1980 a n d H . D. Murphy.

-

Z. V. Dash

...............................

Dispersion i n Tracer Flow i n F r a c t u r e d Geothermal Systems F. J. Rodriguez

-

97

R. N. Horne and

................................

Low-Temperature Geothermal Resource Assessment i n t h e United S t a t e s andM. J. Reed

-

103

M. L. Sorey

................................

109

Reservoir Chemistry and Physics Geothermal Well Logging and Its I n t e r p r e t a t i o n Measuring S a l t Water P e r m e a b i l i t i e s

-

-

.... .................

S. Hirakawa and S. Yamaguchi

B. D. Gobran

115 121

Si02 P r e c i p i t a t i o n Accompanying F l u i d Flow through G r a n i t e Held i n a Temperature Gradient - D. E. Moore, C. A. Morrow and J . D. Byerlee

127

S t r e s s Induced Release of Rn222 and CH4 t o P e r c o l a t i n g Water i n G r a n i t i c Rock - C. G . Sammis, M. Banerdt and D. E. Hammond

133

.............

......... .........

I n t e r s t i t i a l F l u i d P r e s s u r e S i g n a l Propagation Along F r a c t u r e Ladders

-

G. Bodvarsson

Measured Enthalpy Combined with Chemical Concentration Data t o Diagnose Reservoir Behaviour - M. S a l t u k l a r o g l u

..........................

139 143

Modeling

Heat T r a n s f e r i n F r a c t u r e d Geothermal R e s e r v o i r s w i t h B o i l i n g Simulation of Flow i n F r a c t u r e d Porous Media

-

-

...... Pinder . . . .

K. P r u e s s

A. M. Shapiro and G. F.

- S. M. Benson, .......................

151 157

Reservoir Engineering of Shallow Fault-Charged Hydrothermal Systems G. S. Bodvarsson and D. C. Mangold

161

Experimental and F i n i t e Element Analysis of t h e S t a n f o r d Hydrothermal Reservoir Model - L. W. Swenson, Jr. and A. Hunsbedt

169

...................

Cold Water I n j e c t i o n i n t o Two-Phase Geothermal R e s e r v o i r s J. W. P r i t c h e t t

-

S. K. Garg and

................................

175

A n a l y t i c Approach t o t h e Simulation of Laboratory Steam-Flow Experiments A. F. Moench and W. N. H e r k e l r a t h

179

L i s t of P a r t i c i p a n t s

183

-

...................... ................................

-iv-

PREFACE

I c e l a n d , and t e c h n i c a l developments a t Baca, New Mexico and L a r d e r e l l o , i n I t a l y .

The Seventh Workshop on Geothermal R e s e r v o i r Engineering convened a t S t a n f o r d U n i v e r s i t y on December 15, 1981. Attendance a t t h i s workshop remained c o n s t a n t a t 104. The continued growth of f o r e i g n p a r t i c i p a t i o n was e v i d e n t w i t h 16 v i s i t o r s from 5 c o u n t r i e s .

Other noteworthy a r e a s were t h e r e p o r t s on t r a c e r s t u d i e s i n f r a c t u r e d r e s e r v o i r s and t h e developments i n t h e r o l e o f geochemistry i n geothermal r e s e r v o i r e n g i n e e r i n g . The s e s s i o n on Modeling showed t h e continued advances being made t o understand t h e p h y s i c a l and chemical p r o c e s s e s o c c u r r i n g i n geothermal r e s e r v o i r s .

The Seventh Workshop w a s noteworthy i n b o t h looking backward w i t h t h e f i r s t s e s s i o n on Overviews of t h e Developments i n Geothermal Reservoir Engineering o v e r t h e p a s t s i x workshops and i n l o o k i n g forward w i t h t h e theme of t h e p a n e l s e s s i o n on F u t u r e D i r e c t i o n s of Geothermal R e s e r v o i r Engineering Development.

I n summary, t h e y e a r 1981 showed a continuous r e c o r d of advances i n geothermal r e s e r v o i r e n g i n e e r i n g and t h e p r o s p e c t s f o r f u t u r e development appear b r i g h t .

The e x c e l l e n t r e s u l t s of t h e workshop c l e a r l y confirm t h e major o b j e c t i v e s o f t h e S t a n f o r d Geothermal R e s e r v o i r Engineering Workshop i n b r i n g i n g t o g e t h e r t h e a c t i v e r e s e a r c h e r s and e n g i n e e r s i n t h e development o f geothermal energy as a v i a b l e e l e c t r i c a l energy s o u r c e and i n p r o v i d i n g a forum f o r prompt r e p o r t i n g of p r o g r e s s and exchange o f i d e a s . T h i s l a t t e r i s e s p e c i a l l y important i n t h i s e r a of government retrenchment and c o n t i n u o u s p r e s s u r e t o f i n d a l t e r n a t e energy s o u r c e s f o r t h e a n t i c i pated d e c l i n e s i n f o s s i l f u e l resources.

One of t h e main r e a s o n s t h a t t h e Seventh Workshop was w e l l organized and r a n smoothly and s u c c e s s f u l l y w a s t h e c a r e f u l p l a n n i n g of o u r Program Managers and SGP s t a f f . T h i s y e a r i n p a r t i c u l a r , P r o f e s s o r s Ramey, Horne, Brigham, Miller and I are extremely g r a t e f u l f o r t h e g r e a t o r g a n i z a t i o n a l e f f o r t s of D r . I a n G. Donaldson, who u n f o r t u n a t e l y ( f o r us) r e t u r n e d t o New Zealand b e f o r e t h e workshop and of D r . J o n S. Gudmundsson who f o r t u n a t e l y ( f o r us) i s v i s i t i n g from I c e l a n d . We a l s o a r e g r a t e f u l t o o u r competent s t a f f J e a n Cook, Joanne H a r t f o r d , and Marilyn King f o r running t h e Workshop so e f f i c i e n t l y , and t o Terri Ramey and Amy Osugi f o r h e l p i n g p r e p a r e t h e Proceedings.

The overviews on p a s t developments o u t l i n e d t h e e f f o r t s of many s e c t o r s of t h e geothermal community: academic, n a t i o n a l l a b o r a t o r y , government agency, r e s o u r c e d e v e l o p e r s , and u t i l i t i e s . The growing r o l e of c o o p e r a t i v e r e s e a r c h w i t h many f o r e i g n c o u n t r i e s w a s c l e a r l y v i s i b l e as a n important 'lresource" f o r f u t u r e geothermal development i n t h e United States.

We e x p r e s s o u r a p p r e c i a t i o n t o t h e Department of Energy f o r s u p p o r t i n g t h e workshop, and we a r e underway i n planning Workshop Eight f o r December 14-16, 1982.

The expanding l i s t of geothermal f i e l d s w a s noted i n t h e growth of t h e program s e s s i o n s d e d i c a t e d t o F i e l d Development. The two s e s s i o n s included reviews o f Cerro P r i e t o and Los Azufres i n Mexico; Reykjanes and K r a f l a i n

Paul Kruger Stanford University March 15, 1982

-V-

Proceedings Seventh Workshop Geothermal Reservoir Engineering Stanford, December 1981. SGP-TR-55.

STANFORD GEOTHERMAL WORKSHOPS:

THE FIRST SIX YEARS

I a n G. Donaldson* and Paul Kruger

Stanford Geothermal Program Stanford U n i v e r s i t y S t a n f o r d , CA 94305 I n t r o d u c t i o n The F i r s t Stanford Geothermal Reservoir Engineering Workshop w a s convened i n December, 1975. I t s s u c c e s s i n compiling t h e s c a t t e r e d information on geothermal rese r v o i r engineering r e s u l t e d i n i t s e s t a b l i s h ment as a n annual event of t h e Stanford Geothermal Program. I n t h i s seventh Workshop, i t i s a p p r o p r i a t e t o l o o k back over t h e e f f o r t s of t h e p a s t six y e a r s and t o e v a l u a t e t h e o v e r a l l r e s u l t s . Three q u e s t i o n s may be raised: (1) A r e t h e Workshops achieving t h e same aims and o b j e c t i v e s of t h e i n i t i a l m e e t i n g ? ; (2) What p r o g r e s s i n geothermal reserv o i r engineering h a s been achieved over t h e s e six y e a r s ? ; and (3) Have t h e Workshops developed any s p e c i a l v a l u e s of t h e i r own t h a t d i s t i n g u i s h them from o t h e r geothermal meetings?.

w i t h t h e i n t r o d u c t i o n of a panel s e s s i o n . The p a n e l s e s s i o n i s now a n i n t e g r a l p a r t of t h e workshop program; t h e s p e c i f i c theme f o r each workshop i s chosen t o r e f l e c t a t o p i c most a p p r o p r i a t e t o t h e s t a t e of geothermal r e s e r v o i r e n g i n e e r i n g a t t h a t t i m e . The t o p i c s d i s c u s s e d a t t h e f o u r p r i o r annual workshops are l i s t e d i n Table I.

The development of geothermal r e s e r v o i r e n g i n e e r i n g i s a l s o r e f l e c t e d i n t h e changes i n sponsorship of t h e Stanford Geothermal Program, i n d i c a t i n g t h e g e n e r a l change i n government support of geothermal energy r e s e a r c h and. development over t h i s time p e r i o d . The sponsorship of t h e workshop program i s l i s t e d i n Table 11. A i m s and O b j e c t i v e s During t h e p a s t six y e a r s of t h e Workshop, t h e aims and o b j e c t i v e s have been k e p t r a t h e r c o n s t a n t . In the Introd u c t i o n t o t h e Proceedings of t h e F i r s t Workshop, they were c l e a r l y defined: "The purpose of t h e Workshop convened h e r e a t Stanford t h i s December, 1975, i s twofold. F i r s t , t h e Workshop w a s designed t o bring together researchers active i n the v a r i o u s p h y s i c a l and mathematical branches of t h i s newly-emerging f i e l d so t h a t t h e p a r t i c i p a n t s could l e a r n about t h e v e r y many s t u d i e s underway and s h a r e e x p e r i e n c e s through a n exchange of r e s e a r c h r e s u l t s . The second purpose w a s t o p r e p a r e t h e s e Proceedi n g s of t h e Workshop s o t h a t t h e i n t e g r a t e d information could be disseminated t o t h e gsothermal community r e s p o n s i b l e f o r t h e d e v d o p ment, u t i l i z a t i o n , and r e g u l a t i o n a s p e c t s of the industry."

The development of t h e Stanford Geothermal Engineering Workshops i s w e l l recorded i n i t s Proceedings. A t t h e f i r s t Workshop, some 50 p a p e r s were p r e s e n t e d over a three- day p e r i o d . These p a p e r s , i n c l u d i n g two overviews, covered t h e following g e n e r a l areas of geothermal engineering : (1) Reservoir P h y s i c s - s t u d i e s t o e v a l u a t e the physical processes occurring i n geothermal r e s e r v o i r s ( 2 ) Well T e s t i n g - techniques used i n spef i c and g e n e r i c f i e l d s t o determine v o l u m e t r i c and e x t r a c t i v e c h a r a c t e r i s t i c s of a r e s e r v o i r (3) F i e l d Development - methods f o r optimum commercialization of producing f i e l d s (4) Well S t i m u l a t i o n techniques f o r im proving energy and f l u i d recovery from geothermal r e s o u r c e s (5) Modeling - mathematical methods t o study geothermal r e s e r v o i r s .

-

These purposes may be c o n t r a s t e d t o t h e object i v e s as s t a t e d i n t h e I n t r o d u c t i o n t o t h e

During t h e ensuing f i v e Workshops, t h e weighting given t o t h e s e g e n e r a l a r e a s have changed; d i f f e r e n t areas were introduced a t v a r i o u s t i m e s , f o r example t h e a r e a of w e l l t e s t i n g w a s expanded t o i n c l u d e r e s e r v o i r t e s t i n g and formation e v a l u a t i o n ; and s p e c i a l sess i o n s were added f o r t o p i c s such as prod u c t i o n e n g i n e e r i n g , geopressured systems, and r i s k a n a l y s i s . The three- day format of t h e Workshop h a s been r e t a i n e d ; and i n 1977, a major change i n program c o n t e n t occurred

*

current address:

Proceedings of t h e S i x t h Workshop i n 1980: "The o b j e c t i v e s of t h e Workshop, t h e b r i n g i n g t o g e t h e r of r e s e a r c h e r s , e n g i n e e r s , and managers involved i n geothermal r e s e r v o i r study and development and t h e p r o v i s i o n of a forum f o r t h e prompt and open r e p o r t i n g of p r o g r e s s and f o r t h e exchange of i d e a s , cont i n u e t o be m e t . " I n r e t r o s p e c t , t h e s t a t e d o b j e c t i v e s of t h e Workshop have been c o n s t a n t , b u t have they indeed been m e t ? T h i s can b e s t be answered by looking a t t h r e e a s p e c t s of t h e Workshop:

DSIR, New Zealand

-1-

Table I SGP Workshop Panel Discussion Topics Workshop

Year -

Third

1977

D e f i n i t i o n s of Geothermal Reserves

Fourth

1978

Geochemistry

Topic Theme

Fifth

1979

Reservoir Models--Simulation v s . R e a l i t y

Sixth

1980

Numerical Model Intercomparison Study

Table I1 Sponsors of t h e SGP Geothermal Workshops

Year -

Workshop

First

*

Sponsor

1975

Second

1976

Third

1977

Fourth Fifth Sixth

N a t i o n a l Science Foundation:

RANN Program

{ N a t i o n a l Science Foundation: RA" Program Energy R e s . and Dev. Adm.: Div. Geoth. En. Dept. of Energy:

Div. Geoth. En. ( t h r u LBL)

1978

Dept. of Energy:


1979

Dept. of Energy:


1980

Dept. of Energy:

Div. Geoth. En. ( t h r u SFOO)

(1) t h e people who p a r t i c i p a t e by p r e s e n t i n g p a p e r s and a t t e n d i n g t h e s e s s i o n s , (2) t h e coverage of t h e p a p e r s o f f e r e d , and (3) t h e s t a t u s of t h e Proceedings.

p a p e r s ; one t h a t h a s been maintained through t h e six y e a r s . One l a s t i n g f e a t u r e of t h e Workshops h a s been t h e set of Proceedings t h a t have become a n important p a r t of t h e l i t e r a t u r e on geothermal r e s e r v o i r e n g i n e e r i n g . From t h e s t a r t , c o p i e s have been i n demand t o such a n e x t e n t t h a t r e p r i n t i n g s have been r e q u i r e d f o r some and i n c r e a s e d f i r s t p r i n t i n g s are now e s s e n t i a l . P a p e r s i n t h e Proceedings are r e g u l a r l y c i t e d i n t h e p r o f e s s i o n a l l i t e r a t u r e as w e l l as i n review a r t i c l e s and i n books. The Proceedings are o f t e n t h e o n l y p u b l i c s o u r c e of i n f o r m a t i o n rel a t i n g t o some r e s e a r c h and t o some a s p e c t s of f i e l d development.

Throughout t h e six y e a r s , Workshop p a r t i c i p a n t s have come from a wide spectrum of r e s e a r c h and development groups--from government a g e n c i e s (such a s t h e Department of Energy and t h e U.S. Geological Survey), from t h e u n i v e r s i t i e s , from t h e N a t i o n a l Laboratories, and from t h e many s e c t o r s of t h e indust r y ( d e v e l o p e r s , u t i l i t i e s , and c o n s u l t a n t s ) . I n a d d i t i o n , t h e r e h a s been c o n s i d e r a b l e i n p u t and p a r t i c i p a t i o n from abroad. For example, 2 1 a t t e n d e e s from 11 f o r e i g n c o u n t r i e s p a r t i c i p a t e d i n t h e 1979 Workshop; 18 a t t e n d e e s from 6 n a t i o n s p a r t i c i p a t e d i n t h e 1980 Workshop. This 20% a t t e n d a n c e from abroad i n d i c a t e s a s t r o n g i n t e r n a t i o n a l n a t u r e of t h e Workshop. A s i g n i f i c a n t f e a t u r e i s t h a t t h e s e p a r t i c i p a n t s keep coming back.

I n reviewing t h e Proceedings, a major o b s e r v a t i o n becomes r e a d i l y a p p a r e n t . In each of t h e t o p i c areas, t h e c o n t e x t of t h e p a p e r s shows a marked t r a n s i t i o n from rep o r t s on " r e s e a r c h i n t e n t " t o r e p o r t s of " s i g n i f i c a n t achievements. 'I T h i s i s espec i a l l y t r u e i n t h e t o p i c s of f i e l d evaluat i o n and f i e l d development, where s u c c e s s e s (and problems) i n b r i n g i n g new f i e l d s on l i n e have been s h a r e d among t h e p a r t i c i p a n t s . The s e s s i o n s on modeling a l s o show a r a p i d a c c e l e r a t i o n from "How t o " p a p e r s t o a n a l y s i s of complex f i e l d s . A s p e c i a l p a r t of t h e 1980 Workshop Proceedings (SGP-TR-42) w a s i s s u e d s e p a r a t e l y on t h e code i n t e r c o m p a r i s o n s t u d y sponsored by t h e Department of Energy i n which i t w a s noted t h a t six independently c o n s t r u c t e d s i m u l a t o r s could a r r i v e a t r e a s o n a b l e agreement of r e s u l t s g i v e n t h e same i n p u t i n f o r m a t i o n .

P r o g r e s s i n Geothermal R e s e r v o i r Engineering During t h e six annual Workshops, t h e d i s t r i b u t i o n of g e n e r a l t o p i c s covered h a s become d i s c e r n a b l e . Table 111 shows t h e number of p a p e r s by t h e headings t h a t have been used i n t h e Proceedings' programs f o r t h e six Workshops. The t o t a l s f o r t h e l a s t t h r e e a l s o inc l u d e p a n e l d i s c u s s i o n p a p e r s . The a v e r a g e s i n d i c a t e a program of 2 overview p a p e r s , 9 r e s e r v o i r science papers, 1 0 f i e l d e v a l u a t i o n p a p e r s , 9 f i e l d development p a p e r s , 3 s t i m u l a t i o n p a p e r s , and 11 modeling p a p e r s . By d e c i s i o n , a f e a t u r e of t h e Workshops i s a balance between t h e o r e t i c a l and p r a c t i c a l

- 2-

Table 111 D i s t r i b u t i o n of Papers by Topics i n t h e SGP Annual Workshop Proceedings Workshop

Topic

Second Third F o u r t hF i f t hSixth T o t a l. --

. .F i r s t

Overviews

2

3

2

4

1

Reservoir Physics Reservoir Chemistry

9

8

8

7

4

10

5

Well T e s t i n g Well & R e s e r v o i r T e s t i n g Well Test & Formation Eval.

2.2

12 4

48 4

8.1

3

18 12 15 15

15 h

9

9

Well S t i m u l a t i o n

6

6

14

13

7

9

38

6

4 6

I2

11

9

10

50

33

44

The S p e c i a l Values of t h e Workshops I n c o n t r a s t t o t h e many o v e r a l l meetings on geothermal r e s e a r c h and development, t h e SGP Workshops might be c l a s s i f i e d a s relat i v e l y small and s p e c i a l i z e d . The r e s u l t h a s been a set of Workshops i n which a l l p a r t i c i p a n t s are a b l e t o be involved over t h e whole meeting. With a n average of about 100 a t t e n d e e s , t h e meetings are i n f o r m a l , w i t h much c r o s s - d i s c u s s i o n s of i s s u e s r a i s e d during t h e p r e s e n t a t i o n s . Over t h e three- day p e r i o d , t h e p a r t i c i p a n t s can g e t t o know o r renew a c q u a i n t a n c e with a l a r g e p r o p o r t i o n of t h e a t t e n d e e s . A l a r g e amount of i n f o r m a t i o n t r a n s f e r and exchange of i d e a s o c c u r s through t h i s open structure.

45

43

4

10.0

9 .o

18

3 .O

9

66

11.0

2

2

0.3

50

265

44.2

6

Risk A n a l y s i s

Totals

13

15

F i e l d Development Geopressured Systems P r o d u c t i o n Engineering

Modeling

1

12

Pressure Transient Analysis

.Average

The t h i r d s p e c i a l v a l u e f e a t u r e of t h e Workshops are t h e meeting p r e p r i n t s and t h e Workshop Proceedings. These have proved a n exc e l l e n t means of c i r c u l a t i n g r e c e n t r e s e a r c h f i n d i n g s and f i e l d development i n f o r m a t i o n q u i c k l y t o t h e many s c i e n t i s t s , e n g i n e e r s , and managers r e s p o n s i b l e f o r t h e development of geothermal energy. T h i s y e a r , t h e advent of t h e "camera-ready" copy f o r p r e p r i n t s and Proceedings should r e s u l t i n even f a s t e r communication of r e s u l t s t o t h e geothermal community.

A b a s i c p r i n c i p l e of t h e SGP Workshops i s t h a t a l l p a p e r s a c c e p t e d f o r t h e Workshop be publ i s h e d i n t h e Proceedings. T h i s i s now espec i a l l y important today i n t h a t n o t a l l p a p e r s o f f e r e d can be p r e s e n t e d o r a l l y a t t h e sess i o n s . Although t h i s p r i n c i p l e could l e a d t o p u b l i c a t i o n of i d e a s t h a t might n o t be a c c e p t ed f o r p u b l i c a t i o n i n a r c h i v a l j o u r n a l s w i t h f u l l reviewing p r o c e s s , t h e p u b l i c a t i o n i n t h e Workshop Proceedings p u t s them on r e c o r d . L i k e t h e Workshop i t s e l f , t h e Proceedings are a forum f o r d i s c u s s i o n and hence new o r altern a t i v e v i e w p o i n t s should c o n t i n u e t o be acceptable.

Although c e n t e r e d about t h e e n g i n e e r i n g asp e c t s of t h e geothermal r e s e r v o i r , t h e Workshop i s s t i l l broad enough t o a t t r a c t p a p e r s from a wide spectrum of d i s c i p l i n e s : social, economic, environmental. The b a l a n c e between t h e o r e t i c a l and p r a c t i c a l a s p e c t s of geotherm a l r e s e r v o i r developments a l l o w s a wide deg r e e of p e r s p e c t i v e t o each p a r t i c i p a n t . A second s p e c i a l v a l u e f e a t u r e of t h e Workshops is t h e p a n e l d i s c u s s i o n . To d a t e t h e p a n e l s have had e x c e l l e n t s u p p o r t from both t h e audience and t h e p a n e l i s t s . F o r t u n a t e l y , t h e r e h a s been l i t t l e d i f f i c u l t y s o f a r i n f i n d i n g t o p i c s a p p r o p r i a t e t o t h e r e s e a r c h and development c l i m a t e e x i s t i n g a t t h e time of each Workshop. With t h e n a t i o n ' s energy p i c t u r e s t i l l n o t i n s h a r p f o c u s , t h i s situat i o n is l i k e l y t o remain y e t f o r some time.

Achievements of t h e Workshops The S t a n f o r d Geothermal R e s e r v o i r Workshops have made " s i g n i f i c a n t achievements" i n t h e i r s h o r t l i f e t i m e . They c e r t a i n l y have brought t o g e t h e r t h o s e i n t e r e s t e d i n geothermal r e s e r v o i r r e s e a r c h and development and provided t h e p a r t i c i p a n t s w i t h a forum f o r e x p r e s s i o n and exchange of i d e a s and r e s u l t s . Through t h e Proceedings, t h e s e i d e a s and r e s u l t s a r e maintained f o r l a t e r r e f e r e n c e .

- 3-

Another s i g n i f i c a n t achievement r e s u l t s from t h e i n f o r m a l i t y and openness of t h e meeting. The Workshop h a s become a c c e p t e d as t h e medium f o r r e p o r t i n g new r e s e a r c h i d e a s and development information- - well i n advance of more formal p u b l i c a t i o n , i f any. S e v e r a l r e s e a r c h themes have continued through s e v e r a l workshops, i n d i c a t i n g a feedback mechanism from exposure and d i s c u s s i o n of i d e a s t o development i n t o r e s e a r c h programs and r e s u l t s . A major case i n p o i n t w a s t h e panel d i s c u s s i o n of t h e F i f t h Workshop i n 1979, "Geothermal R e s e r v o i r Models-Simulation v s . R e a l i t y , " which i n e f f e c t set t h e s t a g e f o r t h e Department of Energy sponsored numerical modeling intercomparison s t u d y c a r r i e d o u t over t h e n e x t year and reviewed i n t h e p a n e l d i s c u s s i o n of t h e S i x t h Workshop i n 1980.

t h e i r a v a i l a b i l i t y should be expanded. The avenues f o r wider c i r c u l a t i o n t o t e c h n i c a l l i b r a r i e s and o t h e r geothermal r e f e r e n c e systems are being examined. The S t a n f o r d Geothermal R e s e r v o i r Engineeri n g Workshops have played a s i g n i f i c a n t r o l e i n t h e c u r r e n t development of geothermal energy. The momentum i s forward, and m a i n t a i n i n g a p e r s p e c t i v e of t h e growth of geothermal e x p l o i t a t i o n w i l l be a n i n t e r e s t i n g p a r t of f u t u r e S t a n f o r d Geothermal Workshops. Bibliography SGP-TR-12 Geothermal R e s e r v o i r Engineering, December, 1975. SGP-TR-20 Summaries, Second Workshop, Geothermal R e s e r v o i r Engineering, December, 1976. SGP-TR-25 Proceedings, Third Workshop, Geothermal R e s e r v o i r Engineering, December, 1977. SGP-TR-30 Proceedings, F o u r t h Workshop, Geothermal R e s e r v o i r Engineering, December, 1978. SGP-TR-40 Proceedings, F i f t h Workshop, Geothermal R e s e r v o i r Engineering, December, 1979. SGP-TR-42 Proceedings, S p e c i a l Panel on Geothermal Model Intercomparison Study, December, 1980. SGP-TR-50 Proceedings, S i x t h Workshop, Geothermal R e s e r v o i r Engineering, December, 1980.

Summary The S t a n f o r d Geothermal R e s e r v o i r Workshops are now f i r m l y e s t a b l i s h e d as a n i n t e r n a t i o n a l forum f o r t h o s e i n t e r e s t e d i n t h e geothermal r e s e r v o i r , i t s s t u d y , and i t s development. The i n d u s t r y i s y e t i n i t s infancy with respect t o i t s f u t u r e p o t e n t i a l i n meeting t h e n a t i o n ' s energy requirements. Thus, t h e problems of geothermal r e s e r v o i r e n g i n e e r i n g need t o be i n v e s t i g a t e d f o r some t i m e y e t . New a p p r o p r i a t e t o p i c s f o r f u t u r e workshops w i l l n o t be i n s h o r t supply. The Proceedings of t h e Workshops have become a n important p a r t of t h e b a s e l i n e of c u r r e n t knowledge of geothermal r e s e r v o i r s . Howe v e r , t h e d i s t r i b u t i o n t o d a t e h a s been r a t h e r l i m i t e d . With wider r e f e r e n c e s t o them appearing i n t h e g e n e r a l l i t e r a t u r e ,

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Proceedings Seventh Workshop Geothermal Reservoir Engineering Stanford. December 1981. SGP-TR-55.

OVERVIEW

- UNIVERSITY FESEARCH

I N GEOTHERMAL RESERVOIR ENGINEERING

Roland N. Horne

Stanford University

University research into reservoir e n g i n e e r i n g a s p e c t s of geothermal u t i l i s a t i o n has undergone a long development s i n c e work f i r s t g a t h e r e d speed i n t h e e a r l y 1970's. University research, a p p r o p r i a t e l y enough, h a s remained somewhat academic throughout t h a t t i m e although with a c o n s t a n t watch over the philosophically troubling problems a s s o c i a t e d with i n d u s t r i a l u t i l i s a t i o n of t h e resource. Research in the university environment i s charged with f i n d i n g t h e answer "why?" as much as "how?".

Analytical mode l l i n g is somewhat i n a p p r o p r i a t e l y named s i n c e most examples use numerical methods, however t h e term has c o m e t o be used with reference to lumped parameter, l i n e a r i s e d or otherwise s i m p l i f i e d models. Some of t h e s e models such as U n i v e r s i t y of Colorado's fault- charged model (Kassoy and Goyal, 1979) grew o u t of earlier convection work, while o t h e r s a t S t a n f o r d University originated from petroleum engineering material balance techniques (Brigham and N e r i , 1980, and Westwood and Castanier- - this conference). Prof. Gunnar Bodvarsson of Oregon S t a t e U n i v e r s i t y has a l s o c o n t r i b u t e d a wide range of conceptual The models from h i s geophysical backqround. arguments concerning t h e a p p r o p r i a t e n e s s of t h i s t y p e of model compared t o f u l l scaLe d i s t r i b u t e d parameter models have y e t t o be r e s o l v e d , and it is l i k e l y t h a t work i n bohh areas w i l l c o n t i n u e i n r e s e a r c h , a s it does i n industry.

The P a s t One of t h e e a r l i e r popular areas of u n i v e r s i t y r e s e a r c h was t h e behaviour of convection i n porous media--a r e g i o n of i n t e r e s t s t i m u l a t e d by fundamental q u e s t i o n s of how geothermal r e s e r v o i r s form and what mechanisms are important in their behaviour. Work i n this a r e a was done a t the U n i v e r s i t y of Hawaii by Prof. Ping Cheng, a t U n i v e r s i t y of Colorado by Prof. David Kassoy, a t S t a n f o r d U n i v e r s i t y by P r o f s . George Homsy and Roland Horne, and a t UCLA by Prof. George Schubert and D r . Joe S t r a u s . Physical s i t u a t i o n s i n which nonisothermal f l u i d flow through porous media is important also appear i n o t h e r e n g i n e e r i n g f i e l d s and have been studied i n other u n i v e r s i t i e s with frequent c r o s s r e f e r e n c e s t o geothermal energy. Much p r o d u c t i v e work w a s generated i n t h e area of convection, however most a c t i v i t y d r e w t o a c l o s e with a s u b t l e change of emphasis towards more p r a c t i c a l problems, perhaps o r i g i n a t i n g w i t h ERDA' s 1975 catch- phrase of "megawatts on- line".

P h y s i c a l modelling of f u l l scale systems has never been undertaken i n u n i v e r s i t y r e s e a r c h ( d e s p i t e s u g g e s t i o n s of sand box models a t Colorado S t a t e U n i v e r s i t y Fort Collins), b u t l a b o r a t o r y s t u d i e s of l a r g e scale h e a t t r a n s f e r has been undertaken i n t h e chimney model a t Stanford.

-

With t h e change i n t h e l a t e r 1970's towards more immediate p r a c t i c a l problems, g r e a t e r a t t e n t i o n was p l a c e d upon w e l l t e s t i n g and w e l l test analysis. Research i n t h i s a r e a had been i n p r o g r e s s a t S t a n f o r d under the d i r e c t i o n of Prof. Ramey s i n c e c o n s i d e r a b l y b e f o r e t h a t t i m e , however S t a n f o r d w a s j o i n e d by t h e U n i v e r s i t y of Hawaii and o t h e r nanuniversity establishments i n t h e renewed interest. W e l l t e s t i n g is an a r e a on which a l l sectors of t h e geothermal r e s e r v o i r e n g i n e e r i n g area depend, i n c l u d i n g numerical s i m u l a t i o n , economics, r e s e r v o i r modelling, s t a t i o n design, p i p e l i n e design and even environmental impact i n v e s t i g a t i o n .

Methods f o r r e s e r v o i r modelling have been i n v e s t i g a t e d continuously from t h e beginning and s t i l l a r e of c u r r e n t i n t e r e s t . The f i e l d can be s p l i t somewhat u n t i d i l y i n t o t h e r e g i o n s of a n a l y t i c a l modelling, p h y s i c a l modelling and numerical modelling. Full scale numerical modelling has been undertaken George a t t h e u n i v e r s i t y l e v e l by Prof. P i n d e r a t P r i n c e t o n U n i v e r s i t y and Prof. Paul Witherspoon a t U. C. Berkeley who have made major c o n t r i b u t i o n s i n p h i l o s o p h i c a l i n s i g h t s i n t o a s p e c t s of t h e f i e i d . Nuts- and- bolts code development h a s remained p r i n c i p a l l y o u t s i d e t h e academic environment and h a s been t a c k l e d by commercial companies and t h e national laboratories.

I n t e r e s t i n fundamental rock and f l u i d p r o p e r t i e s has been maintained because of t h e continued importance t h a t p h y s i c a l and chemical behaviour have i n e x p l o r a t i o n , w e l l test a n a l y s i s and simulation. Work i n f l u i d flow has been followed a t S t a n f o r d U n i v e r s i t y

-5-

i n the bench scale experiments i n v e s t i g a t i n g a b s o l u t e p e r m e a b i l i t y , steam/water r e l a t i v e p e r m e a b i l i t y and a d s o r p t i o n . The impact of t h e a d s o r p t i o n experiments has been f a r reaching in the engineering of vapor dominated systems, but t h e determination of s t e a m / w a t e r r e l a t i v e p e r m e a b i l i t y (which is of major importance i n r e s e r v o i r s i m u l a t i o n ) The d i f f i c u l t y i n c o n t i n u e s t o be e l u s i v e . measuring r e l a t i v e p e r m e a b i l i t i e s may w e l l arise philosophically i n t h e i r definition, making t h e i r study an i d e a l c a n d i d a t e f o r academic r e s e a r c h .

behaviour of f r a c t u r e d systems is a l s o i n progress.

The chemical behaviour of geothermal f l u i d flow has a l s o been i n v e s t i g a t e d i n u n i v e r s i t y r e s e a r c h , with t h e m i n e r a l o g i c a l and s t a b l e i s o t o p e s t u d i e s performed a t U. C. R i v e r s i d e and the non condensable gas studies at Stanford.

"A Depletion Brigham, W. E . , and N e r i , G.: Model o f t h e Gabbro Zone," Proc. Second DOE-ENEL Workshop Cooperative Research Geothermal Energy. LBL report 11555, January 1981.

The P r e s e n t and Future The p r i n c i p a l p h i l o s o p h i c a l problem s u f f e r e d by geothermal r e s e r v o i r e n g i n e e r i n g i s t h e a p p l i c a t i o n of techniques developed f o r porous media t o systems which have substantial fracture permeability. A g r e a t d e a l of e f f o r t i n u n i v e r s i t y r e s e a r c h is now b e i n g a p p l i e d i n t h i s direction. T r a c e r t e s t i n g h a s been p e r c e i v e d as one of t h e clearest means t o d e f i n e f r a c t u r e systems and t h e design and i n t e r p r e t a t i o n of a major tracer f i e l d test has been undertaken by Stanford in cooperation with the Instituto de I n v e s t i g a c i o n e s E l e c t r i c a s i n Mexico. Well test analysis, and non condensable gas monitoring t e c h n i q u e s are being developed f o r this use also, and the heat transfer

I n September 1980, LBL d r a f t e d a n updated p l a n (Howard, Goldstein and G r a f , 1980) f o r t h e s u p p o r t of geothermal r e s e r v o i r e n g i n e e r i n g r e s e a r c h by t h e U.S. Department of Energy. Twenty t o p p r i o r i t y r e s e a r c h needs were i d e n t i f i e d and p r e s e n t l y 8 of t h e s e r e s e a r c h t o p i c s are under i n v e s t i g a t i o n ( e i t h e r f o r m a l l y or informally) a t universities. References

Howard, J. H . , G o l d s t e i n , N . E . , and Graf, A. N.; "An Updated Plan f o r Support of Research Related t o Geothermal Reservoir Engineering," LBL r e p o r t 10807, September 1980. Kassoy, D. R., and Goyal, K. P.; "Modelling H e a t and Mass T r a n s f e r a t t h e Mesa Anomaly, I m p e r i a l V a l l e y , C a l i f o r n i a , " LBL, GRFNP S e r i e s # 3 (LBL-87841, 1979. Westwood, J. D . , and Castanier, L.M.; " Application of a Lumped Parameter Model t o t h e Cerro P r i e t o Geothermal F i e l d , " Geothermal Resources Council, T r a n s a c t i o n s , 2 (1981) (also t h i s meeting).

-6-


THE ROLE OF THE NATIONAL LABORATORIES I N GEOTHERMAL RESERVOIR ENGINEERING

P.A.

Witherspoon and C.F.

Tsang

E a r t h Sciences Division Lawrence Berkeley Laboratory Berkeley, C a l i f o r n i a 94720

INTRODUCTION The n a t i o n a l l a b o r a t o r i e s , s i n c e t h e beginning of t h e n a t i o n a l geothermal energy development program, have played an important r e s e a r c h r o l e f o r t h e U. S. Department of Energy (DOE) and i t s predecessor agencies. These labo r a t o r i e s , s p e c i f i c a l l y , Lawrence Berkeley Labo r a t o r y ( L B L ) , Lawrence Livermore N a t i o n a l Labo r a t o r y (LLNL), IDS Alamos N a t i o n a l Laboratory (LANL) , Sandia National Laboratory (SNL) , Brookhaven National Laboratory (BNL) , Argonne National Laboratory (Am), and t h e Idaho N a t i o n a l Eng. Laboratory ( I N E L ) , have l a r g e , m u l t i d i s c i p l i n e d s c i e n t i f i c , e n g i n e e r i n g and t e c h n i c a l support s t a f f , and e x c e l l e n t r e s e a r c h f a c i l i t i e s , ena b l i n g t h e l a b o r a t o r i e s t o conduct and manage r e s e a r c h p r o j e c t s beyond t h e a b i l i t i e s of univ e r s i t y departments and many geothermal developers. LBL h a s t h e unique f e a t u r e of being loc a t e d a d j a c e n t t o t h e Berkeley campus of t h e U n i v e r s i t y of C a l i f o r n i a . A s a r e s u l t , f a c u l t y , g r a d u a t e s t u d e n t s and s t a f f augment t h e Laborat o r y s t a f f and make s i g n i f i c a n t c o n t r i b u t i o n s .

3. Because p u b l i c s e r v i c e and highq u a l i t y r e s e a r c h are c e n t r a l t o t h e f u n c t i o n i n g of n a t i o n a l l a b o r a t o r i e s , t h e y are capable Of p r o v i d i n g independent and unbiased assessments and s o l u t i o n s t o problems brought t o them. They a r e committed t o t h e t r a n s f e r of technology t o t h e p r i v a t e s e c t o r .

In g e n e r a l , t h e boundary r e l a t i o n s h i p between t h e n a t i o n a l l a b o r a t o r i e s and i n d u s t r y i s d e f i n e d by t h e l a b o r a t o r i e s ' focus on longterm, h i g h - r i s k g e n e r i c r e s e a r c h and t h e i r f a c i l i t i e s and s p e c i a l c a p a b i l i t i e s , many of which a r e l a c k i n g i n i n d u s t r i a l l a b o r a t o r i e s and a r e n o n e x i s t e n t w i t h i n t h e o r g a n i z a t i o n s of many p r i v a t e geothermal energy developers. At t h e same t i m e , t h e n a t i o n a l l a b o r a t o r i e s maint a i n c l o s e c o n t a c t with i n d u s t r y , t h u s e n s u r i n g relevancy of t h e r e s e a r c h and t h e t r a n s f e r of technology.

R e s e r v o i r Engineering The primary work i n qeothermal r e s e r v o i r e n g i n e e r i n g i s b e i n g done by Lawrence Berkeley Laboratory (LBL) LBL i s a l s o c a r r y i n g o u t s t u d i e s t h a t c o r r e l a t e geop h y s i c s with r e s e r v o i r engineering. Details on LBL's role i n r e s e r v o i r e n g i n e e r i n g f o l l o w i n t h e next section.

2. With t h e i r m u l t i d i s c i p l i n a r y s t a f f s , t h e n a t i o n a l l a b o r a t o r i e s have the proper mix of s c i e n t i s t s , e n g i n e e r s , t e c h n i c i a n s and mana g e r s needed t o conduct l o n g e r - t e r n r e s e a r c h , and t o respond m o r e q u i c k l y t o t h e emergency needs of DOE and o t h e r government agencies.

RECENT ACCOMPLISHMENTS O F THE NATIONAL LABORATORIES The c u r r e n t work performed by t h e national laboratories addresses various aspects of geothermal energy developnent i n n e a r l y all important areas. These areas and t h e c o r r e s ponding involvement of t h e v a r i o u s l a b o r a t o r i e s a r e reviewed below, w i t h emphasis being p l a c e d on t h e l a b o r a t o r i e s most involved w i t h t h e r e s p e c t i v e r e s e a r c h area.

.

Becausewe are most f a m i l i a r w i t h LBL and t h e r e s e a r c h performed t h e r e , and because LBL h a s been t h e l e a d l a b o r a t o r y i n geothermal reservoir e n g i n e e r i n g r e s e a r c h , t h i s paper w i l l d e a l l a r g e l y w i t h LBL's r o l e . From our perspect i v e , however, w e see t h e f o l l o w i n g b a s i c s t r e n g t h s w i t h i n most of t h e n a t i o n a l laborat o r ies:

Geochemistry Lawrence Livermore N a t i o n a l Labo r a t o r y (LLNL) h a s c a r r i e d o u t a comprehensive R&D program a t t h e S a l t o n Sea geothermal f i e l d i n t h e I m p e r i a l V a l l e y , C a l i f o r n i a . The program included r e s e r v o i r e v a l u a t i o n , s c a l e c o n t r o l , c o r r o s i o n , H2S abatement, and b r i n e i n j e c t i o n s t u d i e s . More r e c e n t l y LLNL a s s e s s e d t h e i n j e c t a b i l i t y of b r i n e s and methane extract i o n from geopressured r e s o u r c e s of t h e Gulf Coast.

1. S t a r t i n g with i n i t i a l strengths i n computer technology and advanced e n g i n e e r i n g f a c i l i t i e s , t h e l a b o r a t o r i e s are a b l e t o d e s i g n , b u i l d , and t e s t new or improved t o o l s t o g e t h e r w i t h s u p p o r t i n g methodologies and numerical analyses.

High Temperature D r i l l i n g and Completion Technology and Tools The development of d r i l l i n g completion and w e l l t e s t i n g technology h a s been c o n s i d e r a b l y advanced by t h e work of t h e national laboratories. New d r i l l i n g hardware and f l u i d s have k e n developed by Sandia

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p u t e r programs a v a i l a b l e t o i n t e r e s t e d u s e r s i n a l l areas of energy research. NESC m a i n t a i n s a s o f t w a r e l i b r a r y with f u l l documentation. softw a r e codes can be r e q u e s t e d by a l l types of o r g a n i z a t i o n s worldwide, and NESC'S services are provided a t v e r y nominal cost with no roya l t y charges f o r t h e programs. A s an example, NESC h a s e v a l u a t e d and adapted f o r d i s t r i b u t i o n f i v e computer programs developed a t LBL f o r geothermal r e s e r v o i r e n g i n e e r i n g a p p l i c a t i o n s ( r e s e r v o i r s i m u l a t i o n , wellbore flow, w e l l t e s t a n a l y s i s ) . Numerous r e q u e s t s f o r t h e s e programs have shown t h a t NESC can be e f f e c t i v e i n t r a n s f e r r i n g new methodology i n t o engineering practice.

N a t i o n a l Laboratory (SNL) They have made advances i n completion technology t h a t i n c l u d e downhole p e r f o r a t i o n , cementing, and w e l l cleaning. A s t h e l e a d l a b o r a t o r y f o r high- temperature component developnent, SNL h a s been i n s t r u mental i n p r o v i d i n g , through b o t h in- house p r o j e c t s and s u b c o n t r a c t o r s , components needed f o r logging high- temperature geothermal w e l l s . They have advanced hybrid c i r c u i t technology, and developed p r o t o t y p e logging t o o l s (275OC) and a metal s h e a t h c a b l e . Sandia h a s a l s o worked with t h e General E l e c t r i c Company t o produce a 275OC m u l t i p l e x e r and w i t h t h e H a r r i s Semiconductor Company t o develop hightemperature (275OC) e l e c t r o n i c components.

M a t e r i a l s Brookhaven N a t i o n a l Laboratory (BNL) h a s developed and t e s t e d s e v e r a l high-temperat u r e polymer c o n c r e t e systems f o r cementing BNL h a s a l s o i n v e s t i g a t e d geothermal w e l l s . nonmetallic m a t e r i a l s such as p l a s t i c s , ceramics, and r e f r a c t i n g cements f o r h a n d l i n g h o t b r i n e s and steam.

LBL'S ROLE AND ACCOMPLISHMENTSIN THE FIELD OF GEOTHERMAL RESERVOIR ENGINEERING S i n c e t h e emphasis of t h i s conference i s on reservoir e n g i n e e r i n g , t h i s s e c t i o n w i l l be l i m i t e d t o a summary of LBL's p a s t and p r e s e n t work i n t h i s a r e a . T h i s work i s c l a s s i f i e d i n t o s e v e r a l s e c t i o n s and d i s c u s s e d below.

D i r e c t - U s e Idaho N a t i o n a l Eng. Laboratory ( I N E L ) h a s developed s e v e r a l d i r e c t - u s e demons t r a t i o n p r o j e c t s a t R a f t River, Idaho. The

Cerro P r i e t o Cooperative P r o j e c t

LBL i s coord i n a t i n g t h e U.S. t e c h n i c a l a c t i v i t i e s being c a r r i e d o u t a t Cerro P r i e t o under an agreement between WE and Comisioh Federal de Electrici dad of Mexico. T h i s m u l t i d i s c i p l i n a r y program i n c l u d e s g e o l o g i c a l , geophysical, geochemical, subsidence, and r e s e r v o i r e n g i n e e r i n g s t u d i e s w i t h t h e purpose of developing a hydrogeol o g i c a l model of t h e geothermal system and t o a n a l y z e i t s response t o l a r g e - s c a l e f l u i d production.

l a b o r a t o r y h a s a l s o provided t e c h n i c a l a s s i p t a n c e f o r t h e d i r e c t - u s e geothermal p r o j e c t s . H o t Dry Rock Sponsored by DOE/DGE, t h e Hot Dry Rock (HDR) Geothermal Ehergy Development Pro-

gram is managed through a f i e l d o f f i c e a t t h e Los Alamos N a t i o n a l Laboratory (LANL) The HDR Program was e s t a b l i s h e d t o investigate- - and, i f p o s s i b l e , t o demonstrate- - the u s e f u l n e s s of n a t u r a l h e a t i n t h e e a r t h ' s c r u s t as a c o m e r c i a 1 source of energy.

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Some of t h e more important r e s u l t s o b t a i n e d by t h e v a r i o u s American (e.g., U.S.G.S., U.C. R i v e r s i d e and LBL) and Mexican groups involved i n t h e program are a s f o l l o w s :

Fenton H i l l , l o c a t e d i n a geothermal area approximately 30 km w e s t of Los A l a m o s , i s t h e s i t e of LANL's p i o n e e r i n g HDR h e a t - e x t r a c t i o n experiments. In May 1980, HDR technology w a s used to produce e l e c t r i c i t y i n an i n j e c t i o n A demonstration experiment a t Fenton H i l l . 60-kVA, binarycycle e l e c t r i c a l g e n e r a t o r was i n s t a l l e d i n t h e Phase I s u r f a c e system and h e a t from about 3 kg/s of geothermal f l u i d a t 132OC w a s used t o b o i l Freon R-114. The produced vapor was used t o d r i v e a turboalternator.

( 1 ) The l a t e r a l boundaries and t h e temperature d i s t r i b u t i o n w i t h i n t h e geothermal anomaly have been g e n e r a l l y e s t a b l i s h e d . (2) The g e n e r a l d i s t r i b u t i o n of d e l t a i c and marine l i t h o f a c i e s i n t h e f i e l d h a s been d e t e r mined and i s continuously r e v i s e d as new well d a t a become a v a i l a b l e .

( 3 ) A g e n e r a l p i c t u r e of t h e h e a t and mass flow p a t t e r n i n t h e system h a s been developed. The h o t f l u i d s are recharged from t h e east and n o r t h e a s t and are moving l a t e r a l l y towards t h e w e s t along t h e base of s h a l e u n i t s . The f l u i d s move upward through gaps i n t h e s e s h a l e u n i t s u n t i l t h e y f i n a l l y l e a k t o t h e s u r f a c e . After l a r g e - s c a l e production began i n 1973, c o l d e r w a t e r s began moving i n t o t h e geothermal reserv o i r from shallower a q u i f e r s and from t h e edges of t h e f i e l d . A more d e t a i l e d flow regime i s b e i n g p r e s e n t l y developed.

A Phase I1 system t h a t should approach com-

mercial requirements, c o n s i s t i n g of a pair of 3-km deep wells i n t o g r a n i t e a t 275OC, i s now o t h e r work being c o n s t r u c t e d a t Fenton H i l l . a t Fenton H i l l i n c l u d e s environmental monitori n g , development of equipment, i n s t r u m e n t s and materials f o r t e c h n i c a l s u p p o r t , developnent of s e v e r a l kinds of mathematical models, and a n a l y s i s of d a t a c o l l e c t i o n from e x t e n s i v e r e s o u r c e i n v e s t i g a t i o n s which was t h e n assemb l e d i n t o a geothermal g r a d i e n t map of t h e U.S.

( 4 ) Repeated s u r f a c e geophysical surveys ( d i p o l e - d i p o l e r e s i s t i v i t y ) have monitored changes i n t h e r e s e r v o i r caused by i t s exploitation. Apparent r e s i s t i v i t y i n c r e a s e s have

N a t i o n a l Energy Software C e n t e r (NESC) Argonne N a t i o n a l Laboratory (ANL) operates t h e N a t i o n a l Energy Software Center (NESC) which makes com-

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mural r e s e a r c h program w a s implemented, and LBL administered and t e c h n i c a l l y monitored r e s e a r c h c o n t r a c t s and disseminated t h e r e s u l t s t o t h e geothermal i n d u s t r y .

been d e t e c t e d over t h e o l d e r (western) part of t h e f i e l d a t depths of 1 km or g r e a t e r . Cm t h e o t h e r hand, l a r g e zones of decreased a p p a r e n t r e s i s t i v i t i e s have been observed w e s t and east of t h a t area. Modeling s t u d i e s a r e being made t o e s t a b l i s h whether changes i n s a l i n i t y , temperature, and/or steam s a t u r a t i o n s of t h e r e s e r v o i r f l u i d s can e x p l a i n t h e observed changes.

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(5) Based on chemical and r e s e r v o i r engineeri n g d a t a it w a s concluded t h a t mixture with c o o l e r w a t e r s r a t h e r t h a n b o i l i n g i s t h e domin a n t c o o l i n g p r o c e s s i n t h e n a t u r a l s t a t e , and t h a t production c a u s e s displacement of h o t water by c o o l e r water, and n o t by vapor. Local b o i l i n g occurs near most w e l l s i n response t o p r e s s u r e d e c r e a s e s , b u t no g e n e r a l vapor zone h a s formed. I t i s f e l t t h a t t h e s u c c e s s of a m u l t i d i s c i p l i n a r y p r o j e c t l i k e t h e Cerro P r i e t o s t u d y w i l l s t r o n g l y depend on t h e t e c h n i c a l and managerial c a p a b i l i t i e s of t h e o r g a n i z a t i o n c o o r d i n a t i n g t h e e f f o r t and flow of information between t h e p a r t i c i p a t i n g groups, a s w e l l a s t h i s organizat i o n ' s a b i l i t y t o be a c t i v e l y involved i n t h e r e s e a r c h phase. A n a t i o n a l l a b o r a t o r y with i t s r e s o u r c e s i n manpower, l a b o r a t o r i e s , and shops i s w e l l s u i t e d f o r such t e c h n i c a l and managerial roles.

Low-Temperature Research In 1981, DOE asked LBL and INEL t o develop and implement j o i n t l y a new program i n r e s e r v o i r e n g i n e e r i n g methodology and r e s e r v o i r assessment t e c h n i q u e s s p e c i f i c t o l o w - and moderate- temperature hydrothermal systems. U n t i l t h a t t i m e , l i t t l e e f f o r t had been devoted t o t h e understanding of t h e s e r e s e r v o i r s . Accomplishments t o d a t e i n c l u d e development of a computational model f o r t h i n , f a u l t - c h a r g e d hydrothermal r e s e r v o i r s , development of w e l l t e s t i n g i n s t r u m e n t a t i o n , compilat i o n and p u b l i c a t i o n of c a s e s t u d i e s of low t o moderate- temperature r e s e r v o i r s and p r e p a r a t i o n o f a handbook with r e s e r v o i r e n g i n e e r i n g guidel i n e s f o r p o t e n t i a l developers. High- temperature Research S i n c e 1976, LBL h a s conducted a s i z e a b l e r e s e a r c h e f f o r t i n hightemperature geothermal r e s e r v o i r engineering. Some of t h e h i g h l i g h t s are as follows:

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( 1 ) International cooperation Through a number of i n t e r n a t i o n a l c o o p e r a t i v e p r o j e c t s ( I t a l y , I c e l a n d , Mexico, N e w Zealand), it h a s been p o s s i b l e t o o b t a i n a c c e s s t o geothermal o p e r a t i n g experience and f i e l d performance d a t a . T h i s h a s f a c i l i t a t e d r e c o g n i t i o n of technologi c a l problems and h a s made p o s s i b l e t h e development of novel methods f o r geothermal r e s e r v o i r engineering. Much v a l u a b l e i n f o r m a t i o n h a s been made a v a i l a b l e t o t h e geothermal community i n t h e United States.

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(2) GREMP LBL developed t h e Geothermal R e s e r v o i r Engineering Management Plan (GREMP) f o r DOE a s a guidance f o r R&D p o l i c y i n geothermal r e s e r v o i r engineering. A comprehensive e x t r a -

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( 3 ) Reservoir Modeling L B L ' s in- house res e a r c h e f f o r t s have i n t e g r a t e d t a l e n t from v a r i o u s d i s c i p l i n e s (geology, hydrology, geop h y s i c s , r e s e r v o i r e n g i n e e r i n g , p h y s i c s , mathematics) t o develop q u a n t i t a t i v e methods f o r geothermal r e s e r v o i r e v a l u a t i o n and performance a n a l y s i s . Einphasis h a s been p l a c e d on t h e developnent of s o p h i s t i c a t e d y e t easy-to-use comp u t e r programs such a s SHAFT79 and PT ( o r C C C ) , which would be v i a b l e t o o l s f o r e n g i n e e r i n g a p p l i c a t i o n s . Novel methods have been demons t r a t e d through g e n e r i c s t u d i e s , a s w e l l a s through a p p l i c a t i o n s t o f i e l d d a t a . A workshop was h e l d to i n s t r u c t e n g i n e e r s from t h e geothermal i n d u s t r y i n t h e use of t h e s e computer programs. LBL s t a f f c o n t i n u e s t o a d v i s e i n t e r e s t e d i n d i v i d u a l s and o r g a n i z a t i o n s i n t h e a p p l i c a t i o n of advanced geothermal r e s e r v o i r e n g i n e e r i n g methods.

W e b e l i e v e t h a t our r e s e r v o i r modeling work h a s a l r e a d y had a s i g n i f i c a n t impact on geothermal development planning i n t h e United S t a t e s , and w i l l continue t o f a c i l i t a t e d e s i g n and optimiz a t i o n of e x p l o i t a t i o n s t r a t e g i e s .

Energy U t i l i z a t i o n - The U t i l i z a t i o n Technology Group a t LBL h a s had a s i t s f o c u s t h e binaryc y c l e energy conversion p r o c e s s . The binarycycle i s t h e leading candidate f o r u t i l i z a t i o n o f m o d e r a t e and low- temperature r e s o u r c e s f o r e l e c t r i c i t y generation. Optimization of t h e working f l u i d and c y c l e s t a t e p o i n t as funct i o n s of r e s o u r c e temperature and economic cond i t i o n s h a s been s t u d i e d u s i n g t h e powerful code "GEOTHM" developed a t LBL. Cycles u s i n g superc r i t i c a l m i x t u r e s of l i g h t hydrocarbons have been shown t o be o p t i m a l a t p r e s e n t c o s t l e v e l s . Heat exchangers c o n s t i t u t e a major p o r t i o n of t h e binarycycle power p l a n t c o s t , and t h e performances of both novel and conventional h e a t exchangers have been s t u d i e d under f i e l d condit i o n s . Laboratory measurements have a l s o been performed to e s t a b l i s h h e a t t r a n s f e r c o e f f i c i e n t s f o r butane/isopentane mixtures. A 500-kW d i r e c t - c o n t a c t h e a t exchanger p i l o t p l a n t h a s been designed, b u i l t , and t e s t e d a t E a s t Mesa. LBL i s p r e s e n t l y s u p p o r t i n g t h e Heber Binary Demonstration Power P l a n t by p a r t i c i p a t i n g i n d e s i g n reviews and i n t h e d e s i g n of t h e d a t a a c q u i s i t i o n system f o r t h e p l a n t .

F i e l d T e s t i n g - A t t h e o n s e t of government programs t o s t i m u l a t e t h e developnent of g e e thermal r e s o u r c e s i n t h e United States, t h e r e w a s a need t o develop improved methodology f o r t e s t i n g geothermal w e l l s and geothermal reserv o i r s . Because w e l l t e s t i n g i s t h e most comnon and r e l i a b l e technique f o r p r o v i d i n g d a t a on t h e i n s i t u r e s e r v o i r parameters, it i s essent i a l to have a well- developed t e s t methodology. As p a r t o f t h e LBL Geothermal Reservoir Engin e e r i n g Program, w e l l t e s t i n g methods, procedure,

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i n s t r u m e n t a t i o n , and i n t e r p r e t a t i o n have been investigated.

(2) R a c t u r e d porous media s t u d i e s Because most geothermal reservoirs are h i g h l y f r a c t u r e d , it i s important t h a t p i o n e e r i n g work on f r a c t u r e d porous media done a t LBL b e continued.

W e l l t e s t i n g methods developed f o r geothermal

w e l l t e s t i n g have followed t h e l e a d of t h e petroleum i n d u s t r y . Three t y p e s of t e s t s production t e s t s , i n j e c t i o n t e s t s , and i n t e r f e r e n c e t e s t s - - a s s i m i l a t e d from t h e petroleum i n d u s t r y were determined t o be t h e most s u i t a b l e t o geothermal r e s e r v o i r s . To make t h e s e t e c h n i q u e s a p p l i c a b l e t o geothermal r e s e r v o i r s , t h e t h e o r y h a s been modified t o i n c l u d e t h e e f f e c t s of two-phase ( steam-water) n o n i s o t h e r m a l flow i n t h e r e s e r v o i r , two-phase and noni s o t h e r m a l wellbore flow w i t h v a r i a b l e r a t e , and f r a c t u r e flow. For h i g h l y s a l i n e o r g a s eous r e s e r v o i r s , an a c c u r a t e e q u a t i o n of s t a t e must be developed and included i n t h e c a l c u l a t i o n s . To d a t e very l i t t l e d a t a e x i s t on t h e p r o p e r t i e s of s a l i n e and gaseous b r i n e s a t e l e v a t e d temperatures and p r e s s u r e s . This is a major a r e a of on-going r e s e a r c h intended t o f a c i l i t a t e t h e d e v e l o p e n t of geothermal reservoirs.

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( 3 ) Geophysics There i s continued need f o r t h e monitoring of reservoir changes by geophysi c a l means.

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( 4 ) Geochemical s t u d i e s There i s g r e a t potent i a l f o r f u t u r e developnent and a p p l i c a t i o n of geochemical methods i n r e s e r v o i r engineering.

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G a s chemistry ( 5 ) G a s chemistry e f f e c t s e f f e c t s on r e s e r v o i r a n a l y s i s must b e m o r e thoroughly s t u d i e d .

In t h e past, a major o b s t a c l e t o t h e implementat i o n of w e l l t e s t i n g t e c h n i q u e s h a s been t h e l a c k of i n s t r u m e n t a t i o n and measurement d e v i c e s f o r high- temperature and high- pressure a p p l i c a t i o n . Temperature, p r e s s u r e , and flow must be measurable both downhole and a t t h e s u r f a c e , a t s u f f i c i e n t accuracy t o o b t a i n r e l i a b l e estimates of t h e r e s e r v o i r parameters. In 1976, t h e r e were o n l y two systems a v a i l a b l e f o r o b t a i n i n g downhole p r e s s u r e : a mechanical d e v i c e designed f o r t h e o i l and gas i n d u s t r y , and an e l e c t r o n i c d e v i c e r a t e d t o 150OC. Obtaining downhole p r e s s u r e , temperature, and flow d a t a a t tempera t u r e s g r e a t e r than 150°C w a s l i m i t e d by l a c k o f high temperature e l e c t r o n i c s , t r a n s d u c e r s , seals, c a b l e heads, and c a b l e . N o w , a s t h e r e s u l t of t h e development of high temperature e l e c t r o n i c s a t Sandia, t h e p o t e n t i a l e x i s t s t o develop i n s t r u m e n t s s u i t a b l e f o r use a t t e m p e r a t u r e s of up t o 275OC. Concurrent t o t h e Sandia program, t h e r e s e r v o i r e n g i n e e r i n g group a t LBL h a s c o n c e n t r a t e d on developing t o o l s t h a t use only surface electronics. Using t h i s p h i l osophy, a downhole pressure/temperature/flow t o o l r a t e d t o 225OC h a s been developed. I t inc o r p o r a t e s a Bell and Howell CEC 1000-4 series p r e s s u r e t r a n s d u c e r , an RTD, and a modified Kuster spinner. A s p i n n e r (downhole flow meter) and a temperature t o o l , both r a t e d t o 300°C, have a l s o been developed and f a b r i c a t e d .

FUTURE POSSIBLE RESEARCH AREAS A g r e a t d e a l of important work remains t o be done by t h e n a t i o n a l l a b o r a t o r i e s i n geothermal r e s e r v o i r engineering. F u t u r e c h a l l e n g i n g areas of research are: ( 1 ) I n j e c t i o n s t u d i e s - Many geothermal f i e l d s r e q u i r e i n j e c t i o n i n t h e very near f u t u r e , and many important problems i n t h i s a r e a have t o be addressed a s soon a s p o s s i b l e .

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Acknowledgments we acknowledge t h e i n p u t from v a r i o u s members of t h e geothermal group a t LBL, i n p a r t i c u l a r , Norman G o l d s t e i n , Karsten P r u e s s , Marcel0 L i p p a n n , S a l l y Benson, Robert Fulton, and Mary Bodvarsson. T h e i r a s s i s t a n c e and coo p e r a t i o n is g r a t e f u l l y a p p r e c i a t e d . T h i s work was supported by t h e A s s i s t a n t S e c r e t a r y f o r Conservation and Renewable Energy, O f f i c e of Renewable Technology, Division of Geothermal and Hydropower Technologies of t h e U.S. Department of Energy under C o n t r a c t No. W-7405-ENG-48.


GEOTHERMAL RESERVOIR ENGINEERING: THE ROLE OF THE U.S. GEOLOGICAL SURVEY

Wendell A. Duffield Geological Survey 345 Middlefield Rd. Menlo Park, CA 94025

U.S.

Introduction The U.S. Geological Survey has a legislated mandate t o assess and inventory the Nation's geothermal resources. Accordingly , principal objectives of the Survey's Geothermal Research Program are: ( 1 ) t o determine the geologic, geochemical , geophysical , and hydrologic characteristics of a l l types of geothermal systems; ( 2 ) t o improve and develop techniques of exploration f o r and assessment of geothermal resources; ( 3 ) t o characterize the seismicity, ground deformation, and hydrologic changes t h a t may be induced by the production of geothermal f l u i d s and t h e i r subsequent injection back i n t o the subsurface; and ( 4 ) t o determine the amount of geothermal energy t h a t e x i s t s as a national resource and periodically t o update t h i s assessment as new information becomes available. Research t h a t addresses objectives 1 , 2 , and 3 is designed t o provide information t h a t permits an increasingly accurate assessment of the Nation's geothermal resources.

behavior of a vapor-dominated hydrothermal system; computer-assi sted numerical modeling has also been applied t o hot-water and two-phase hydrothermal systems. Precise measurement of ground deformation a t The Geysers, California, has suggested a correl a t i on between steam production and subsidence. Similarly, precise measurements of gravity a t The Geysers have suggested a correl a t i on between steam production and changes i n the strength of the gravity f i e l d . Research i n well-logging equipment has resulted i n development of an acoustic televiewer t h a t can map the size, orientation, and position of fractures i n high-temperature geothermal boreholes. Laboratory measurements of the electrical and magnetic properties of rocks under simul ated geothermal conditions he1 p constrain a1 1 modeling o f geothermal reservoirs. Geologic studies of the Survey's Geothermal Research Program have provided information on the nature of the heat source, the lithology of reservoir rocks, and the inferred nature of permeability f o r several geothermal systems. Several geoel e c t r i c techniques have been used t o map the electrical conductivity of the c r u s t and upper mantle; re1 atively deep probing techniques have provided information on heat sources, and shallower probing techniques have helped t o outline the s i z e and position of geothermal reservoirs themselves. Research u s i n g active and passive seismic techniques has identified crustal zones of low seismic velocity t h a t may r e f l e c t the presence of magma, has outlined structures important t o del ineating geothermal reservoirs, and has discovered apparent anomalies i n the e l a s t i c properties of rocks t h a t may constitute geothermal reservoirs. Monitoring of seismicity a t The Geysers has shown a correlation between areas of steam production and small-magnitude earthquakes. Measurements of temperature gradient and thermal conductivity i n crustal rocks have delineated several broad regions of characteristic and differing heat flow and have provided considerable information on the thermal budget of specific hydrothermal convection systems. Hydrologic studies have

Reservoir-Related Research Since i t s formal establishment i n I Y / I , the Survey's Geothermal Research Program has supported f i e l d , laboratory, and theoretical studies i n geology, geophysics, geochemistry, and hydrology. Duffield and Guffanti (1981a,b) have recently sumnarized the history of the program from 1971 through 1980. The program's broad-based, mu1 tidisciplinary character r e f l e c t s t h e f a c t t h a t any comprehensive understanding of geothermal systems requires studies of rocks, f l u i d s , and a host of canplex interactions between the two. I f reservoir engineering, the subject of this workshop, i s defined i n a broad sense t o include a l l studies t h a t either directly or indirectly help t o characterize a geothermal reservoir, most of the research of the Survey's Geothermal Research Program addresses some aspect of today's topic. Some of the Geological Survey's geothermal

research i s quite directly related t o reservoir engineering Laboratory experiments, development of theory, and computer-assisted numerical modeling have attempted t o describe the characteristic

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d e f i n e d t h e water budget o f some geothermal areas and, coupled w i t h chemical analyses o f waters, have provided a powerful t o o l f o r d e f i n i n g t h e o r i g i n and t r a c i n g t h e general path o f f l u i d s i n hydrothermal systems.

Geological Survey's geothermal research has been o f t h e surface v a r i e t y . Recent research d r i l l i n g , however, r e f l e c t s program p o l i c y t o seek o p p o r t u n i t i e s t o make measurements i n and study samples from boreholes. Geological Survey research d r i l l i n g funded by t h e U.S. Department o f Energy demonstrated i n J u l y 1981 a p o t e n t i a l geothermal resource f o r n o n e l e c t r i c a l use i n t h e v i c i n i t y o f Mount Hood, Oregon; a pumping t e s t produced 380 L h i n from an 80°C a q u i f e r a t about 1,200-m depth. A t Newberry caldera, Oregon, research d r i l l i n g funded and c a r r i e d o u t by t h e Geological Survey i n 1981 discovered a 265% permeable zone i n t h e bottom few meters o f a 932-m-deep borehole; a 20-hour f l o w t e s t from t h i s zone produced a sample o f geothermal f l u i d whose chemical a n a l y s i s should p r o v i d e considerable i n f o r m a t i o n on the composition and thermodynamic s t a t e o f t h e r e s e r v o i r f l u i d . The recovery o f n e a r l y continuous c o r e from t h e Newberry h o l e provides abundant o p p o r t u n i t i e s f o r studies o f t h e r e s e r v o i r and o v e r l y i n g rocks.

Geochemical study o f f l u i d s probably has been t h e most f r u i t f u l s i n g l e area o f research f o r c h a r a c t e r i z i n g hydrothermal systems; many widely used geochemical techniques have been pioneered by t h e Geological Survey. The chemistry o f surface f l u i d samples has been used t o d i s c r i m i n a t e between vapor-dominated and hot- water systems, t o estimate subsurface temperatures, t o c o n s t r a i n t h e p o s s i b l e o r i g i n s o f f l u i d s and t h e i r recharge areas, and t o estimate t h e residence time o f f l u i d s i n many hydrothermal systems. I n recognition o f t h e i r cost- effective e x p l o r a t i o n p o t e n t i a l , a v a r i e t y o f chemical geothermometers a r e used i n t h e e a r l y stages o f geothermal e v a l u a t i o n worldwide. Techniques have a l s o been developed t o a d j u s t geothermometer temperatures f o r d i l u t i o n o f thermal water w i t h cool shallow ground water d u r i n g convective f l o w t o t h e surface. Analyses f o r deuteriun, oxygen-18, and t r i t i u m commonly a r e used t o estimate t h e o r i g i n and residence time o f hydrothermal f l u i d s . C u r r e n t research holds some promise f o r developing a d d i t i o n a l means o f e s t i m a t i n g residence time, through analysis o f various radionuclides i n hydrothermal f l u i d s . Moreover, geochemical m o n i t o r i n g o f f l u i d s from areas under production- - for example, a t L a r d e r e l l o , I t a l y , and Cerro P r i e t o , Mexico--has provided considerable i n s i g h t i n t o t h e f u n c t i o n i n g o f geothermal r e s e r v o i r s ; time-dependent changes i n t h e composition of produced f l u i d s , e s p e c i a l l y when considered together w i t h such more t r a d i t i o n a l production data as temperature and pressure , p r o v i de in f ormat ion c r iti c a l t o successful long- term e x p l o i t a t i o n s t r a t e g i e s o f such r e s e r v o i r s . D u f f i e l d and Guff a n t i (1981a,b) have sumnarized t h e f i n d i n g s o f o t h e r Geological Survey research t h a t addresses various aspects o f geothermal r e s e r v o i r s .

Such sampling o f r e s e r v o i r f l u i d s and rocks i s t h e most d i r e c t way t o t e s t inferences made from surface studies. The Survey's Geothermal Research Program i s phi1 o s o p h i c a l l y comnitted t o seeking o p p o r t u n i t i e s f o r research d r i l l i n g t h a t make t h i s type o f d i r e c t sampling possible. The Survey p l a y s an a c t i v e r o l e i n promoting t h e goals o f t h e Continental S c i e n t i f i c D r i l l i n g Comnittee o f t h e U.S. National Academy o f Sciences, s p e c i f i c a l l y t h e proposal o f t h i s o r g a n i z a t i o n ' s Thermal Regimes Panel t o d r i l l i n t o t h e r o o t s o f a hydrothermal system a t roughly 500% and 6- t o 7-km depth. Research d r i l l i n g i n t o i n c r e a s i n g l y h o t t e r and deeper p a r t s o f geothermal r e s e r v o i r s may be t h e t o o l needed f o r major advances i n our understanding o f hydrothermal systems. References D u f f i e l d , W. A., and G u f f a n t i , Marianne (1981 a), The Geothermal Research Program o f t h e Geological Survey: U.S. Geological Survey Open-File Report 81-564, 108 p.

Research Trends F i e l d - o r i e n t e d s t u d i e s o f geothermal r e s e r v o i r s may conveniently be grouped i n t o surface ( i n c l u d i n g shallow heat- flow d r i l l i n g ) and subsurface categories. With r a r e exception, t h e

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(1981b) , The Geothermal Research Program o f t h e Geological Survey: U.S. Geological Survey C i r c u l a r , i n press.

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GEOTHERMAL RESERVOIR ENGINEERING DEVELOPMENT THROUGH INTERNATIONAL COOPERATION

F. G. Miller H. J. Ramey, Jr.

Stanford U n i v e r s i t y S t a n f o r d , CA USA

I n t r o d u c t i o n About twenty y e a r s ago when t h e Geysers Geothermal Steam F i e l d i n C a l i f o r n i a f i r s t s t a r t e d producing steam i n s i g n i f i c a n t q u a n t i t i e s f o r t h e g e n e r a t i o n of e l e c t r i c i t y t h e r e w e r e probably no mxe than f i v e or s i x people i n t h e U.S.A. who could q u a l i f y as geothermal r e s e r v o i r engineers. Then as now, t h e Geysers f i e l d w a s t h e only f i e l d i n t h e U.S.A. producing steam i n s i g n i f i c a n t q u a n t i t i e s on a commercial b a s i s . W e are now e n t e r i n g a new era, however, and can expect steam p r o d u c t i o n i n t h i s country t o r i s e markedly i n t h e next few years.

modeling, and s t u d i e s of p h y s i c a l models i n the laboratory. All t h i s i s f o r t h e purpose of developing and applying techniques t o f o r e c a s t w e l l and r e s e r v o i r d e l i v e r a b i l i t i e s and u l t i m a t e economic r e c o v e r i e s under dLff e r e n t operating conditions. I n some i n s t a n c e s the concept of r e s e r v o i r e n g i n e e r i n g i s much broader, encompassing a d d i t i o n a l f u n c t i o n s which may i n c l u d e land management, d r i l l i n g , e n g i n e e r i n g , production geology, production engineering, pipeline t r a n s p o r t a t i o n of p r o d u c t s t o local s t o r a g e f a c i l i t i e s or power p l a n t s , and managerial decision- making r e g a r d i n g such t h i n g s a s redrills, s e l e c t i o n of s i t e s f o r new w e l l s , p r o c e s s i n g f a c i l i t i e s , or power p l a n t s . This broad concept p l a c e s t h e r e s e r v o i r engineer i n t h e role of r e s e r v o i r manager and i n t h i s c a p a c i t y he would also i n i t i a t e f e a s i b i l i t y s t u d i e s and a r r a n g e f o r r e s e a r c h along l i n e s t h a t appear best t o him.

I n 1960 t h e r e were a t l e a s t a few thousand persons i n t h e U.S.A. who w e r e p r a c t i c i n g r e s e r v o i r e n g i n e e r i n g i n t h e petroleum indust r y and i n groundwater management organizations. Thus t h e U.S.A. had a t i t s d i s p o s a l a great d e a l of t a l e n t and e x p e r t i s e b u t very l i t t l e experience i n e i t h e r t h e o p e r a t i o n of n a t u r a l underground steam r e s e r v o i r s o r i n t h e development of needed geothermal reserv o i r e n g i n e e r i n g technology. In contrast, v a r i o u s o t h e r c o u n t r i e s had been producing n a t u r a l underground steam r e s e r v o i r s f o r many years and had developed from p r a c t i c a l experience a c o n s i d e r a b l e fund of knowledge Hcswever, these about r e s e r v o i r behavior. c o u n t r i e s had no r e s e r v o i r e n g i n e e r s i n t h e s e n s e w e now conceive t h i s p r o f e s s i o n , and t h e r e was no common awareness of the need for new technology.

I n t h e geothermal community t h e conventional concept of r e s e r v o i r e n g i n e e r i n g has been adopted g e n e r a l l y although it h a s been rubj e c t t o change, and o f t e n t o a d d i t i o n s as more is l e a r n e d about new f a c t o r s a f f e c t i n g r e s e r v o i r behavior. B i n a t i o n a l c o o p e r a t i v e geothermal r e s e a r c h programs were developed as a l o g i c a l means t o b e n e f i t both t h e U.S.A. and t h e c o u n t r i e s Some b i n a t i o n a l prop a r t i c i p a t i n g with it. grams covered a number of areas of r e s e a r c h on geothermal energy, b u t from t h e o u t s e t , r e s e r v o i r e n g i n e e r i n g became widely recogn i z e d as an important s u b j e c t of g e n e r a l concern.

What w e conceive a s r e s e r v o i r e n g i n e e r i n g is of importance i n t h e p r e p a r a t i o n and unders t a n d i n g of i n t e r n a t i o n a l c o o p e r a t i v e agreements i n geothermal energy. Reservoir engin e e r i n g is r e l a t e d t o o t h e r branches of engin e e r i n g involved i n t h e development and opera t i o n of geothermal- fluid r e s e r v o i r s . As examples, it i s r e l a t e d t o d r i l l i n g , product i o n , and p r o c e s s engineering. I t is a l s o r e l a t e d t o management f u n c t i o n s p e r t a i n i n g t o development and production.

E f f o r t s were made e a r l y in b i n a t i o n a l s t u d i e s t o develop new geothermal r e s e r v o i r engineeri n g technology through a p p l i c a t i o n s and m d i f i c a t i o n s of e x i s t i n g petroleum and groundwater technology. These s t u d i e s l e d t o a b e t t e r understanding of t h e p h y s i c a l n a t u r e of geothermal r e s e r v o i r s , a deeper appreciat i o n of t h e p h y s i c a l d i f f e r e n c e s between steam, petroleum, and groundwater r e s e r v o i r s ; and they helped r e s e a r c h e r s avoid d u p l i c a t i o n of e f f o r t

The c o n v e n t i o n a l concept l i m i t s its scope mainly t o : (1) Analyses of w e l l l o g s , ( 2 ) Analyses of r e s e r v o i r p r e s s u r e , temperature, production and w e l l - e f f l u e n t composition h i s tories, ( 3 ) The design implementation, and a n a l y s i s of f i e l d tests, ( 4 ) theoretical s t u d i e s of r e s e r v o i r behavior, r e s e r v o i r

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Laboratory s t u d i e s of p h y s i c a l models were made, computer models w e r e developed, and reservoir engineering f i e l d studies w e r e made. Generally accepted methods of approach w e r e followed i n making f i e l d s t u d i e s . F i e l d performance d a t a w e r e used t o formulate hypot h e s e s r e g a r d i n g t h e n a t u r e of r e s e r v o i r s . These hypotheses were then t e s t e d u s i n g physi c a l o r mathematical models, o r both. The p h y s i c a l laws involved i n t h e performance of t h e r e s e r v o i r s could t h e n be recognized so t h a t p e r t i n e n t e n g i n e e r i n g e q u a t i o n s could be d e r i v e d and solved. I f r e s u l t s from such e q u a t i o n s appear v a l i d based on comparisons t o f i e l d performance d a t a , t h e n it is poss i b l e t o study v a r i o u s methods of f i e l d development and production, and t o f o r e c a s t performance.

problems which might arise. They should reco g n i z e t h a t s o c i a l , c u l t u r a l , and economic differences a r e ever present but w i l l not cause d i f f i c u l t i e s i f both s i d e s a c t i n good f a i t h and d i s p l a y good w i l l . The ways of doing b u s i n e s s i n one country a r e not t h e Researchers and r e s e a r c h same as i n another. managers should a c c e p t t h i s f a c t and c o n s i d e r t h e i r p u r s u i t of j o i n t o b j e c t i v e s as a contest t o be won through p a t i e n c e and t h e b e s t use of their a b i l i t i e s and resources. International Cooperative Studies In t h e ensuing paragraphs w e d i s c u s s some of t h e i n t e r n a t i o n a l c o o p e r a t i v e work t h a t has been done i n geothermal energy i n r e c e n t years. By n e c e s s i t y our treatment of t h i s s u b j e c t is sketchy because t h e r e have been so many cooperative arrangements, formal and informal. Published information on many of t h e s e a p p a r e n t l y i s e i t h e r s c a r c e o r nonexistent. Moreover, we recognize that compiling available data for a more comprehensive t r e a t m e n t could become a v i r t u a l l y impossible t a s k , but would not a l t e r our views i f t h e c o o p e r a t i v e arrangements covered i n our d i s c u s s i o n are r e p r e s e n t a t i v e of t h e whole. L a s t l y , w e admit t h a t making our sel e c t i o n was n a t u r a l l y i n f l u e n c e d by p r o j e c t s w e are most f a m i l i a r with and t h o s e a r e t h e ones which have involved S t a n f o r d e i t h e r directly or indirectly. W e do not mean t o i m p l y i n any way t h a t S t a n f o r d ' s r o l e i n t h e development of geothermal r e s e r v o i r engineeri n g has been any m r e important than t h o s e of many o t h e r o r g a n i z a t i o n s concerned w i t h geothermal energy problems.

B i l a t e r a l programs a c t i v e d u r i n g r e c e n t y e a r s have been t h e source of a s u b s t a n t i a l fund of knowledge, u s e f u l new technology, and t r a n s f e r s of technology t o t h e U.S.A. These t e c h n i c a l g a i n s f o r p a r t i c i p a t i n g count r i e s would not be p o s s i b l e without w e l l understood w r i t t e n agreements c a r e f u l l y prepared i n advance of j o i n t r e s e a r c h a c t i v i ties. Some comments on t h e s e agreements f o l low. Comments on The Execution of I n t e r n a t i o n a l Cooperative Agreements I n t e r n a t i o n a l coopera t i v e r e s e a r c h h a s been government-sponsored, an outgrowth of p r i v a t e c o n s u l t i n g , o r a necessity for industrial development. Regardless of how they came about or how t h e y w e r e implemented, b i n a t i o n a l programs in which t h e U.S.A. has been a p a r t i c i p a n t have of benefit t o t h i s country.It is been important that participants sign international agreements with a s p i r i t of good will. Both s i d e s should be w e l l aware of t h e non- technical problems which can c o n f r o n t Joint them. There are many such problems. determination of t h e s p e c i f i c r e s e a r c h t o be undertaken is one of them. Another i s t h e manner i n which j o i n t r e s e a r c h p r o p o s a l s are t o be submitted t o government sponsors. Anot h e r i s naming i n t h e p r o p o s a l s the researchers from t h e p a r t i c i p a t i n g c o u n t r i e s and d e l i n e a t i n g t h e d i s t r i b u t i o n of work among them. L e s s s i g n i f i c a n t b u t a l s o important a r e t h e p l a n s t h a t must be made f o r working t o g e t h e r i n each o t h e r s home o f f i c e s and laboratories. T h i s involves t h e need for t r a v e l plans. I t has become clear t h a t b i n a t i o n a l it is t o be cooperative research, if s u c c e s s f u l , demands t h a t p a r t i c i p a n t s work t o g e t h e r a t f a i r l y f r e q u e n t i n t e r v a l s and w r i t e t h e i r reports together. Doing so i s unmistakable evidence of cooperation. If each s i d e works mainly a l o n e and a mere exchange of r e p o r t s t a k e s p l a c e a t t h e end of a c o n t r a c t p e r i o d , t h e r e s e a r c h cannot be judged as t r u l y cooperative.

Our purpose i s t o d i s c l o s e b e n e f i t s t o participating countries, t o indicate the p r a c t i c a l i t y of t h e s e b e n e f i t s , t e c h n i c a l l y and economically, and t o show t h a t i n t e r n a t i o n a l c o o p e r a t i v e work can a c c e l e r a t e t h e development of t h e geothermal i n d u s t r y and thereby ease energy s h o r t a g e problems.

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I n 1980 P r o f e s s o r H . J . Ramey, Costa R i c a Jr., of S t a n f o r d U n i v e r s i t y c o n s u l t e d f o r t h e government of Costa Rica on t h e M i r a v a l l e s geothermal f i e l d . He made p r e s s u r e t r a n s i e n t a n a l y s e s , designed w e l l t e s t s , and made engin e e r i n g assessments. H e p r e s e n t e d a t a l k on t h e M i r a v a l l e s f i e l d i n t h e f a i l of 1980 i n one of t h e weekly seminars of t h e S t a n f o r d Geothermal Program. One of the geothermal engineers at M i r a v a l l e s , Eduardo Granados, an employee of t h e Costa Rican government u t i l i t y , I C E , was i n v i t e d t o come t o S t a n f o r d d u r i n g t h e w i n t e r After comof 1980-81 as a v i s i t i n g s c h o l a r . p l e t i n g h i s v i s i t M r . Granados a p p l i e d f o r admission t o t h e graduate geothermal s t u d y program a t S t a n f o r d and w a s admitted. This k i n d of i n t e r a c t i o n with p r a c t i c i n g e n g i n e e r s throughout t h e world y i e l d s worthwhile benef i t s n o t only t o graduate s t u d e n t s and facu l t y a t S t a n f o r d , but also t o t h e U.S.A. as a whole. I t provides an important i n d i r e c t a i d

Researchers and r e s e a r c h managers on both s i d e s should be t o l e r a n t r e g a r d i n g language

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t o our n a t i o n a l geothermal development program. E l Salvador

should be a t t a c k e d . Moreover, no s p e c i a l funding f o r the work w a s provided f o r . Thus, i n t e r e s t waned.

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I n 1975 P r o f e s s o r W.E. Brigham of S t a n f o r d U n i v e r s i t y d i d some c o n s u l t i n g work f o r E l Salvador on t h a t c o u n t r y ' s Ahuachapan geothermal f i e l d . T h i s work concerned t h e p o s s i b i l i t y of i n c r e a s i n g t h e s i z e of the power p l a n t , t h e l o n g e v i t y of t h e f i e l d , loc a t i o n s f o r new w e l l s , and p o s s i b l e reinject i o n of produced water. The q u e s t i o n s which a r o s e c h a r a c t e r i z e d t h e t y p e of development planning t h a t u s e s r e s e r v o i r e v a l u a t i o n s as a basis f o r decision- making. The Ahuachapan f i e l d is t h e f i r s t h o t water f i e l d i n the western hemisphere t h a t produced from volc a n i c sediments. S i n c e 1975 a number of o t h e r s i m i l a r f i e l d s have been found, however, i n C e n t r a l America and t h e U.S.A.

Some b e n e f i t s t o both c o u n t r i e s d i d accrue, however, as a result of informal c o o p e r a t i v e arrangements. Lawrence Berkeley Laboratory (LBL) i n the U.S.A. and v a r i o u s i n s t i t u t i o n s i n I c e l a n d worked t o g e t h e r on t h e K r a f l a high- temperature a r e a of n o r t h e a s t Iceland. I c e l a n d w a s i n t e r e s t e d i n having work &ne a t LBL on t h e troublesome K r a f l a r e s e r v o i r . It w a s t h e f i r s t high- temperature r e s e r v o i r i n t h e country t o produce less t h a n what w a s expected o r i g i n a l l y . I c e l a n d has not y e t developed t h e experience t o d e a l with complex r e s e r v o i r s except f o r low- temperature r e s e r v o i r s where c l a s s i c a l groundwater hydrology can be a p p l i e d . Icel a n d e r s working i n geothermal energy have broad experience, but only a l i m i t e d p a r t is s p e c i f i c a l l y i n r e s e r v o i r engineering.

Consulting assignments of t h e k i n d t a k e n by P r o f e s s o r Brigham i n E l Salvador expand t h e experience background of r e s e r v o i r e n g i n e e r s , augment t h e i r understanding of geothermal r e s e r v o i r behavior and improve t h e s k i l l s they need to forecast reservoir performance. The new technology gained i s of primary value t o the U.S.A. a s an a i d i n t h e development of t h e American geothermal industry.

Although t h e problems a t K r a f l a have n o t y e t been solved t h e c o o p e r a t i v e e f f o r t with LBL h a s provided some i n s i g h t i n t o t h e p r o c e s s e s gains u n d e r l y i n g t h e s e problems. The U.S.A. t e c h n o l o g i c a l l y from i t s involvement w i t h t h e s e problems.

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S e v e r a l y e a r s ago there w a s a f o r Iceland m a l agreement i n e f f e c t between t h e Atomic and t h e IceEnergy Commission i n t h e U.S.A. land Energy Authority. This agreement c a l l e d f o r cooperation between t h e two c o u n t r i e s i n t h e f i e l d of geothermal energy. I t w a s broad i n scope and d e a l t more with conventional e n g i n e e r i n g m a t t e r s a t t h e ground s u r f a c e t h a n with geosciences. Apparently it w a s i n f o r c e f o r a p e r i o d of about f i v e y e a r s b u t used i n o n l y a few i n s t a n c e s . Experience d i s c l o s e d t h a t formal procedures were n o t necessary f o r t h e s e .

I n t h e e a r l y part of the 1970's, ProItaly f e s s o r H . J . Ramey, Jr. of S t a n f o r d U n i v e r s i t y d i s c u s s e d p o s s i b l e c o o p e r a t i v e r e s e a r c h between t h e U.S.A. and Ente Nazionale p e r 1'Energia E l e t t r i c a (ENEL) i n I t a l y . H e del i v e r e d l e c t u r e s i n I t a l y on r e s e r v o i r engin e e r i n g and geothermal r e s e r v o i r behavior. I n t h e s p r i n g of 1975 D r . Graziano Manetti and Engineer Antonio Barelli, both with ENEL came t o t h e Petroleum Engineering Department of S t a n f o r d U n i v e r s i t y as v i s i t i n g s c h o l a r s , f o r a p e r i o d of about two months. Cooperat i v e work between the U.S.A., through Stanf o r d , and ENEL was d i s c u s s e d a t length. Tent a t i v e p l a n s w e r e made a f t e r many d i s c u s s i o n s and seminars.

By means of t h e agreement t h e U.S.A. was s e e k i n g access t o t h e e x t e n s i v e background knowledge I c e l a n d had accumulated on none l e c t r i c a l a p p l i c a t i o n s such as d i r e c t h e a t i n g with geothermal water. The U.S.A. used no particular method to gain its objectives. Generally t h e a c t i v i t i e s of t h e w e r e l i m i t e d t o v i s i t s t o p l a n t s and U.S.A. t o other installations. These v i s i t s a l s o could have been a r r a n g e d informally.

L a t e r t h e proposed program was reviewed and d i s c u s s e d on a more formal b a s i s w i t h D r . R a f f a e l C a t a l d i of ENEL and some of h i s associates. Dr. Cataldi had already d i s c u s s e d with P r o f e s s o r Famey i n I t a l y the p r o s p e c t i v e Stanford-ENEL c o o p e r a t i v e e f f o r t , b e f o r e t h e v i s i t of D r . Manetti and Engineer Barelli. During t h e p e r i o d of D r . C a t a l d i ' s v i s i t j o i n t meetings were held. Those pres e n t included P r o f e s s o r s F.G. M i l l e r and H.J. Ramey, Jr. from S t a n f o r d , P r o f e s s o r P.A. Witherspoon and D r s . R. Schroeder and J - H Howard from LBL, D r . L.J.P. Muffler from t h e USGS, and D r . C a t a l d i and h i s a s s o c i a t e s from

Reservoir e n g i n e e r i n g w a s not mentioned i n t h e agreement, probably because t h i s branch of e n g i n e e r i n g was r e l a t i v e l y unknown i n t h e geothermal community a t t h e t i m e and it had been a p p l i e d t o few geothermal systems.At about t h e end of t h e 1970's decade t h e agreement w a s due f o r renewal, but n e i t h e r of t h e two c o u n t r i e s took t h e i n i t i a t i v e t o see t h a t t h i s w a s accomplished. Their passive a t t i t u d e may have been due t o t h e agreement's It w a s q u i t e general, o u t l i shortcomings. ning no s p e c i f i c programs which should be undertaken o r any s p e c i f i c problems which

ENEL.

These a c t i v i t i e s l e d t o a five- year agreement on c o o p e r a t i v e r e s e a r c h i n geothermal energy which w a s s i g n e d i n 1975 by both c o u n t r i e s and extended i n 1980 f o r a second f i v e years. The agreement was between Ente Nazio-

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n a l e p e r l ' E n e r g i a E l e t t r i c a (ENEL), I t a l y , and t h e Energy Research and Development Administration (ERDA), now the U.S. Dept. of Energy ( D O E ) , U.S.A. S i x major areas f o r One of these, j o i n t r e s e a r c h were involved. P r o j e c t 3 , w a s on " r e s e r v o i r p h y s i c s and e n g i n e e r i n g and r e s o u r c e assessment" , and is t h e one of i n t e r e s t here.

Energy Development Organization (NEDO). This is approximately organization in Japan e q u i v a l e n t t o t h e Department of Energy in t h e U.S.A. P r o f e s s o r Horne a l s o d e l i v e r e d v a r i ous l e c t u r e s and made a number of s i t e v i s its. This i n t e r a c t i o n proved i n v a l u a b l e from t h e U.S.A. geothermal s t a n d p o i n t because a wide range of Japanese geothermal e x p e r i e n c e h i t h e r t o b u r i e d i n Japanese language publications came t o light. The subsequent p r e s e n t a t i o n of t h e impact of r e i n j e c t i o n experience i n Japan h a s evoked controversy i n U.S.A. geothermal r e s e r v o i r e n g i n e e r i n g circles and h a s s t i m u l a t e d new r e s e a r c h a s t h e apparent i m p l i c a t i o n s a r e confirmed o r disproved.

Through t h i s agreement and r e s u l t a n t DOE cont r a c t s , S t a n f o r d and LBL with ENEL, have used t h e L a r d e r e l l o r e g i o n of geothermal s t e a m f i e l d s i n I t a l y as an experimental l a b o r a t o r y t o develop new r e s e r v o i r e n g i n e e r i n g technology. F i e l d d a t a are a v a i l a b l e back t o 1945. Under t h e c o o p e r a t i v e r e s e a r c h program f i e l d t e s t s have been designed and implement e d and t h e r e s u l t s analyzed. Important res u l t s have been published, a t t e n t i o n being i n v i t e d p a r t i c u l a r l y t o P r o j e c t 3 papers pres e n t e d a t two ENEL-DOE Workshops f o r Cooperat i v e Research i n Geothermal Energy. The f i r s t w a s h e l d a t ENEL f a c i l i t i e s i n Lardere l l o , I t a l y , Sept. 12-16, 1977. The papers p r e s e n t e d were p u b l i s h e d i n t h e Workshop Proceedings and i a t e r i n a s p e c i a l i s s u e of thermics.

Contact between Stanford U n i v e r s i t y and Japanese geothermal agencies is now e x t e n s i v e . These agencies include NEDO, New Energy Found a t i o n , U n i v e r s i t y of Tokyo, Kyushu Univers i t y Geological Survey of Japan, E l e c t r i c Power Development Company, Kyushu E l e c t r i c Power Company, Japan Metals and Chemical Company, M i t s u b i s h i Metals Corporation, Mitsubishi Heavy Industries, Toshiba I n t e r n a t i o n a l , Nippon S t e e l Corporation, Japan O i l Engineering Company and West Geothermal Energy Company. I n January 1982 S t a n f o r d U n i v e r s i t y w i l l welcome i t s f i r s t Japanese H e i s from t h e geothermal exchange v i s i t o r . E l e c t r i c Power Development Company and w i l l j o i n t w o Japanese s t u d e n t s c u r r e n t l y i n residence.

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The second Workshop w a s h e l d a t Lawrence Berk e l e y Laboratory, Berkeley, C a l i f o r n i a , October 20-23, 1980. P r e s e n t e d papers are published i n t h e Workshop Proceedings. An example of an important t r a n s f e r of technology t o t h e U.S.A. is a s u c c e s s f u l new method developed to forecast steam production. I t was developed from s t u d i e s made i n t h e Gabbro f i e l d i n I t a l y , and can be a p p l i e d t o s i m i l a r f i e l d s i n t h e U.S.A. The r e s u l t s a r e published i n t h e 1980 Workshop Proceedings.

Cooperation between t h e geothermal communit i e s of t h e U.S.A. and Japan i s formalized a l s o through Japanese f i n a n c i a l and t e c h n i c a l p a r t i c i p a t i o n i n t h e Los Alamos Hot Dry Rock Program. The enthusiasm and t e c h n i c a l expert i s e which Japan is applying t o geothermal u t i l i z a t i o n is c e r t a i n t o be of b e n e f i t t o t h e U.S.A. i f j o i n t r e l a t i o n s continue a t t h e i r p r e s e n t or an i n c r e a s e d l e v e l .

Another example of a t r a n s f e r of technology t o t h e U.S.A. i s a method of e n g i n e e r i n g a n a l y s i s developed t o estimate flow p a t t e r n s and f r a c t u r e t r e n d s i n c e r t a i n geothermal steam r e s e r v o i r s i n which a p r i n c i p a l produci n g w e l l p e n e t r a t e s a v e r t i c a l f r a c t u r e ext e n d i n g p a r t way t o t h e bottom of a reserv o i r , hypothesized a s a b o i l i n g water i n t e r face. The method w a s developed from w e l l i n t e r f e r e n c e s t u d i e s made on t h e T r a v a l e steam f i e l d i n I t a l y . R e s u l t s a r e published in the foregoing special issue of Geothermics.

Japan

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Mexico Two c o o p e r a t i v e agreements have been i n e f f e c t i n r e c e n t years. One of t h e s e , signed i n 1977, was between t h e U.S. Energy Research and Development Administration (now DOE), r e p r e s e n t e d by Lawrence Berkeley Labora t o r y , and t h e Comision Federal de E l e c t r i c i dad (CFE) , Mexico. The o t h e r agreement, signed i n 1980, and supported by t h e IX)E, i s between t h e Petroleum Engineering Department of Stanf o r d U n i v e r s i t y and t h e I n s t i t u t o de I n v e s t i g a ciones E l e c t r i c a s , Mexico. Together t h e s e agreements involve f i e l d t e s t s , r e s e r v o i r modeling and l a b o r a t o r y r e s e a r c h .

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Contacts between S t a n f o r d U n i v e r s i t y and t h e Japanese geothermal i n d u s t r y were i n i t i a t e d a s a r e s u l t of p a r t i c i p a t i o n i n t h a t i n d u s t r y by p o s t g r a d u a t e s . An i n f o r m a l c o o p e r a t i o n has been i n e f f e c t f o r t h e l a s t two years. P r o f e s s o r Roland N. Horne of S t a n f o r d has s p e n t t w o months i n Japan d u r i n g H i s a c t i v i t i e s included c o n s u l t that time. i n g arrangements f o r r e s e r v o i r e v a l u a t i o n s and t h e t e a c h i n g of a geothermal r e s e r v o i r e n g i n e e r i n g s h o r t course with 63 a t t e n d e e s , done i n cooperation with t h e Japan Geothermal Energy Center which i s now a p a r t of the N e w

The c o o p e r a t i v e r e s e a r c h of LBL and CFE i s based on a formal b i - n a t i o n a l agreement. Stanford- IIE c o o p e r a t i v e r e s e a r c h i s based on a memorandum of understanding, a k i n t o a l e t t e r of i n t e n t o r gentlemen's agreement. For t h e j o i n t purposes of S t a n f o r d and I I E t h i s more informal arrangement, which w a s recommended by WE, seems more p r a c t i c a l .

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O b j e c t i v e s of t h e Stanford- IIE proposed program were d i s c u s s e d with t h e U.S. Division of Geothermal Energy i n e a r l y 1980 by P r o f e s s o r s Miller and Ramey, of S t a n f o r d U n i v e r s i t y , and D r . Pablo Mulas del Pozo of I I E . I n the earl y p a r t of 1980 t h r e e e n g i n e e r s from I I E , D r . F r a n c i s c o Cordoba, Ing. V i c o r Arellano, and A l b e r t 0 Yanez, s p e n t about t h r e e months a t Stanford a s v i s i t i n g scholars p a r t i c i p a t i n g i n many seminars r e l a t i n g t o t h e p r o s p e c t i v e cooperative research e f f o r t . This l e d t o the DOE-Stanford C o n t r a c t f o r j o i n t r e s e a r c h w i t h IIE. Prior t o this, however, Stanford Miller and Heber Cinco-Ley, p r o f e s s o r s F.G. i n 1979, performed c o n s u l t i n g s e r v i c e s f o r t h e United Nations, a s s i s t i n g I I E with i t s p l a n s f o r r e s e a r c h f a c i l i t i e s and a r e s e a r c h program.

l a n d he field.

similar

work

on

the

Wairakei

What i s b e l i e v e d t o be the f i r s t r e s e r v o i r e n g i n e e r i n g study of a N e w Zealand geothermal reservoir was made by P r o f e s s o r s R.E. Whiting and H.J. Ramey, Jr., i n t h e e a r l y 1960'$. Both men were f a c u l t y members of Texas A&M a t the time. A r e p o r t on t h e i r work, p u b l i s h e d by t h e S o c i e t y of Petroleum Engineers of A I M E , d e a l t with t h e development of m a t e r i a l and-energy balance e q u a t i o n s f o r the Wairakei field.

Two r e c e n t managers of t h e S t a n f o r d Geotherm a l Program came from New Zealand, P r o f e s s o r Roland N. Horne who is now a f a c u l t y member of t h e S t a n f o r d Petroleum Engineering Department, and D r . Ian G. Donaldson, who is a s c i e n t i s t w i t h t h e DSIR i n New Zealand. Both men have made important c o n t r i b u t i o n s t o the development of geothermal r e s e r v o i r engineeri n g technology a p p l i c a b l e i n t h e U.S.A.

Long t e r m r e s e r v o i r performance d a t a needed by t h e U.S.A. t o develop new technology are being made a v a i l a b l e from the C e r r o P r i e t o Geothermal Steam F i e l d and from o t h e r Mexican fields.

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Nicaragua I n 1977 H. Dykstra and R.H. A d a m s , both formerly with t h e Standard O i l Company of C a l i f o r n i a w e r e engaged by t h e Nicaraguan government t o make a r e s e r v o i r e n g i n e e r i n g study and conduct f i e l d t e s t s i n t h e Momotombo geothermal r e s e r v o i r . Both of t h e s e men are experienced and h i g h l y q u a l i fied.

During f i s c a l y e a r 1981, Stanford- IIE work included i n v e s t i g a t i o n s o f : ( a ) t h e use of pressure g r a d i e n t s and p r o f i l e s in w e l l a n a l y s i s , (b) Tracer a n a l y s i s f o r f r a c t u r e d systems, ( c ) I n t e r f e r e n c e tests i n f l a s h i n g r e s e r v o i r s , and, ( d ) Lumped parameter modeli n g of t h e Cerro P r i e t o r e s e r v o i r . New Zealand

did

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Most of t h e s c i e n t i f i c i n t e r change between t h e U.S.A. and New Zealand is probably a t t h e p e r s o n a l l e v e l . New Zealand government l a b o r a t o r i e s f r e q u e n t l y provide o f f i c e space f o r s h o r t term appointments of t h e f e l l o w s h i p t y p e t o s c i e n t i s t s of t h e U.S.A. wishing t o work c l o s e l y with New Zeai a n d colleagues. S i m i l a r l y , the U.S. Geologi c a l Survey (USGS) and U.S. u n i v e r s i t i e s f r e q u e n t l y provide o p p o r t u n i t i e s f o r New Zealand pursus c i e n t i s t s t o spend t i m e i n t h e U.S.A. i n g t h e i r r e s e a r c h and exchanging i d e a s with American c o l l e a g u e s .

Dykstra r e p o r t e d t h e p r i n c i p a l r e s u l t s of t h e i r s t u d i e s a t S t a n f o r d ' s Third Workshop on Geothermal Reservoir Engineering h e l d i n December 1977. Flow tests and p r e s s u r e measurements w e r e made on a group of f i v e w e l l s i n t h e Momotombo r e s e r v o i r . The purpose w a s t o e v a l u a t e t h i s hot water reserv o i r , t o determine w e l l i n t e r f e r e n c e e f f e c t s , t o determine r e s e r v o i r boundary c o n d i t i o n s and t o o b t a i n mass flow rates and enthalpy. Bottom h o l e p r e s s u r e s were measured i n f o u r w e l l s , s t a t i c p r e s s u r e s i n t h r e e of t h e s e and both flowing and shut- in buildup p r e s s u r e i n the fourth. Flow t e s t s were made on a l l f i v e wells.

A major c o n t r i b u t i o n of New Zealand t o t h e U.S.A. geothermal program is an open supply of d a t a from developed geothermal f i e l d s i n New Zealand. Data from Wairakei, f o r examp l e , a r e a v a i l a b l e i n t h e U.S.A. through an exchange arrangement i n which t h e USGS is t h e U.S. coordinator.

Although Dykstra and Adams could not accompl i s h a l l t h e i r o b j e c t i v e s throuqh a n a l y s i s of t h e i r c a r e f u l l y planned and executed tests, t h e y were a b l e t o shed l i g h t on t h e p e r f o r mance behavior of t h e Momotombo r e s e r v o i r and t o e x p l a i n i n l o g i c a l f a s h i o n why t h e reserv o i r behaves as it Qes.

P r o f e s s o r Michael 0' S u l l i v a n of the Theoreti c a l and Applied Mechanics Department of t h e U n i v e r s i t y of Auckland s p e n t a year r e c e n t l y a t t h e Lawrence Berkeley Laboratory doing Earlier r e s e a r c h on r e s e r v o i r s i m u l a t i o n s . he d i d s i m i l a r work on New Zealand's Wairakei and Broadlands f i e l d s . Conversely, Dr. Michael Sorey who i s an American s c i e n t i s t with t h e USGS r e c e n t l y s p e n t t w o y e a r s with the Department of S c i e n t i f i c and I n d u s t r i a l Research (DSIR) i n New Zealand. D r . Sorey had e a r l i e r gained experience w i t h r e s e r v o i r s i m u l a t i o n work on t h e Long Valley geothermal While he w a s i n New Zeaa r e a of C a l i f o r n i a .

important t h e y brought back t o t h e some v a l u a b l e experience on a h o t water r e s e r v o i r i n a v o l c a n i c environment which adds t o our meager fund of knowledge on t h e subject. Most U.S.A.

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P r o f e s s o r H.J. k e y , Jr. of StanTaiwan f o r d U n i v e r s i t y v i s i t e d Taiwan on a c o n s u l t i n g assignment i n 1979. H e made s t u d i e s on t h e Chingshui geothermal steam f i e l d , near t h e n o r t h e a s t e r n coast of Taiwan. This f i e l d

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f i e l d tests can be i n t e r p r e t e d mre e a s i l y and w i t h more confidence? How can w e e x t r a c t h e a t economically from r e s e r v o i r rocks a f t e r rates of f l u i d production have become uneconomic? W e have, of course, developed p a r t i a l answers but more complete answers are needed.

produces h o t w a t e r with some carbon dioxide from s i x w e l l s . C a r l Chang of t h e Chinese Petroleum Corporat i o n and P r o f e s s o r Ramey performed p r e s s u r e t r a n s i e n t t e s t s j o i n t l y and analyzed t h e results. T h e i r work l e d t o f o u r p u b l i c a t i o n s a t t h e F i f t h Workshop on Geothermal Reservoir Engineering a t S t a n f o r d i n December 1979. These p u b l i c a t i o n s a l l r e l a t e d t o t h e Chingshui field. They d e a l t w i t h a p r e l i m i n a r y s t u d y of t h e Chingshui geothermal a r e a , a well interference test, p r e s s u r e buildup t e s t s , and an a p p l i c a t i o n of t h e Horner method t o t h e e s t i m a t i o n of s t a t i c r e s e r v o i r temperature during drilling operations. These p u b l i c a t i o n s p r e s e n t p r e s s u r e and t e m p e r a t u r e t r a n s i e n t data from f i e l d t e s t i n g , and d a t a i n t e r p r e t a t i o n . F i e l d information of t h i s k i n d i s v a l u a b l e t o e n g i n e e r s and s c i e n t i s t s i n t h i s a r e a of r e s e a r c h .

I n t e r n a t i o n a l cooperation g e n e r a l l y o f f e r s economic i n c e n t i v e s which should not be minimized. A t l e a s t as an approximation, t h e work f o r c e on a b i n a t i o n a l r e s e a r c h i n v e s t i g a t i o n is twice what it would be for e i t h e r of t h e t w o p a r t i c i p a t i n g c o u n t r i e s considered alone. Easy access t o each o t h e r ' s experience background promotes r a p i d growth of new ideas.The growth t h a t we can a n t i c i p a t e can be s i m i l a r t o t h e explosion i n technology which occurred i n t h e 1 9 3 0 ' s and 1 9 4 0 ' s i n W e can f o r e s e e t h a t t h e petroleum i n d u s t r y . t h e p r o s p e c t s f o r c o n t i n u i n g an expanded geothermal energy development should remain very good i f enough encouragement and support a r e forthcoming from t h e governments and p r i v a t e companies involved. I n t e r n a t i o n a l cooperat i o n can be an important f a c t o r i n t h i s growth.

Following t h e 1979 Workshop, C a r l Chang s p e n t a q u a r t e r i n r e s i d e n c e a t S t a n f o r d as a v i s i t i n g scholar. It i s c l e a r l y e v i d e n t Concluding Remarks from t h i s b r i e f review of i n t e r n a t i o n a l coo p e r a t i o n i n geothermal energy development t h a t t h e c o u n t r i e s involved have made import a n t a d d i t i o n s t o t h e i r fund of knowledge on geothermal energy. E f f o r t s made a r e e a s i n g energy s h o r t a g e s , r e v e a l i n g t h i s source a s a v i a b l e a l t e r n a t i v e t o o i l and gas throughout many p a r t s of t h e world.

T e c h n i c a l l y , much has been gained from new e n g i n e e r i n g methods and s c i e n t i f i c techniques which shed l i g h t on t h e p h y s i c a l and chemical n a t u r e of geothermal- f l u i d r e s e r v o i r s . These methods and t e c h n i q u e s h e l p e x p l a i n why r e s e r v o i r s behave as they do and they thereby f a c i l i t a t e f o r e c a s t s of performance- Geothermal energy development appears t o be on t h e t h r e s h h o l d of a new expansion. Much h a s been l e a r n e d and t h e r e i s s t i l l much t o be learned. Looking ahead, we must c o n s i d e r problems now A few q u e s t i o n s which a r i s e are surfacing. how can w e make b e t t e r use of geochemical d a t a t o e x p l a i n p a s t r e s e r v o i r behavior and e x p l a i n f u t u r e behavior? To what e x t e n t can t h e s e d a t a be b e s t a p p l i e d t o determine underground flow p a t t e r n s ? How can w e b e s t advance r e s e r v o i r a n a l y s i s by a p p l y i n g t o steam zones t h e knowledge we now have on vapor pressure lowering and liquid adsorption? How can we b e s t design tracer s t u d i e s so t h a t they w i l l y i e l d needed i n f o r mation on r e s e r v o i r s i z e and c o n f i g u r a t i o n and on fracture size, orientation, and distribution? What compaction and f r a c t u r e e f f e c t s can be expected from t h e r e i n j e c t i o n of cool water? Can land subsidence become a major problem i n r e s i d e n t i a l , i n d u s t r i a l or farm a r e a s ? What a r e t h e economic implicat i o n s ? What should be done t o f u r t h e r develop w e l l t e s t i n g t h e o r y so t h a t t h e r e s u l t s of

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RESOURCE DEVELOPMENT

C. L. Ritz Republic Geothermal, Inc. 11823 E. Slauson Avenue Santa Fe Springs, CA 90670

RESOURCE DEVELOPMENT *

emphasis on stepwise, modular development rather than a large scale, all-at-once exploitation of a geothermal resource. As an example, various development scenarios of the East Mesa field will be presented along with advantages and disadvantages of each approach. Recommendations for future studies of resource development will also be given.

The resource assessment techniques utilized in geothermal reservoir engineering are a combination derived from the disciplines of groundwater hydrology and petroleum engineering. As the geothermal industry has developed, these techniques have been modified to take into account phenomena normally ignored in aquifer management and oilfield practice. Details of multiphase flow and heat transfer need to be incorporated into the geothermal reservoir engineer's repertoire. The purpose of this presentation is to (1) comment on how several established groundwater and petroleum reservoir engineering techniques (reservoir modeling and wellbore flow simulation) are currently utilized in the geothermal industry to speed the development of large and small prospects, and (2) suggest areas where future government-funded research might be directed in order to assist the geothermal industry in the development of geothermal resources.

In comparison to the development of a large geothermal resource such as East Mesa, the smaller, low temperature geothermal resources capable of only direct heat utilization rather than electric power generation present an interesting challenge. The reason for the interest is that the nature of the flow of the hot water is incompletely understood and must be inferred from a combination of subsurface geology and well test data. This, in turn, leads to questions of delicately balancing resource recharge with production and reinjection. An example of a small resource, along with the reservoir parameters needed for well-test design and resource assessment, will be given. Recommendations for future studies regarding the exploitation of small geothermal resources will also be presented.

Mathematical models of wellbore flow have been used to extend our understanding and make predictions on well productivity, heat loss, and the relationship of temperature to pressure in a flowing and flashing well. Examples of satisfactory fits of theory to experiment in a well with very high salinity (200,000 ppm TDS) and high concentration of C02 will be presented. Suggestions for improvements to currently available wellbore flow models will be made. Sophisticated numerical simulators have been used to predict reservoir performance and compare resource output as a function of well placement and development scenarios. Unfortunately, such studies have not increased the utilities or investors confidence in a reservoir's ability to deliver its estimated reserves. This has resulted in a reassessment of field development plan and a subsequent

Concluding remarks will deal with the present versus future role of the geothermal reservoir engineer in evaluating and predicting reservoir performance. Emphasis will be on recommendations that can put the credibility of the geothermal reservoir engineer on a level equal to that given to reservoir engineers in the petroleum industry. The useful role that government funding can play is raising the credibility of the geothermal reservoir engineer. The point will be raised that funding efforts need to be increased in the areas of field studies and case histories, development of high temperature well logging instrumentation, and evaluation of materials capable of withstanding geothermal conditions.

* This is only a summary o f the actual presentation.

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GEOTHERMAL RESERVOIR ENGINEERING

-

UTILITY INDUSTRY PERSPECTIVE

Vase1 W. Roberts

Electric power %search I n s t i t u t e P. 0. Box 10412 Palo Alto, CA 94303

The p e r s p e c t i v e o f t h e u t i l i t y i n d u s t r y , conc e r n i n g geothermal energy, h a s n o t changed d r a m a t i c a l l y d u r i n g t h e p a s t y e a r . There have been minor changes, but mst w e r e p o s i t i v e and a l l were small increments. T h i s i s somewhat s u r p r i s i n g given t h e dramatic down t u r n i n t h e f e d e r a l r e s e a r c h and development program and d e l a y s encountered by t h r e e o f t h e key pacing geothermal power p l a n t p r o j e c t s . It indicates a n unexpected s t r e n g t h i n t h e i n d u s t r y .

D i f f e r e n t geothermal f l u i d s may have d i f f e r e n t temperatures and e n t h a l p i e s , and do n o t a l l have t h e same i n t r i n s i c v a l u e . A f i r s t o r d e r estimate of t h e v a l u e of a p a r t i c u l a r geothermal h e a t source from t h e u t i l i t y p e r s p e c t i v e can be e s t a b l i s h e d by comparing conversion e f f i c i e n c i e s . Geothermal energy must be conv e r t e d t o e l e c t r i c i t y a t temperatures s i g n i f i c a n t l y lower t h a n f o r f o s s i l and n u c l e a r f u e l s , t h e r e f o r e , t h e conversion e f f i c i e n c y i s lower, , t h e h e a t rate h i g h e r . For example, t h e H e b e r b i n a r y p l a n t i s expected t o have a n o v e r a l l thermal e f f i c i e n c y o f a b o u t 1 2 p e r c e n t while a conventional f o s s i l p l a n t would be around 36 p e r c e n t , o r p o s s i b l y h i g h e r . Therefore, a H e b e r BTU i s worth o n l y o n e - t h i r d t h a t o f a f o s s i l BTU i n t h a t area f o r power g e n e r a t i o n . T h i s concept can be f u r t h e r r e f i n e d by includi n g second o r d e r f a c t o r s f o r such t h i n g s as differences i n inherent a v a i l a b i l i t y factors f o r d i f f e r e n t p o w e r p l a n t t y p e s , c a p i t a l cost, o p e r a t i o n s and maintenance c o s t s , e t c . The a l t e r n a t e energy source might range from "avoided c o s t , " t o dominant energy s o u r c e o r a mix o f a l l energy s o u r c e s , depending on t h e needs o f t h e u t i l i t y , t o e s t a b l i s h an equival e n t v a l u e f o r geothermal energy. The v a l u e t h u s e s t a b l i s h e d can t h e n be used as t h e basis f o r negotiating price.

Some o f t h e key i s s u e s and changes i n u t i l i t y i n d u s t r y p e r s p e c t i v e are d i s c u s s e d h e r e i n , recognizing t h a t a consensus o f o p i n i o n i s sometimes slow t o form and t h a t most measures o f p e r s p e c t i v e are i n d i r e c t and n o t v e r y accurate. Strong I n t e r e s t Continues U t i l i t y i n t e r e s t i n geothermal energy h a s never been g r e a t e r . Evidence o f t h i s i n t e r e s t i s m a n i f e s t i n modest i n c r e a s e s i n t h e u t i l i t i e s estimates o f f u t u r e g e n e r a t i n g c a p a c i t y , roughly 1 0 p e r c e n t l a s t y e a r . The problems a s s o c i a t e d w i t h f i n d i n g a n adequate steam supply f o r t h e 50 M we power p l a n t a t B a c a , t h e n e c e s s i t y t o s h u t down t h e E a s t Mesa b i n a r y p l a n t f o r m o d i f i c a t i o n , and t h e n e g a t i v e report by t h e C a l i f o r n i a Public U t i l i t i e s Commission on t h e H e b e r f l a s h p l a n t were d i s a p p o i n t i n g o f c o u r s e b u t appare n t l y have n o t diminished i n t e r e s t . These e v e n t s simply t e n d t o confirm t h e view o f some t h a t n o t a l l i s s u e s have been r e s o l v e d and s u g g e s t t h a t s t r o n g c o n t i n u i n g commitments w i l l be necessary . f o r t h e commercialization o f geothermal resources. Most o f t h e geothermal u t i l i t i e s are w i l l i n g t o c o n s i d e r such commitments.

R e l i a b i l i t y o f Energy Supply Perhaps t h e most f r e q u e n t l y c i t e d concern r e l a t e s t o t h e reliab i l i t y and l o n g e v i t y o f new reservoirs. This h a s been a p e r s i s t e n t i s s u e and e f f o r t s t o res o l v e it have been s l o w . EPRI with t h e h e l p o f S t a n f o r d h a s been t r y i n g t o p a r t i a l l y overcome t h i s problem by developing a u t i l i t y o r i e n t e d r e s e r v o i r assessment manual. The i d e a i s t o make t h e u t i l i t i e s more comfortable w i t h t h e s u b j e c t matter and i n c r e a s e t h e i r c a p a b i l i t y i n t h i s area. The problem with most of t h e e x i s t i n g l i t e r a t u r e i s t h a t it i s n o t t u t o r i a l and i s n o t geared t o u t i l i t y needs.

Value v s P r i c e One o f t h e key i s s u e s i s s t i l l t h e cost o f geothermal p o w e r . While t h e c o s t o f geothermal h e a t i s u s u a l l y c a l c u l a t e d by a c c o u n t a n t s and economists and t h e p r i c e est a b l i s h e d by n e g o t i a t i o n , b o t h are based o n r e s e r v o i r performance data developed by t h e reservoir e n g i n e e r . Most u t i l i t i e s d o n o t have s u f f i c i e n t i n f o r m a t i o n o r knowledge a b o u t t h e reservoir t o calculate t h e c o s t o f produci n g geothermal energy and must p r e p a r e f o r p r i c e b a r g a i n i n g based on i t s v a l u e t o t h e company. The v a l u e may n o t be t h e same f o r d i f f e r e n t companies.

The u t i l i t y h a s a v i t a l i n t e r e s t i n t h e i t e r a t i v e p a t h t h a t combines t h e v a r i o u s t e c h n i c a l d i s c i p l i n e s and d i a g n o s t i c a c t i v i t i e s designed t o assess t h e v a l u e of s p e c i f i c geothermal energy d e p o s i t s . I n a l o g i c a l sequence o f dec i s i o n s , t h e u t i l i t y contemplating a geothermal p r o j e c t must be a b l e t o assess t h e p r o b a b i l i t y

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o f success o f t h e p r o j e c t . I t must a l s o determine whether t h e p r o j e c t i s a sound business venture and a l l o c a t e some l e v e l o f importance t o t h e p r o j e c t w i t h i n t h e company. To accomp l i s h t h e s e e f f o r t s , c l o s e i n t e r a c t i o n between t h e u t i l i t y c o u n t e r p a r t and t h e r e s e r v o i r engineering a c t i v i t y i s e s s e n t i a l . Physical and thermal models of t h e r e s e r v o i r t o g e t h e r with r e s e r v o i r c a p a c i t y and s u s t a i n a b l e prod u c t i o n rates a r e key sets of information neede d t o convey confidence t h a t a p r o j e c t can be s u c c e s s f u l . Estimates o f t h e r e l i a b i l i t y o f energy supply are d i f f i c u l t t o develop b u t may be i n f e r r e d i n a crude way from r e s e r v o i r d a t a on producing p o t e n t i a l .

t h e economics of d i s t r i b u t e d systems compared t o c e n t r a l p l a n t s . I n t e r e s t i n t h i s concept s t e m s from s p e c u l a t i o n t h a t t h e economics of q u a n t i t y might outweigh economics of s c a l e f o r some geothermal systems. Also small u n i t s a r e more e a s i l y recycled i n t h e e v e n t of r e s e r v o i r o r w e l l f a i l u r e . This a s p e c t may be a t t r a c t i v e where t h e producing p o t e n t i a l of t h e r e s e r v o i r h a s n o t y e t been proven.

P l a n t Type and S i z e Once a r e s e r v o i r has been shown t o be i n t e r e s t i n g enough f o r a power p l a n t p r o j e c t , f e a s i b i l i t y s t u d i e s w i l l follow. During t h i s phase, t h e q u a l i t y o f t h e energy i s t h e next most important s e t o f information. Information about temperature, e n t h a l p y , pres u r e and well production r a t e s a r e necessary t o a l l o w t h e u t i l i t y t o determine t h e type and s i z e of p l a n t t o be b u i l t . Generally it i s n o t a quescion o f s e l e c t i n g a power p l a n t type, b u t matching the t y p e with t h e thermodynamic c h a r a c t e r i s t i c s of t h e geothermal f l u i d f o r optimum busbar c o s t and r e s o u r c e u t i l i z a t i o n .

Other I s s u e s The concern f o r t h e r e l i a b i l i t y of long term energy supply i s one of t h e main i s s u e s , a s noted, and busbar energy c o s t runs a c l o s e second. Other i s s u e s of high p r i o r i t y with t h e u t i l i t i e s i n c l u d e c a p i t a l a v a i l a b i l i t y , land u s e , and f u t u r e p o t e n t i a l of t h e resource. While l i c e n s i n g can be a d i f f i c u l t i s s u e , environmental p r o t e c t i o n i s thought t o be p r a c t i c a l s i n c e t h e t h r e a t t o t h e environment i s low and p r e s e n t environmental c o n t r o l technology appears t o be capable of meeting most e x i s t i n g s t a n d a r d s . I s s u e s t h a t a r i s e l a t e r i n t h e project include plant type, plant s i z e , c o o l i n g water a v a i l a b i l i t y and s c a l i n g and c o r r o s i o n . EPRI’s Geothermal Program i s attempting t o a d d r e s s a number o f t h e s e i s s u e s , as a p a r t o f i t s c u r r e n t r e s e a r c h and development p l a n .

Accurate and complete geothermal f l u i d chemist r y i s e s s e n t i a l f o r developing requirements f o r s c a l e and c o r r o s i o n c o n t r o l , and a l s o r e requirements f o r environmental c o n t r o l systems and d e s i g n c r i t e r i a f o r t h e s e systems.

The u t i l i t y p e r s p e c t i v e on power p l a n t s i z e has changed somewhat d u r i n g t h e past y e a r . While most u t i l i t i e s s t i l l p r e f e r 50 MWe p l a n t s , o r l a r g e r , f o r commercial u s e , some have an i n t e r e s t i n small p l a n t s down t o one N e and most a r e now i n t e r e s t e d i n s m a l l e r p l a n t s from W e as t h e f i r s t u n i t on each one MWe t o 20 M new f i e l d . A s t r o n g i n t e r e s t i n wellhead u n i t s has a l s o emerged, as a means of achievi n g e a r l y involvement i n f i e l d development, a s s e s s i n g r e s e r v o i r p o t e n t i a l , and developing d e s i g n c r i t e r i a f o r l a r g e r p l a n t s t o follow. Wellhead u n i t s can a l s o be u s e f u l i n a s s e s s i n g

Conclusion While t h e c a p a b i l i t y of t h e u t i l i t i e s i s s t i l l d e f i c i e n t i n t h e a r e a of geothermal r e s e r v o i r assessment, i n t e r e s t i n geothermal power i s high. F i l l i n g t h i s gap by c o o p e r a t i v e exchange, c o n s u l t a t i o n and i n c r e a s i n g in- house c a p a b i l i t y can o n l y a c c e l e r a t e geothermal development.

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APP1,ICATIONOF A LUMPED PARAMETER MODEL TO THE CERRO PRIETO GEOTHERMAT, FIELD

J. D. Westwood

*

and L. M. Castanier

Petroleum Engineering Dept. Stanford University Stanford, California 94305

In this paper, a lumped parameter approach to geothermal modeling is investigated. The amount of data required is minimal compared to finite-difference models. The Cerro Prieto geothermal field in Mexico was the subject of the modeling study; the lumped parameter model is most appropriate here since the field is still "young,11having entered its eighth year of commercial production.

Abstract In this paper, a lumped parameter mathematical model for hot water geothermal reservoirs is developed and applied to the Cerro Prieto geothermal field in Mexico. The production and pressure histories of Cerro Prieto were assembled from field data. A computer program was then used to perform sensitivity studies on reservoir size, porosity, aquifer recharge, and temperature of recharge fluid. Two types of depletion schemes were investigated; in the first, the reservoir remains essentially one-phase liquid, and in the second, the reservoir becomes two-phase at a point early in its history. A satisfactory match of the history of the field has been obtained. This paper shows the usefulness of a lumped parameter model in clarifying the basic behavior of a hot water geothermal system and in giving focus to more complex two- and threedimensional modeling efforts. The results obtained specifically for the Cerro Prieto field will also be of value to the scientists and engineers studying this reservoir.

Some examples of lumped parameter models can be found in the literature. Whiting and Ramey (8) first used this concept in 1969 to model the Wairakei reservoir in New Zealand. Brigham and Morrow (2) in 1974 developed three models appriate for closed, vapor-dominated reservoirs. In 1979 Brigham and Neri (3) modeled the Gabbro zone o f the Larderello field. Castanier, Sanyal, and Brigham ( 4 ) included heat transfer in the recharge region to simulate the behavior of the East Mesa reservoir.

Introduction Typically, a geothermal power plant must have a lifetime of thirty years to be economic. Because of the large increments of investment necessary at each successive stage of development, it is especially important to be able to forecast the future performance of a field from existing knowledge. Geothermal reservoir models attempt to serve this predictive function. In the beginning of the life of a field, when relatively little is known, the simplest models, which require the least amount of information, are appropriate. Later, as more information is accumulated about the geologic, geochemical, hydrodynamic, and thermodynamic characteristics of the field, these models may be refined and a better understanding of the resource achieved. One of the simplest types of mathematical models is the so-called "lumped parameter" model. The purpose of this kind of simulator is to match average reservoir behavior. The reservoir is treated as a homogeneous body, whose characteristics change as quantities of mass and energy enter and exit. The value of a particular parameter throughout the model reservoir is the average value of that parameter in the real system. .L

n

Now with Marathon Oil.

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To date, there have been no lumped parameter studies of the Cerro Prieto field similar to those reviewed above. However, a few preliminary simulation efforts have been made. In 1978, Lippmann, Bodvarsson, et al. ( 6 ) , formulated a simplified three-dimensional, finitedifference model of the reservoir. In 1979, Lippmann and Goyal presented the results of two three-dimensional, finite-difference, hydrogeoloeic models of Cerro Prieto (7). Liguori(5) in 1979 used a simplified finite-difference reservoir model coupled to a wellbore model. The Cerro Prieto Field The Cerro Prieto field is located about 30 km south of Mexicali, Mexico. This study is concerned with modeling the area of the field shown in Fig. 1, which supplies Units 1 and 2 and which has been in production since 1973. The wells have a perforated interval of 100-200 m at an average depth of 1200-1300 m. However, because of the complex interbedding, it is not obvious what the thickness of the reservoir is in this area Porosity ranges from .15 to .35 in sand or sandstone, but is lower in shales. Various estimates of the permeabilities range from 40 to 100 md. The temperature of most wells in the Unit 1 and 2 area is about 300°C. Noncondensable gases, predominantly Cog and HzS, are present in sufficient quantity to affect both the compressibility and the phase behavior.

\

110

Time (yrs) 0 2 4 6

1-114

106

Pressure (kg/cm2> 109.1 101.2 97.5 94 .0

102

98

94

90 0

1

2

3

4

5

6

7

T I M E (YRS)

Fig. 2: History of Average Field Pressure The initial temperature in the production zone is calculated from the enthalpies of the producing wells in 1973. The enthalpy should correspond to that of liquid water because the steep decline in pressure during this period indicates that the reservoir fluid was essentially one-phase liquid. After neglecting the cold wells M-9, M-34, and M-39, and M-29 because hey were producing from a cold shallow zone during this time, the average liquid enthalpy was calculated to be 316 kcal/kg. This enthalpy corresponds to an initial temperature of 296OC. It is generally believed that the temperature has declined steadily at a rate of 1-3OC per year to about 285OC in 1979.

Fig. 1: The Units 1 and 2 Production Area In the lumped parameter model, reservoir and fluid properties are considered to be uniform throughout. During a time step, quantities of mass and heat enter and exit, changing the reservoir from its initial state to its final state. The model in this paper neglects conductive heat transfer and convective movement of mass and heat across reservoir boundaries. For a complete description of the model, please see references 4 and 8. Data Analysis Modeling requires two general types of data: field properties and field history. Some field properties such as reservoir area, rock matrix compressibility, density and heat capacity are known well. Others, such as thickness, porosity, and fluid compressibility of the production zone as well as geometry, permeability, and temperature of the recharge aquifer are not known accurately.

If the reservoir is uniformly two-phase, then the pressure and temperature must follow the saturation line. Unfortunately, the vapor pressure curve is not certain because of the effects of water salinity, non-condensable gases, and capillarity in the pore space. With this in mind, it is more appropriate to determine the position of the saturation curve empirically.

The history of reservoir behavior is deduced from logging, geochemical, and well production data. The model requires the history of mass flowrate, specific enthalpy of produced fluid, average reservoir pressure, and average reservoir temperature. The history of average pressure in the production zone was developed from a set of isobaric maps which were presented by Bermejo, et al. (1) Using the four maps intheir paper, the average pressure was determined in the production zone during 1973, 1975, 1977, and 1979. These four points are the basis for the curve of observed pressure history which is drawn for reference in Fig. 2.

-24-

After examining the enthalpy and pressure histories, it was decided that if the reservoir flashed at all (in the sense of a lumped parameter model), it flashed sometime around the beginning of 1974. It is at this time that the initial steep decline in pressure due to liquid decompression perhaps levels out somewhat because of the growth of two-phase conditions. The fluid enthalpy also rises above the average enthalpy of liquid water at the initial temperature. The specification of two-phase conditions starting at the beginning of 1974 determines the initial point on the "pseudo" vapor pressure curve as 104.7 kg/cm3 and 296OC. The rest of the curve is constructed according to

t h e shape of t h e a c t u a l vapor p r e s s u r e l i n e f o r water. The r a t e of temperature d e c r e a s e , which i s d i c t a t e d by t h e r a t e of average p r e s s u r e d e c l i n e , matches t h e observed r a t e ( F i g . 3).

-

106 '

u

;102"

1

5

v

98

100

'

m

a

b 90

94,.

90

1

0

1

2

3

4

5

6

1

T i m e (years)

80

110

J

L

2

70

106

cp

r"

102

"

v

0

60 275

y

2

280

285

290

295

300

98 '

Ql

94"

TEMPERRTURE

901

(DEC C)

300

I

320

:

:

:

340 3 60 Enthalov (kcal/kn)

:

380

:

*

400

.I

F i g . 3:

Vapor P r e s s u r e Curves Fig. 4 :

H i s t o r y M a t c h i I s I n t h e s i m u l a t i o n of a twophase s c e n a r i o f o r t h e r e s e r v o i r , t h e behavior o u t l i n e d above w a s assumed. A s a n a l t e r n a t i v e f o r comparison purposes, t h e r e s e r v o i r w a s a l s o modeled as a one-phase system w i t h a l a r g e compressibility.

H i s t o r y Match

o b s e r v a t i o n s . A s shown by t h e r e s u l t s of t h e s e n s i t i v i t y s t u d i e s presented later, a s a t i s f a c t o r y match of t h e r e s e r v o i r by a one-phase scheme i s impossible. A two-phase scheme pres e n t s a sharp decline i n pressure u n t i l t h e r e s e r v o i r r e a c h e s t h e pseudo-vapor p r e s s u r e curve. I t t h e n f l a s h e s and t h e p r e s s u r e dec l i n e rate decreases while the enthalpy i n t h e r e s e r v o i r rises. I n o r d e r t o match b o t h t h e p r e s s u r e and t h e produced- enthalpy c u r v e s , i t i s n e c e s s a r y t o assume a l a r g e a q u i f e r recharge. A t t h e same t i m e , t h i s would slow t h e p r e s s u r e d e c l i n e r a t e a t t h e beginning of p r o d u c t i o n and t h e n lower t h e e n t h a l p y a t t h e end by reducing t h e steam q u a l i t y i n t o t h e r e s e r v o i r .

Various s e n s i t ] - v i t y s t u d i e s were performed und e r each d e p l e t i o n scheme i n o r d e r t o i n v e s t i g a t e t h e effect: of v a r y i n g r e s e r v o i r parameters. S e v e r a l f a c t o r s were c o n s i d e r e d i n o b t a i n i n g a s a t i s f a c t o r y match of t h e h i s t o r y a t Cerro P r i e t o . On t h e b a s i s of t h e s e n s i t i v i t y studies, i t w a s f e l t t h a t a two-phase scheme would o f f e r t h e b e s t chance of matching t h e p r e s s u r e behavior.

A s e n s i t i v i t y s t u d y on t h e temperature of recharge water showed t h a t r a i s i n g t h e temperat u r e of f l u i d i n f l u x would cause t h e p r e s s u r e d e c l i n e t o b e more g r a d u a l . Decreasing t h e r e s e r v o i r t h i c k n e s s would c o u n t e r a c t t h i s e f f e c t , and would c a u s e t h e e n t h a l p y r i s e t o be slower. I n o r d e r t o o b t a i n agreement w i t h t h e e n t h a l p y h i s t o r y , a l a r g e a q u i f e r would be n e c e s s a r y t o p r o v i d e a c o o l mass of r e c h a r g e water. A s t h e volume of t h e production zone became smaller r e l a t i v e t o t h e c o l d a q u i f e r , t h e d e c l i n e i n p r e s s u r e along t h e s a t u r a t i o n l i n e would i n c r e a s e . F i g u r e 5 shows an example of a s e n s i t i v i t y s t u d y i n which t h e temperature of t h e r e c h a r g e water i s v a r i e d , assuming a two-phase d e p l e t i o n s c e n a r i o . I n t h i s case, t h e s i z e of t h e a q u i f e r was t o o small t o match e i t h e r t h e p r e s s u r e o r t h e e n t h a l p y observed.

F i g u r e 4 shows t h e r e s u l t s of h i s t o r y matching. The match i s q u i t e good over t h e e n t i r e 7.5 y e a r p e r i o d . The parameters used i n o b t a i n i n g t h i s match are r e a l i s t i c except f o r a q u i f e r s i z e . The t h i c k n e s s of t h e p r o d u c t i o n zone is 380 m; t h e p o r o s i t y i s 22%; and t h e r e c h a r g e temperature i s 26OOC. However, t h e c r o s s s e c t i o n a l area of t h e a q u i f e r i s 1 8 x lo6 m2, which i s a b o u t 3.5 times t h e t o t a l s u r f a c e area of t h e p r o d u c t i o n zone. The s t r e n g t h o f t h e recharge i n d i c a t e s t h a t a r a d i a l o r s p h e r i c a l i n f l u x may b e c l o s e r t o a c t u a l c o n d i t i o n s . I n any c a s e , t h e r e s u l t s i n t h i s r e p o r t w i t h a l i n e a r r e c h a r g e system i n d i c a t e t h a t s t r o n g r e c h a r g e is o c c u r r i n g a t Cerro P r i e t o . One add i t i o n a l p o s s i b i l i t y i s bottom water i n f l u x i n t o t h e r e s e r v o i r . This would e x p l a i n t h e behavior shown by t h e p r e s s u r e and e n t h a l p y

-25-

-5 I\. . r" 110

A

3.6.10'

110 1

m2

Temperature P. C .)

1

296

-

106

=

Cum*

," 102

v

$

98

m a Iy

c

94

90

-

0

1

2

3 4 Time ( y e a r s )

4

5

6

90 1 0

7

1

2

3

4

5

6

7

Time (yfsr's)

110 1

9

o

300

Fig. 5:

L

320

: : 340 360 Enthalpy (kcal/kg)

I

. 380

.

320

400

340

360

-

400

380

Enthalpy ( k c a l / k g )

Fig. 6:

S e n s i t i v i t y Study of Temperature of Recharge F l u i d (Two-Phase)

F i g u r e 6 shows t h e e f f e c t of a change i n aquifer size.

S e n s i t i v i t y Study of Aquifer S i z e (Two-Phase)

110

An a d d i t i o n a l p o i n t t o a d d r e s s i n t h i s work i s

106

t h e s e n s i t i v i t y of t h e model t o t h e l o c a t i o n of t h e a r b i t r a r y zones; p a r t i c u l a r l y , t h e i n t e r mediate zone e x t e n t may e f f e c t t h e v a l i d i t y of the results.

h

0

g

>

102

v

u

F i g u r e 7 shows t h e drop i n p r e s s u r e and t h e r a t e of rise of produced e n t h a l p y when t h e l e n g t h of t h e i n t e r m e d i a t e zone i n c r e a s e s . Geological d a t a and temperature surveys must be used c a r e f u l l y i n o r d e r t o i n p u t t h i s parameter properly.

98

a m

:

94

0

1

2

3 4 Time ( y e a r s )

5

6

7

Due t o t h e complex f a u l t i n g and d i f f i c u l t geology of Cerro P r i e t o , t h e t h i c k n e s s of t h e producing i n t e r v a l i s d i f f i c u l t t o estimate. I n Fig. 8 , w e v a r i e d t h i s parameter from 250 t o 750 m. The b e s t h i s t o r y match was o b t a i n e d f o r a t h i c k n e s s of 380 m. The p o r o s i t y was changed from .15 t o .25. A s can be expected, t h e model is n o t v e r y s e n s i t i v e t o t h i s parameter, because the t o t a l heat content i n t h e reservoir i s n o t v e r y dependent on t h e p o r o s i t y . The h i s t o r y match o b t a i n e d h e r e is n o t unique. Other matches, which were n o t as good, were p o s s i b l e f o r a smaller r e s e r v o i r and a w a r m e r and s t r o n g e r recharge. A temperature of 26OoC is t h e minimum f o r which i t is p o s s i b l e t o g e t good e n t h a l p y and p r e s s u r e matches. Below t h i s temperature, a smaller a q u i f e r must be

-26-

-

400

E n t h a l p y (kcallkg)

Fig. 7 :

S e n s i t i v i t y Study of Length of Thermal Zone (Two-Phase)

-

2) A p o t e n t a q u i f e r r e c h a r g e has prevented t h e enthalpy of t h e f l u i d i n t h e r e s e r v o i r from r i s i n g i n r e s p o n s e t o h i g h rates of production from a r e l a t i v e l y small volume. The geometry of t h e r e c h a r g e system i s probably r a d i a l o r spherical, r a t h e r than l i n e a r .

Ti. = 210°C A

CW.

3.6.10' m2 7hieknc.r

(m)

0

1

2

3

4

5

6

3) The temperature of t h e r e c h a r g e water i s about 260OC. I f t h e temperature i s below 260OC i n t h e model, t h e n e i t h e r t h e p r e s s u r e d e c l i n e is too steep o r t h e enthalpy i n t h e reservoir i n c r e a s e s . I f t h e temperature i s above 26OoC, t h e h i s t o r y matches are n o t as good, and t h e r e s e r v o i r d e s c r i p t i o n becomes less p h y s i c a l l y realistic.

7

Time (yeers)

Acknowledgments The a u t h o r s would l i k e t o thank Coordinadora E j e c u t i v a d e Cerro P r i e t o through i t s Department of S t u d i e s , who, through Lawrence Berkeley Laboratory, k i n d l y c o n t r i buted f i e l d p r e s s u r e and t e m p e r a t u r e d a t a used i n t h i s work. This work forms p a r t of t h e coo p e r a t i v e program between S t a n f o r d U n i v e r s i t y , C a l i f o r n i a and t h e I n s t i t u t o d e I n v e s t i g a c i o n e s E l e c t r i c a s , Mexico, w i t h s u p p o r t from U.S. Department of Energy c o n t r a c t number DE-AT03-80SFl.1459.

110

-

r"

102

Y

01

1

01

References 90 300

Fig. 8:

320

340 360 Enthalpy (kcal/kg)

380

400

1. Bermejo, F. J . , Navarro, F. X . , &. "Press u r e V a r i a t i o n a t t h e Cerro P r i e t o Reserv o i r Durine Production." i n Proc. of t'

S e n s i t i v i t y Study of R e s e r v o i r Thickn e s s (Two-Phase)

s p e c i f i e d t o reproduce t h e p r e s s u r e t r e n d , b u t t h e e n t h a l p y becomes t o o h i g h because t h e r e s e r v o i r mass i s d e p l e t e d t o o r a p i d l y . Above t h i s temperature, i t i s p o s s i b l e t o o b t a i n simultaneous p r e s s u r e and e n t h a l p y matches, b u t t h e d i s c r e p a n c y between a q u i f e r s i z e and reserv o i r volume becomes l a r g e r .

2. Brigham, W. E., and Morrow, W. B. "P/Z Beh a v i o r f o r Geothermal Steam R e s e r v o i r s , " SPEJ, Dec. 1977, 407-412.

The h i s t o r y match h a s been e x t r a p o l a t e d o u t t o a 30- year l i f e t i m e . A f l o w r a t e of 2000 t o n s / h r i s assumed, and t h e produced e n t h a l p y i s approxi m a t e l y e q u a l t o t h e e n t h a l p y of t h e r e s e r v o i r f l u i d . No a t t e m p t i s made t o account f o r t h e e f f e c t of p r o d u c t i o n i n o t h e r areas of the f i e l d . After t h i r t y years, t h e pressure has d e c l i n e d t o 67 kg/cm2, t h e t e m p e r a t u r e t o 256OC, and t h e r e s e r v o i r f l u i d i s 5% q u a l i t y steam w i t h a n e n t h a l p y o f 286 k c a l f k g . Conclusions Although t h e i n f o r m a t i o n t h a t was r e q u i r e d f o r t h e lumped parameter model of Cerro P r i e t o U n i t s 1 and 2 is minimal compared t o d i s t r i b u t e d parameter models, c o n s i d e r a b l e judgment w a s s t i l l n e c e s s a r y t o f o r m u l a t e a c o n s i s t e n t s e t of d a t a from t h e d i v e r s e and sometimes c o n f l i c t i n g s o u r c e s t h a t are a v a i l a b l e . However, t h e simple n a t u r e of t h e lumped parameter approach a l l o w s r a p i d i n s i g h t i n t o r e l a t i o n s h i p s between p h y s i c a l r e s e r v o i r chara c t e r i s t i c s and p r o d u c t i o n behavior. The f o l lowing c o n c l u s i o n s may b e drawn:

3. Brigham, W. E., and N e r i , G . " Preliminary R e s u l t s on a D e p l e t i o n Model f o r t h e Gabbro Zone," i n Proc. F i f t h Workshop on Geothermal R e s e r v o i r Engineering, Ramey, H. J . , and Kruger, P . , e d s . SGP-TR-40, 229-240. 4. C a s t a n i e r , L. M., Sanyal, S. K . , andBrigham, W. E . "A P r a c t i c a l A n a l y t i c a l Model f o r Geothermal R e s e r v o i r Simulation,'' SPE 8887, Annual C a l i f o r n i a Regional Meeting of t h e SPE, Pasadena, 1980. 5. L i g u o r i , P. E. " Simulation of t h e Cerro P r i e t o Geothermal F i e l d Using a Mathematical Model," i n Proc. of t h e Second Sym. on t h e Cerro P r i e t o Geothermal F i e l d , M e x i c a l i , 1979, 524-526. 6. Lippmann, M. J . , Bodvarsson, G. S . , e t a l . "Preliminary Simulation S t u d i e s R e l a t e d t o t h e Cerro P r i e t o F i e l d , " i n Proc. of t h e F i r s t Sym. on t h e Cerro P r i e t o Geothermal F i e l d , San Diego, 1978, LBL-7098, 375-383.

7 . Lippmann, M. J . , and Goyal, cal Modeling S t u d i e s of t h e R e s e r v o i r , " i n Proc. of t h e t h e Cerro P r i e t o Geothermal 1979, 497-507.

1 ) A d e p l e t i o n scheme i n which t h e p r o d u c t i o n zone becomes two-phase e a r l y i n i t s h i s t o r y b e s t f i t s observed behavior a t Cerro P r i e t o .

-27-

K. P. "NumeriCerro P r i e t o Second Sym. on Field, Mexicali,

8 . Whiting, R . L . , and Ramey, H. J . , J r . "App l i c a t i o n of Material and Energy Balances to Geothermal Steam Production,'' J . Pet.

Tech. (July,

1969), 893-900.

-28-

Proceedings Seventh Workshop Geothermal Reservoir Engineering S t a n f o r d , December 1981. SGP-TR-55.

WELL LOG ANALYSIS APPLIED TO CERRO PRIETO GEOTHERMAL FIELD M. CaStaneda*, A. Abril**, V. Arellano*, R. L. McCoy***

*Institute De Investigaciones Electricas * *Cornision Federal De Electricidad Coordinadora Ejecutiva De Cerro Prieto Mexicali B. C. Mexico ***Patterson, Powers and Associates, Inc. Houston, Texas

Introduction :

sure that only reliable data could be used in making any interpretation. Special care was taken for any

The Cerro Prieto Geothermal Field is a liquiddominated geothermal system located 30 km southeast of Mexicali Baja, California, Mexico, in t h e Mexicali Valley. Although some wells were drilled and completed in t h e late 60%, i t was April, 1973, when the geothermal power plant began operating with a capacity of 75 M W of electric power. Presently, the power plant installed capacity is 150 M W but this amount is expected to increase as further field development is planned to take place in the coming years. A number of questions are being presently asked as this field development continues. What will the deliverability of this geothermal field be in relation t o planned installations? W i l l reinjection be required t o supplement aquifer recharge? What will be the reservoir life and ultimate recovery?

possible depth shift on logs run in each well and when such depth shift was present, a correction was made for this effect. The Selecting the Reservoir Interval of Study. structural geology of the Cerro Prieto Field has been presented in several proceedings related t o this field and i t is well known. The formation is of sedimentary type with alternating shale and sandstone layers resting on a highly fractured granitic There are some structural base ment. interpretations of this field based on temperature and electrical logs in the literature. For this study, basically the lithologic column presented by Abril and Noble in 1978'l) was selected. Figure 1 presents a typical field cross-section resulting from that work and Figure 2 shows its location on the field. From this lithologic column, intervals La and Reservoir A were of special interest for the following reasons: a) for si mulation studies, these zones will be of interest because most of the are completed here and existing wells consequently, all reservoir data (production data, well tests) come from these zones, and b) although the for mation temperature begins to increase rapidly in Zone N, this zone presents a high content of carbonate ions in solution resulting in many well completion and scaling problems.

This concern has resulted in a joint project, where Comision Federal De Electricidad, Instituto De Investigaciones Electricas and INTERCOMP Resource Development and Engineering, Inc., of Houston, Texas are presently involved in performing reservoir simulation studies on the Cerro Prieto Field. An integral part of this project is the analysis of geophysical well logs t o deter mine basic reservoir parameters. There are three primary sources of data on the petrophysical properties of a reservoir: Core core analysis, well tests and well logs. analysis data a r e limited because of t h e expenses involved in obtaining the core samples and perfor ming t h e analysis. Well test analysis provide reservoir properties averaged over a large volume and therefore is not detailed. Well log analysis then, is the prime means of obtaining detailed data A distribution of material from t h e reservoir. parameters can be obtained from this analysis and the reservoir can be better defined for simulation purposes.

Determination of Effective Porosity. The computer all calculations was program used for INTERCOMP's Log Analysis Program. This program permits t h e use of petrophysical relationships whether they be standard indu$try accepted equations or derived empirical relationships. This program is full explained in Reference 11. For a clean sandstone lithology, the density log is usually the most reliable porosity device. When shale is present, a correction has t o be applied to density log readings for deter mining effective porosity. From the density log:

.

In late 1976, as a result of the Agreement, The Lawrence Berkeley Laboratory (LBL) of the University of California began a syste matic digitization of selected geophysical logs in order t o permit computer analysis of the Cerro Prieto well logs. Selected wells throughout t h e field were chosen for this study. Before any computer techniques were applied, the digitized well logs were visually compared with t h e original blue prints t o make DataGatherin

__T___p DOE CFE Cooperative

=

p

=

Density Log Reading

=

Matrix Density

ma

-29-

'ma-'

8,

P m a - 'f

pf

=

Formation Fluid Density

and:

where:

e' where:

=

D'

-

'sh

Osh

8,

=

Effective Porosity

8,

=

Porosity from Density Log

Vsh

=

ShaleVolume

BDsh =

Shale Porosity

,T i n 9

K

=

6 1 + 0.133T

Rmf

=

Mud Filtrate Resistivity (from log headings and temperature log)

SSP =

Static Value

Spontaneous

Potential

-

Method 11 Evaluates Rw using an electrical log and a porosity log. A t any depth:

The shale volume can be obtained from gamma ray or Sp logs. From the gamma ray log:

Rw

=

R t gem a

m

=

Cementation Factor

a

=

Constant in Archie's Formula F=

Rt

=

True Formation Resistivity

From the SP log:

Method 111 - Uses a Simandeoux water saturation equation (total shale equation)(14). In this case, water saturation is assumed to be 100% and the only remaining unknown is the formation water resistivity, Rw. That is:

In order to obtain maximum and minimum log values, the gamma ray and Sp log responses were histogrammed in each interval. Figure 3 presents a typical histogram for well M27. It is well known that both logs tend to over estimate shale volume. In our case, shale volume was evaluated from both logs and the minimum value w a s used for the effective porosity determinations. The exception to this was the case where the baseline drifts in the self potential log were apparent and this log was not used or when just one of these logs w a s run in a particular well.

Rw

=

0," a(l-Vsh)

(k

Vsh

-

m)

where:

8 Y , m, a,

An average effective porosity was obtained by

Vsh,

Rt are defined before in this

paper.

arithmetically averaging the incremental determined porosity in each zone. Porosity values greater than 40% were disgarded on the averaging procedure. Figure 4 presents the obtained effective porosity values for both intervals. A t this time, no consistent core data was available to verify the reliability of the obtained porosity data. Core samples from selected wells are being presently analyzed at IIE Petrophysical Laboratory. As this is done, reservoir permeability, another basic reservoir parameter, will be determined by means of some sort of porosity-permeability transform.

Rsh =

Shale Resistivity.

After R is obtained from any of the three methods Wdescribed above, total disolved solids (water salinity) can be found from a correlation for Na C1 solutions reported in the literature(7 1

.

Na C1 eq = 10' 3.562 x

=

- Log

(Rw75

- 0.0123)

0.955

and:

Evaluation of Water Salinity. The methods used to determine water salinity from well logs are reported and discussed elsewhere in the literature(399) and will not be reviewed here. Three of those methods that can be applied to the Cerro Prieto Field were selected for this purpose. They are different in the way the formation water resistivity (R,) is determined. A brief description is as follows:

T + 6.77

Rw75=

RHIT

,

T in°F

(75 + 6.77)

Some facts have been taken into account in evaluating the resistivity terms. I t is a we1 1 known e f f e c t t h a t both t h e i n v a s i o n o f d r i l l i n g mud i n t o the formation and temperature measurements o r c a l c u l a t i o n s o f t r u e f o r m a t i o n r e s i s t i -

Method I - Requires only the spontaneous potential log. So at any depth:

v i t y (Rt) and some a ~ t h o r s ( ~ ) h a vproposed e various methods t o overcane t h i s problem. I n our

- 30-

t

Bateman, R. M., Konen, C. E., (19781, "The Log Analyst and the Programmable Pocket Calculator: Part IV, Dual Induction - Late Rolog 8", The Log Analyst, May-June, 1978.

case, when possible, Rt was corrected for mud invasion effects according t o the method presented by Bateman et al. (1978). Regarding temperature effects, this is not a serious problem as long as enough data is available to determine true or initial formation temperature. If the temperature a linear profile deviates radically from relationship, this profile must be considered. Although some methods for determining static reservoir temperature during drillin operations, have been presented in t h e literature 7698,13), they could not be used because there was not enough required drilling data for this purpose. Instead, the stabilized shut-in well temperature profile obtained during the observation period was selected as an approximation to true formation temperature. For some fractured geothermal fields this temperature profile is not representative of reservoir temperature because of the existence of internal flows within the well (10) but Cerro Prieto is of sedimentary type field and i t was suggested recently(4) that this phenomenon was unusual in this field,

Brown, L. S., Gobran, B. D., Sanyal, S. K., (1980), "Determination of TDS in Geothermal Systems by Well-Log Analysis", Proceeding of the Sixth Workshop Geothermal Reservoir Engineering, Stanford University, Stanford, California, Dec. 16-18, 1980.

The water salinities determined from these three methods were compared with laboratory data(12) and the results are shown in Figure 5. As we can observe, water salinities evaluated from the self potential log (Method 1) and those evaluated using the Simandeoux's Equation (Method III) were lower than the laboratory reported data. Water salinities calculated from resistivity logs and density logs (Method II) were closer t o the actual water salinity values. This has been found t o be the case in other geothermal fields(5).

4.

Horne, R. N. (1981), Castaneda, M., "Location of Production Zones With Pressure Gradient Logging", Geothermal Resources Council Trans., Vol. 5, pp. 275-281.

5.

Davis, D. G., Sanyal, S. K. (19791, "Case History Report on East Mesa and Cerro Prieto Geothermal Fields", Infor mal Report LA 7889-MS, Los Alamos Scientific Laboratory, June, 1979.

6.

Dowdle, W. L., Cobb, W. M. (19751, "Static Formation Temperatures From Well Logs: An Empirical Method", J. Pet. Tech., V. 27, pp. 1320-1330.

7.

'

Dresser Atlas (1980), Charts."

"Log Interpretation

8.

Edwardson, M. J., Girner, H. M., Parkinson, H. R., Williams, C. D., (1962), Valculation of Formation Temperature Disturbances Caused by Mud Circulation", J. Pet. Tech., V. 14, pp. 416-426.

9.

Gobran, B. D., Brown, L. S., Sanyal, S. K. (1981), "A Hand-Held Calculator Program for Estimating Water Salinity From Well Logs", The Log Analyst, Vol.'XXII, No. 2, (MarchApril, 1981).

10.

"Interpretation of Grant, M. A. (1979), Downhole Measurements in Geothermal Wells", New Zealand Department of Scientific and Industrial Research, Report AM D-88.

Acknowledgements

11.

The authors would like to thank Coordinadora Ejecutiva De Cerro Prieto through i t s Department of Studies, and also the Instituto De Investigaciones Electricas for the opportunity given to them for presenting this paper.

INTERLOG, A Petrophysical Program: User's Guide, (1981), INTERCOMP Resource Development and Engineering, Inc., Houston, Texas.

12.

Laboratory Department, Internal Report. Coordinadora Ejecutiva De Cerro Prieto, Mexicali, B. C., Mexico.

13.

ROW, B., Sanyal, S. K. (1979), "An Improved Approach to Estimating True Reservoir Temperature From Transient Temperature Data", Proceedings of the Fifth Workshop Geothermal Reservoir Engineering, Stanford University, Stanford, CA., Dec. 12-14.

14.

Schlumberger, (1972), Volume I Principles."

f i n a l Remarks The obtained information will be of great help for the planned reservoir simulation studies of this A s more wells are being geothermal field. analyzed, more data regarding reservoir parameters will be known. A t the present time, some wells a r e being drilled and completed into the deeper reservoir B interval. That portion of the reservoir will also be analyzed and included in t h e simulation study.

REFERENCES 1.

Abril, G. A., Noble, J. E., (1978). "Geophysical Well Log Correlations Along Various Cross-Sections of the Cerro Prieto Geothermal Field," First Symposium on the Cerro Prieto Geothermal Field, Baja, California, Mexico, Sept. 1978, pp. 41-46.

-31-

-

"Log Interpretation,

-

FIGURE 1 TYPICAL FIELD CROSS-SECTION (FROM ABRIL AND NOBLE, 1978)

I

I'

1

-

FIGURE 2 LOCATION OF THE FIELD CROSS-SECTIONS (FROM ABRIL AND NOBLE, 1978)

I,.".,..

Q'.

9..

1

XBL i98-10915

-32-

FIGURE 3

- GAMMA LOG RESPONSE

HISTOGRAM FOR WELL M27

.....................................................................................................

-- 2U.000

0.100

L bO.000

40.100

10.010

100.000

IZO.OOU

I*O.OOO

I~O.OOO

I10.000

200.000

I

...... __.... ......... ......... ............. ._............... ................... ............. .......................... ...

I

...__..._e

2.00

-t

FIGURE 4

- AVERAGE

WELL

POROSITY VALUES FOR INTERVALS L2 AND A

ZONE

M19A M25 M 27

M29 M43 M45 M46 M50 MlOl M102 M107

LA2 L

2

2

INTERVAL (ft)

POROSITY (%)

-

19.7 16 .O 20.2 12.0 19.7 13.2 15.7 13.3 19.0 15.4 16.4 14.5 15.9 11.5 16.5 17 .O 16.5

2900 3750 3800 - 4270 2900 3800 3850 4580 3100 - 4000 4050 - 4210 3450 3700 3750 4180 2930 3730 3800 4090 3100 - 3860 3900 - 4570 3000 - 3800 3850 4650 3050 - 3900 3950 - 4120 3590 - 4350

-

-

- 5350 - 6300 - 6060

3900 5500 4500

-

-33-

19.1 12.9 17.1

-

FIGURE 5

- AVERAGE WATER SALINITY DATA AS

PPM NaCL equiv CALCULATED FROM THREE DIFFERENT METHODS

INTERVAL

METHOD

(ft)

I

3600- 4240 3580- 4590 3600 - 4240 3608- 4250 3772- 4100 3900- 4120 3930- 4640 3750- 4120 6050- 6550

1650 4266 1958 1151 3127 730 1278 1348

WELL Ml9A M25 M27 M29 M43 M45 M46 M50 M53

*WATER SALINITY (PPM) METHOD METHOD LABORATORY II m DATA

-

10465 13499 9306 170 37 14617 10806 10885 9329 15225

*Salinity at reservoir conditions

-34-

6013 6504 4840 9664 8512 4477 5650 3002 8793

13812 15054 11794 13044 13076 11060 10113 13278 14446

Proceedingo Seventh Vorkshop Geothermal Reservoir Eqbeerlng Stanford. Decrber 1981. SGP-TR-55.

RECENT RESULTS OF TEE WELL DRILLING PROGRAM AT CERRO PRIETO

Bernard0 Domfnguez A. ('I,

Marcelo J . L€ppmannC2) and Francisco Bennejo M. (1)

'Coordinadora Ejecutiva d e Cerro P r i e t o ComisiBn Federal d e E l e c t r i c i d a d Mexicali, Baja C a l i f o r n i a , Wixico Abstract

'Lawrence Berkeley Laboratory Earth Sciences Division Berkeley, C a l i f o r n i a 94720

high- pressure turbogenerators). A 30-Hw lowerp r e s s u r e u n i t h a s been undergoing t e s t i n g s i n c e Before going i n t o f u l l operation, mid-1981. some i n s t a l l a t i o n s of t h e f l a s h i n g p l a n t f o r t h i s u n i t w i l l have t o be mo'dified t o improve i t s performance. A t t h i s p l a n t , water a t about 169OC separated from t h e high- pressure steam is flashed a t 4.36 and 2.11 kg/cm2 abs. The c o n s t r u c t i o n of two 220-MW power p l a n t s , each with two 110-MW turbogenerators, has begun. These p l a n t s a r e scheduled t o go i n t o operation during 1983 and 1984, r e s p e c t i v e l y .

The r e s u l t s of t h e 1980 and 1981 w e l l d r i l l i n g a c t i v i t i e s a t t h e Cerro Pr i e t o geot h e r m a l f i e l d a r e summarized. D e t a i l s a r e given on t h e new series of deeper w e l l s comp l e t e d i n t h e western (" older" ) p a r t of t h e f i e l d (Cerro P r i e t o I ) , and on t h e development and step- out w e l l s d r i l l e d i n t h e e a s t e r n p a r t of t h e f i e l d (Cerro P r i e t o I1 and 111). Product ion c h a r a c t e r i s t i c s of on- line and standby w e l l s a r e discussed. Recent changes i n w e l l completion procedures a r e a l s o d e s c r i b e d . Introduction

D r i l l i n g program

D u r i n g t h e l a s t two y e a r s s i g n i f i c a n t advances have been made i n t h e development of t h e Cerro P r i e t o f i e l d . The purpose of t h i s paper is t o update t h e informat ion presented during t h e F i f t h Geothermal Reservoir Engineering Workshop (Alonso e t e l . , 1979).

I n November 1981 t h e r e were f i v e d r i l l i n g r i g s and two work-over r i g s a c t i v e i n the area; a b o u t 96 d e e p w e l l s h a v e been c o m p l e t e d . Between December 1979 and November 1981 27 w e l l s were d r i l l e d . These w e l l s a r e shown i n F i g u r e 1, w i t h t h e e x c e p t i o n of w e l l G- 1, located about 6 km ENE of well NL-1. The t o t a l depths and m a x h u m temperatures measured i n these w e l l s a r e given i n Table 1.

The i n s t a l l e d e l e c t r i c a l power c a p a c i t y a t t h e f i e l d continues t o be 150 MW ( f o u r 37.5-MW

TABLE 1 CERRO PRIETO WELLS DRILLED BETWEEN DECEMBER 1979 AND NOVEMBER 1981 TOTAL DEPTHS AND MAXIMUM MEASURED TEMPERATURES

Total Depth

Well

(m 1

Max. Temp. (OC 1

E-4 E-5 E- 7

1996 338 1945 328 1814 333 1767 333 1966 322 under c o n s t r u c t ion

G-1

3000

219 324 245 355

E-1 E- 2 E- 3

M-47 n-73 M-79 M-lo9

NOTE : * well

Well

(m 1

M-115 M-117 M-118 M-122 M-125 M-132 M-133 M-137 M-139 M-147 M-157 M-172 M-189 T-328 T-364

f i l l e d with d r i l l i n g mud

- 35-

T o t a l Depth

Max. Temp. (OC 1

under c o n s t r u c t ion 2495 360 2664 299 under c o n s t r u c t ion 2315 354 3268 2 84 2356 310 2506 >233 under c o n s t r u c t ion 1908 353 2545 331 3287 282 3495 267 2695 34 9 2926 320

CERRO PRIETO GEOTHERMAL FIELD WELL LOCATIONS NOVEMBER 1981

'\VOLCANO',

\

'\

\

I

\ \

'I

c

'4

0 1 b - 6

2 krn

I

Scole I

Figure 1.

Location of v e l l s a t Cerro P r i e t o . Wells Indicated with concentric c i r c l e s were d r i l l e d between December 1979 and November 1981.

- 36-

XBLBI IO- 121368

I n the e a s t e r n p a r t o f the f i e l d t h e purpose of t h e d r i l l i n g a c t i v i t y h a s been t o inc r e a s e t h e number of product-ion w e l l s f o r t h e power p l a n t s under c o n s t r u c t i o n , and t o e x p l o r e f o r t h e boundaries o f t h e geothermal rystem. The temperature p r o f i l e s obtained in t h e most r e c e n t l y d r i l l e d wells confirmed t h e taperat u r e d is tr ibut ions developed e a r l i e r t h is year by C a s t i l l o et a1. (1981) (See Figures 2 and 3). The w e l l s d r i l l e d during 1980 and 1981 have e s s e n t i a l l y d e l i n e a t e d t h e n o r t h e r n , e a s t e r n and s o u t h e a s t e r n b o u n d a r i e s o f t h e thermal anomaly. Outside of t h i s r e g i o n , t h e 1977 P r i m w e l l (3496 m depth) and t h e r e c e n t G- 1 w e l l (3000 m depth) have shown v e r y low temperatures. In the western p a r t of the f i e l d , new prod u c t i o n and stand-by wells were d r i l l e d f o r t h e e x i s t i n g power p l a n t . In t h a t r e g i o n t h e w e l l s of the deeper "E- series" (average t o t a l depth: 1900 m) have confirmed t h e presence of a h o t t e r a q u i f e r ( a b o u t 335OC) below t h e r e s e r v o i r which has been under e x p l o i t s t ion s i n c e 1973, and whose average temperature and depth a r e about 28OoC and 1250 m, r e s p e c t i v e l y .

Well p r e s s u r e s and production r a t e ? Shut- in wellhead p r e s s u r e s i n t h e northwestern p a r t of the f i e l d (CP I Norte), excluding t h e E -w e l l s , have reached up t o about 800 p s i ; i n t h e southwestern p a r t (CP I Sur) about 900 p s i ; in t h e s o u t h e a s t e r n p a r t (CP IT) about 1300 p s i ; and i n t h e n o r t h e a s t e r n region (CP 111) about 1200 p s i . I n CP I N o r t e ( e x c l u d i n g t h e d e e p e r E - w e l l s ) t h e maximum s t e a m p r o d u c t i o n e v e r I n CP I S u r , measured in a well was 125 t / h . some w e l l s reached 140 t / h o f steam. I n CP 11, where t h e r e s e r v o i r i s a t 2700-3000 m depth, steam productions of up t o 300 t / h have bten measured. I n CP 111, t h e r e s e r v o i r i s a t 2000-2500 m d e p t h , and some w e l l s have produced above 100 t / h o f steam. (Domhguez and SBnchee, 1981). The p r o d u c t i o n c h a r a c t e r i s t i c s of t h e w e l l s supplying steam t o t h e power p l a n t as of August 1981 a r e g i v e n i n T a b l e 2. A t t h a t time, t h e average e l e c t r i c a l power g e n e r a t i o n

TABLE 2 CERRO PRIETO WELLS ON LINE PRODUCTION CHARACTERISTICS

(AUGUST 1981) Well M- 5 M 11 M 14 M 19A M - 25 M - 29 M 30 M 31 M 35 M 42 M 43 M 48 M 50 M 51 M 53 M 84 M 90 M 91 M - 101 M 102 M 103 M 104 M - 105 M 114 M - 130

-

-

E E -

1 2 E - 3

Orifice diam. ( i n ) 7 718 4 3 718 7 718 4 7 718 7 718 5 7 718 8 8 8 8 8 8 8 8 8 8

5 4 6 8 8 8 4 112 3 118 3 112

Pressure ( i n p s i g ) Wellhead Separator 110 119 168 110 240 120 110 106 122 190 108 130 125 122 124 100 108 112 104 100 185 132 130 105 115 410 872 580

98 104 107 100 112 113 100 97 105 104 107 106 108 110 103 95 106 109.5 102 96 104 103 115 104 107 130 131 98 TOTALS:

-37-

Production ( m e t r i c t o n s / h ) Steam Brb e 25.7 13.5 20.5 56.3 38.5 15.8 41.4 16.7 53.8 43.7 18.7 46.1 75.4 78.2 17.3 41.4 41.4 67.4 17.6 22.0 48.5 63.4 56.1 41.6 53.3 103.6 64.4 41.1 1223.4

69.4 47.9 70.5 135.7 73.1 60.5 106.1 38.8 130.1 146.5 54.0 72.3 167.8 133.1 25.2 22.3 129.0 133.1 27.9 8.2 48.3 25.6 81.6 136.8 131.8 172.4 108.7 121.8 2478.5

Enthalpy (callg) 304 281 284 315 344 277

309 318 316 285 300 364 326 35 5 370 489 292 339 361 5 28 418 522 375 287 315 363 361 295

Figure 2. Cerro Prieto; isotherms at 2000 m depth. (modified from Castillo et al., 1981)

ISOTHERMS 2000m depth

Scalr

lkn

Figure 3.

ISOTHERMS

3000 m depth

-

Cerro Prieto; isotherms at 3000 m depth. (modified from Castillo et al., 1981)

t

Scrk

0

lh

)#

-38-

8111-4e70

w a s 112.5 EIW ( o n l y t h r e e o f t h e f o u r turbogenc r a t o r s were on l i n e because of r e p a i r s t o one of the cooling towers). T a b l e 3 shows t h e p r o d u c t i o n c h a r a c t e r i s t i c s o f t h e standby wells. Well completion

A number of m d i f i c a t i o n s have been made i n t h e way t h e w e l l s are completed a t C e r r o P r i e t o , p a r t l y because deeper production and e x p l o r a t i o n wells a r e being d r i l l e d , and p a r t l y t o reduce mechanical and c o r r o s i o n problems i n t h e c a s i n g s (Table 4 ) . The c a s i n g completion d e s c r i b e d by Alonso e t a l . (1979, Table 2 ) u s i n g production c a s i n g s w i t h API N-80 t u b i n g wa8 not very s u c c e s s f u l . The l i f e t h e of t h e s e c a s i n g s is about 6 months (Domhguez e t a l . , 1981). Corrosion, c o l l a p s e s and f r a c t u r e s have been d e t e c t e d .

To reduce c i r c u l a t i o n l o s s e s w h i l e cement-

ing long s t r i n g s of c a s i n g s (up t o 2000 m l o n g ) , low- density cement s l u r r i e s have been used. Recently, good r e s u l t s have been obtained by adding small diameter ceramic s p h e r u l e s t o t h e s l u r r y , reducing i t s s p e c i f i c g r a v i t y t o about 1.3 (10.8 l b l g a l ) . I n some wells, mainly because of c i r c u l e t i o n l o s s e s , none of t h e cement s l u r r y r e t u r n s t o t h e surface. Recently, t h e non-cemented annular space behind c a s i n g s h a s been f i l l e d by pouring f i n e s i l i c a sand. I t not only reduces t h e open space behind t h e p a r t i a l l y cemented c a s i n g , b u t a l s o g i v e s it mechanical s u p p o r t . Up t o now t h e wells where t h i s procedure was used have n o t shown problems.

F i n a l remarks

Up- to- date r e s u l t s h a v e shown t h a t t h e w e l l s completed d u r i n g 1977-78 u s i n g API K-55 production c a s i n g s have performed w e l l . These h e a v i e r , s o f t s t e e l c a s i n g s have shown g r e a t e r r e s i s t a n c e t o mechanical stresses and c o r r o s i o n . The damages observed i n some of t h e 1977-78 w e l l s a r e believed t o be r e l a t e d t o f a u l t y cementing of t h e c a s i n g s caused by c i r c u l a t i o n l o s s e s and/or f a i l u r e of c a s i n g a c c e s s o r i e s d u r i n g t h e cementing o p e r a t ions.

The d r i l l i n g of production and e x p l o r a t i o n w e l l s w i l l c o n t i n u e a t Cerro P r i e t o . The immed i a t e goal is t o d r i l l enough w e l l s t o s a t i s f y t h e long-term steam requirements of the power I t is e s t i m a t e d t h a t t h e e x i s t i n g plants. p l a n t w i l l need 30 w e l l s ( 6 EIW/well), w h i l e each o f t h e two power p l a n t s under c o n s t r u c t i o n w i l l r e q u i r e t h e s t e a m from a b o u t 25 w e l l s ( 8 . 8 MW/well) t o r e a c h a t o t a l g e n e r a t i n g c a p a c i t y of 620 MUe by 1984.

P r e s e n t l y A P I C- 75 g r a d e p r o d u c t i o n c a s i n g s are being i n s t a l l e d a t C e r r o P r i e t o (Table 4 ) . Because of t h e r e c e n t i n s t a l l a t i o n , it h a s as y e t n o t been p o s s i b l e t o e v a l u a t e t h e i r performance.

I n o r d e r t o e s t a b l i s h t h e a r e a l e x t e n t and t h e energy p o t e n t i a l o f t h e s o u t h e r n p a r t s of t h e f i e l d (CP I S u r and CP 11) a number o f w e l l s a r e planned t o be d r i l l e d soon i n t h e a r e a between w e l l s M-101, 93, 1 8 9 and 92.

TABLE 3 CERRO PRIETO STAND-BY WELLS PRODUCTION CBARACTERISTICS

Well

M - 7 M - 73 M - 93 M 94 M 110 M - 120 M 129 M - 147 M 149 M 169 M - 172 T T T

Q

-

366 386 388 757

Date

7/25/79 7/29/81 6/22/79 91031ao 11/18/79 4/21/80 2/12/80 2/05/80 310 2 (80 3/28/80 91 09/61 7/29/79 10/07/81 512 21 80 12/05/79

Orif ice dim. (in) 5 8 8

4 10 10 5 6 8 9 6

8 10 7 3

Wellhead Pressure (PSig) 100 182 170 100 220 129 700

670 104 183 101 291 110 324 101

-39-

Product ion ( m e t r i c t o n s l h ) Steam Water 15.0 100.1 80.7 6.5 185.3 121.7 216.0 297.1

66.4 123.3 23.3 223.5 86.0 169.8 5.8

101.5 165.0 186.0 76.9 335.7 148.7 301.2 249.6 124.8 204.9 117.3 269.5 159.5 278.5 51.4

Enthalpy (callg) 235 357 320 210 346 392 37 6 438 342 35 6 25 3 383 34 3 357 2 22

TABLE 4

CERRO PRIETO PRESENT CASING COMPLETIONS Size (in)

Cam ing

O.D.

API Grade

We igh t

(lblft)

Joint Threds

Approximate Depth (m)

PRODUCTION WEUS Conductor Surf ace Intermediate Product ion Lh e r

30 20 13 318 9 518 7

98.9 106.5 68.0 47.0 29.0

B

5-55

K-55 c-75 c-75

Welded RT.8T.SC.

0

BT

0 0

0

.

SEU .HT. SEU .HT

.

2450

--

-

50

300 1200 2500 3000

EXPLORATION WELLS Conductor Surface Intermediate Product ion Liner Notes BT .

RT.8T.X. SEU.HT. CS.HT.

20 13 318 9 518 7

4 112

H-40 K- 55

94.0 54.5

RT.8T.X.

c-75 c-75 c-75

47.0

SEU.HT. SEU.HT. CS.HT.

29.0 13.5

BT

.

'

0

- 100

1550 3250 0 0

500 1600 3300 3800

Buttress thread = Round t h r e a d , 8 t h r e a d s l i n , s h o r t j o i n t Super E.U. H y d r i l l t h r e a d

C.S. H y d r i l l t h r e a d

Acknowledgments

C a s t i l l o , F . , Bermejo,F.J., Domhguez,B., Esquer, C.A., and Navarro, F.J., 1981. Temperat u r e d i s t r i b u t i o n i n t h e Cerro P r i e t o I n Proceed ings , Third geothermal f i e l d Symposium on t h e CeGo Pr i e t o Geothermal F i e l d , Lawrence B e r k e l e y L a b o r a t o r y , LBL 11967 ( i n p r e p a r a t i o n ) .

The a u t h o r s express t h e i r g r a t i t u d e t o t h e a u t h o r i t i e s and personnel of t h e Coord inadora E j e c u t i v a d e Cerro P r i e t o of t h e Comisibn Feder a l de E l e c t r i c i d a d f o r t h e i r support and encouragement. P a r t of t h i s work was supported by t h e A s s i s t a n t S e c r e t a r y f o r Conservation and Renewable Energy, O f f i c e of Renewable Technology, Division of Geothermal and Hydropower Technologies of t h e U.S. Department of Energy under Contract No. U-7405-ENG-48.

.

Domhguez, B . , and Shnchez, G . , 1981. Comments on some geothermal d r i l l i n g and w e l l comp l e t i o n problems a t Cerro P r i e t o . 3 Proceedings, Third Symposium on t h e Cerro P r i e t o Geothermal F i e l d , Lawrence Berkeley Laboratory, LBL 11967 ( i n p r e p a r a t i o n ) .

References Alonso E.,H., Dmnhguez A., B . , Lippmann, M . J . , H o l i n a r C . , R . , S c h r o e d e r , R . E . , and Witherspoon, P A . , 1979. Update of reserv o i r engineering a c t i v i t i e s a t Cerro Prieto. I n Proceedings, F i f t h Workshop G e o t h e r m a l ~ e s e r v o i rEngineering, Stanford Geothermal Program, SGT-TR-40, pp. 247-256.

Dom$nguez,B., V i t a l , F . , Bermejo,F., and S h c h e z , G . , 1981. Performance o f c a s i n g s i n Cerro P r i e t o production wells. IF Proceedings, Third Symposium on t h e Cerro P r i e t o Geothermal F i e l d , Lawrence Berkeley Laborat o r y , LBL 11967 ( i n p r e p a r a t i o n ) .

-40-


Fduardo R. Iglesias

and

Gerard0 Hiriart

Ccmisidn Federal de Electricidad 0klakn-m. #85, 6' Piso Mkxico 18, D.F., Mikico

Instituto de Investigaciones El6ctricas AJ?artadoPostal 475 cllernavacd, Morelos, Msxico

A tracer test will be conducted at the Los

Azufres geothermal field to detect possible high permeability flowpaths interconnecting reinjection and production wells, in order to asses reinjection feasibility in the Tejamaniles area. This test will be the first of its kind to be conducted in t h i s geothermal field. Tracers will be injected in wells A-7 and A-8; A-2 will constitute the main obsenration well. This study suggests that other wells, including probably A-16, A-1, A-22 and A-18, should also,& mnitored. Continuous prcduction/reinjection will provide the driving force for interwell flow. Tu speed up results, simplify logistics, arkl reduce costs, slugs of two different tracers, each identifying a particular well, will be simultaneously injected. W e chose anions as tracers because they propagate through reservoir formations with negligible absorption, and considering the availability of equipnent for their analysis. Several brcmide, iodide and thiocyanate salts were considered as prospective tracer sources. The unknown background iodide concentrations in the reservoir were detem-ined by us. We estimated the m u n t s of the prospective salts to be injected by means of Lenda and Zuber's mthcd, and computed the corresponding costs. Considering these munt-cost calculations and that brcanide and iodide have been extensively tested in geot h e m l enviromts, we chose these anions as the main tracers for the test. As a canplemnt, we propose to conduct a field experiment to evaluate thicqnate as a geotheml tracer

A-17, in the Tejmiles region. Produced waters are of d m salinity: TIXS % 5000-8000 ppn for separated brines (Tenplos y Garcfa,1981) These brines contain silica (900-1300 p p ) , boron (150-250 ppn), arsenic (15-30 ppn), and traces of mercury (Wrplos y &rcla,1981). such m u n t s of ecologically nocive substances are rot suitable for untreated surface disposal. Thus, reinjection of the spent brines is considered as one alternative for disposal.

&injection of relatively cold spent brines, apart from solving the disposal problem, may be beneficial to maintain reservoir pressure (e.g. Cuellar et al, 1978; Home, 1981). But thermal interference may also result from reinjection (e.g. Einarsson et al, 1978; Home, 1981), with potentially serious economic consequences due to enthalpy draidown. Tu avoid the detrimental effects of t h e m 1 interfexience, tm reinjection strategies may be adopted: (a) to reinject into fomtions hydraulically isolated f m the producing reservoir; or (b) to reinject at a distance from the producing wells such that the reinjected water has time to reheat before being produced. Tnis "safe distance" depends sensitively on the details of the local pemability. For example, in Ahuachapan a tracer arrived at a well 400 m distant f m the reinjection well in tw~ days, but the corresponding arrival times far two other wells located 450 m and 1000 m frcm the injector were several weeks (Einarsson et al, 1978). Thus, identification of high permeability paths interconnecting planned reinjection-and productionwells should preceed the actual inplanentation of a reinjection schene. Tracer tests constitute an unsurpassed reservoir engineering tool to detect such s b r t circuiting flowpaths.

INTFQDWTICN he US Azufres geothermal field is located

19O49'N, 100°39'Wonthe Transmexican Nee-

This p a w reports on the design of a tracer test that will be run in the U s Azufres geo-

Volcanic Axis. The field consists of higuy fractured neo-quaternaq volcanic deposits. Production horizons are mstly andesites, but in saw cases include dacite. Because of the tightness of these igneous rocks the main production is believed to proceed through fractures.

thermal field. The test will involve wells A-7 and A-8 as reinjectors, and well A-2 as the producer. The rain goals are (a) to detect possible high permeability flowpaths linking the planned reinjector wells with A-2; and (b) to interpret the data, either in tenns of fracture conductivity or of matrix p e h i l i t y .

The reservoir is liquid-daminat&,

In the following sections we describe the test area, and discuss the design of t h i s first

but thae is a steam cap located near wells A-6 and

-4 1-

atures but low permeabilities were found. Recently the w e l l was reentered and directional drilling w a n frun a depth of % 500 m. A t the time of t h i s writing the directional w e l l had succesfully intersected the T e j m i l e s fault. The depth and distances to w e l l A-7 and A-8 given i n T a b l e 1 were estimated by us. A t the tim of this writing the productivity test had not started. Thus, it is not krrown whether dry steam o r a tu;o-phase mixture w i l l be produced. I f the produced fluid turns out to be -phase, it is expected that A-16 w i l l be kept bleeding through a small diameter line t h r o q b u t the duration of the tracer test. In that case, the separated water w i l l be mnitord for tracers.

tracer test in IDS Azufres. TEST AfEA

The test area lies in the Tejamaniles region (or module, a s they have been termed), i n the southern p r t i o n of the f i e l d (Fig. 1).

-

0

100

t

'

100

_--

1 -

Farther East from the reinjection area l i e wells A- 6 and A-17. T k s e relatively shallm wells produce dry steam. The chenical tracers we are planning to use in t h i s f i r s t test partition negligibly into the gas phase. Therefore these wells w i l l not be mnitored during the tracer test.

. I

I

FAULT FRACTURE O R INFERRED FAULT

N e x t i n increasing distance fran the reinjection area is well A-1, located to the NE of A-7/A-8 i n the Agua F'ria d u l e (Fig. 1). This w e l l is currently producing 30 tons/h It is exwith a water/steam r a t i o of 2:l. pected t h a t t h i s flawrate w i l l be kept constant throughout the duration of the tracer

Fig. 1. Map of the Tejamaniles and Aqua R i a mdules.

Planned reinjection wells A-7 and A-8 lie on the western edge of the f i e l d , as inidicated by geologic and r e s i s t i v i t y data (Garfias, 1981). Their depths, open intervals, and distances to other wells located i n the Tejamaniles and Aqua R i a d u l e s are given i n Table 1. East fran A-7/A-8 the distribution of temperature is d m - l i k e , with its mximum near w e l l s A-6 and A-17. This is reflected i n the depths of the wells d r i l l e d i n the T e j m i l e s mdule

test. I f t h a t is the case, the separated water w i l l be mnitored for tracers. WELL A.2

WELL

A- 7

WELL

A- 0

a

PORPHYRITIC ANDESITE

I:::::/

.

PlLOTPXlTlC A N I S I T E UlCROLlTlC ANDESITE UlCROCRYSTALLlPE ibSESITE TUFF AlSESi::

Production w e l l A-2 is located several hundred

@ TUFF

meters E a s t from t h e planned r e i n j e c t o r s . The completions and l i t h o l o g i c a l columns of w e l l s A- 2, A- 7 and A- 0 a r e shown i n F i g u r e

/.:I

2. The production and reinjection intervals intersect m s t l y microlitic andesites, w i t h some interspersed dacite and porphyritic andesite i n w e l l A-7. In these t i g h t ingneous rocks the flaw is thought to ccm mainly through fractures. Note that the reinjection intervals l i e deeper than the productioninterval of A-2, except for the upper slotted interval of w e l l A-I which overlaps it. This is a desirable feature for a reinjection scheme because the negative bouyancy of the colder and denser reinjected water tends to delay thermal interference: but m y add scm d i f f i culty to the recovery of tracers. Hmver, there is indication of hydraulic connection of wells A-7 and A-8 W i t h A-2, f m an B Y lier interference test ( H i r i a r t , 1980).

Fig. 2.

DACITE

L i t h l o g i c a l colunms and a a n ~ ? k tions of wells A-2, A-7 and A-8.

Finally, wells A-22 and A-18, in the Aqua Fria and Tejamniles mdules respectively, are the farthest fram the reinjection area. *se w e l l s are expected to be kept bleeding through mll diameter lines throughout the duration of the tracer test. Due to their negigible productions and great distances from the injection pints, there is l i t t l e hop of actua l l y detecting tracers i n these wells. N e v e r theless they w i l l be mnitored on a low priori t y basis throughout the test.

Other neighboring wells, i n addition t o A-2,

were included i n Table 1 for ccsnpleteness. Of these, A-16 is the second closest to the reinjection area. Well A-16 was f i r s t drilled vertically t o a depth of 2500 m.. High tenper-

-42-

Chanicals with high tracer-anion/salt mass ratios are convenient to keep m u n t s of salts injected and costs dawn. Iodide and bnmide have been succesfully used as tracers in geothermal e n v i m m t s (e.g. Mccabe et al, 1980, 1981; Tester et al, 1979); t h i c q n a t e ( a l t b u g h fairly m n in the oil industry) has not, to our kncrwledge.

General &marks

The test will be conducted in multitracer, continuous prcduction/reinjection mile, as follows. Simultaneous prcduction/reinjection will begin ECXIE time before actual tracer injection to allaw as much stabilization of the flaw in the reservoir as practically possible. Separated water being produced frcm A-2 will be condtantly reinjected in wells A-7 and A-8. After a convenient stabilization time has elapsed, short injections of tm different tracers, each identifying a particular well, will be inserted in the lines feeding A-7 and A-8. Sampling of well A-2 will begin m i ately afterwards. During the first several hours samples will be extracted every half hour; subsequently, the rate of sampling will decrease mnotonically. The rationale for this sampling strategy is to record possible early rapid buildup of tracer concentration in A-2 due to high permeability path(s) extending f m one or both reinjection wlls; as arrival times increase so do dispersions, and tracer concentration buildups beccane s l m r , therefore sampling frequency can be safely decreased.

Background Concentrations

The natural (backpund) reservoir concentrations of the chemicals used as tracers are inprtant for tracer test design because they are related to the m u n t s of tracers to be injected, as Shawn later in t h i s paper. In the following paragraphs the background concentrations of iodide, bromide, and thiocyanate in the test area are discussed. Branide concentrations are routinely determined by (=FE's* geochemistry group in IDS Azufres. Average values for separated waters f r m wells A-2, A-7 and A-8 are 0.29 f 0.28 ppn, 0.35 0.10 p p , and 0.2 f 0.2 ppn respectively (Iopez and Tgnplos, 1980).

*

Iodide concentrations are not routinely determined in Los Azufres. Sampling and analysis

was conducted by an IIEt team in October 1981; CFE personnel collaborated taking downhole samples from well A-7. Iodide concentration resulted as follows: for separated water f m A-2, 0.35 f 0.01 ppn; for tm downble samples from A-7, corresponding to depths of 1075 m and 1420 m, 0.18 ?: 0.01 ppn and 0.17 f 0.01 ppn respectively; for water from the disposal pond of well A-18 (samples taken near the discharge tube), 0.23 k 0.01 ppn.

Tracers used in hydrqeolosy and in the oil and geothemnl industries include radioactive substances and chemicals. Use of radioactive tracers involves lengthy proceedings to obtain licenses and equiprent currently unavailable to us- For sirplicity chemical tracers *re adopted. The main advantage of the approach described in the preceding paragraphs is that the multitracer configuration insures a significantly shorter test. Consequently, logistics can be made sinpler, costs (e.g. associated with p r sonnel and equipnt) can be reduced, and results will be available earlier.

Allawing for dilution of the samples from separated water, and considering the known downhole values, we conservatively adopted reservoir "background concentrations" of 0.40 pgm and 0.25 p p for bramide and iodide respectively. Thiocyanate is generally absent from geothermal waters. Its concentration in the r e servoir brine was assumed not to exceed 0.01

Tracers The popular chgnical tracers mentioned in the literature can be broadly divided in fluorescent dyes and salts with detectable cations or anions. Fluorescent dyes tend to be retained in the reservoir rocks (Wagner, 1977) and were not considered for this test. Experience has indicated that cations do not propagate through reservoir formations as easily as anions. Thus, only anions were considered as tracers. After screening, the follcwing candidate salts were selected: sodium bromide (NaBr), sodium iodide (NaI), potassium iodide ( K I ) , m n i u m thiocyanate (NH4SC?J), and sodium thiocyanate (NaSCN). These salts have suitable anion to mlecule mlecular weight ratios and high solubilities (Table 2). High solubilities are desirable because they determine the maxirmrm concentration attainable at the point of injection, which generally should be as high as possible in slug injection tests to insure detectability over long distances.

ppn.

Amunts and costs The m u n t of tracers to be injected were estimated following a method devised by Lenda and Zuber (1970). Normalized breakthrough curves for line injection between two impemable layers ccsnputed by these authors were used. This method assumes that the flow takes place in a hcaoogeneous porous medium with matrix pemability. These assumptions might not apply to the igneous formations characteristic of the test area. Nonetheless, they provide a conservative, "mrst case" approach to the tracer flow: generally, in a two well injection

* ComisiBn Federal de Electricidad t Instituto de Investigaciones Electricas -43-

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configuration w a t e r dispersion of the tracsr is expected in transport through a p m u s m t r i x than i n fractme flow.

advantacp of using as tracers s&stana?s that are naturally absent i n the brines, and f o r which sensitive analysis techniqEs exist.

in its original fonn applies to radioactiE t r a m , for use with ionic tracers d e r i e d from highly liissociating salts. Briefly, the mdified meestimates the mss of salt to be inyected, m, frmn

Table 3 also shows the relative costs of the mass to be injected fbr each salt mnsidered. N3t surprisingly, the srmllest costs, by far, corresp n d to aocyanates. Note also that the relatively low ba&ground cmnoentration of iodide mmpensates f o r its higher price per u n i t W s .

-Ne have adapted Lenda and Zuber's metbd, which

Althom thiocyanates look very attractive from the pint of vied of their low costs, they have m t been extensively tested i n geothernrl e n v i m m t s . Brcanide and iodide are better Iumm as 9 t h tracers, and their costs are reasonable for the planned test (Us $1009 for NaEir i n A-7). Therefore, we conch& that bmmide and iodide are the indicated tracers for the f i r s t test i n U s Azufres.

;here c is the minimal mncentration of the anion a t the lraximrm of the breakthrough curve which assues a proper recording of the whole curve, Q the -ionless peak concentration i n the dirrwsionless breakthrough curve, x the interwell distance, $ the p m s i t y and h the heiat of the confined aquifer, and J& and the dispersion coefficients parallel and n o d to the direction of flow respectively. The dimmionless ratio (D&x), where v is the man velocity of flaw i n the x direction, paramtrizes the breakthrough curves. Whithin the range of m t e r s consi&red CD decreases with increasing v a l e s of (Wvx). Therefore, framequation (1) greater valcles of m correspond to higher values of (4JQ for a gim distance. Values of (hv) range fran a few centimeters for fine sands to about 100 m for fissured rocks (mdaand Z u b e r , 1970). Thus, we adopted CDJ.v) = 100 m i n order to obtain conservative estimates of the mass of t r a e r to be injected.

9.

Finally, a usefull extension of t h i s mrk is suggested by our results. We prapose to conduct a f i e l d experimnt to evaluate thiocyanate a5 a geothermal tracer. ?he experimnt w u l d consist of injecting this anion i n parallel with the main tracers in wells A-7 o r A-8. cbnparison of the corresponding breakthrough cumes muld pmvide valuable i n f o m t i o n about the Ckaracteristics of thiocyanate as a geotherrral traer.

W e thank Ing. C. Mirank from CFE for taking cbwnhole fluid samples, and Dr. D. Nieva from IIE f o r conducting the iodide analysis and for useful discussions.

An extra safety factor is provided by the r a t i o ) a p p a r i n g i n equation (1). For liquids, the0 tical and experimntal values of this r a t i o

(4J

2

l i e between 1 and 20 for Bodenstein nunbrs ( = effective grain diamter x man flow velocity/ coefficient of mlecular diffusion) ranging f r o m 4x10-1 (slow laminar flow) to lo5 ( f a s t turbulent flow) (Mda and Zuber, 1970 and r e f e r e n e s therein). In our calculations we set &/%) = 1, effectively enhancing the safety mrg of our estimates.

l h i s m& fom part of the cxmperative geothermal program betieen I n s t i t u b de Investigaciones

E l e c t r i c a s and Stanford University.

Wllar, G., m u s s y , M., and Esakar, D. (1978) "Extraction-rein jection a t Ahuachapan Mthermal Field", Pmc. 2nd Invitational Welltesting Symp., &port -8883, pp 15-25.

W e adopted c = -k l o € , where is the background a n c a t r a t i o n of the anion in the resemir fluid and E is the resolution of the corresponding mthod of analysis. Standard nethods of analysis adequate for the concentrations of interest here have typical resolutions of 0.02 ppn, O.Olppn, and 0.01 ppm for bromide, iodide, and thiocyanate respectively. The vales of c thus coquted are shawn i n Table 3.

Einarsson, S., Vides, A., and Cuellar, G. (1975) "Disposal of -the& W a s t e Water by Feinjection", Proc. 2nd Mited Nations Symp. on the Developmt and U s e of Ge0therm.l &sour&s, pp 1349-1363. Garfias, A. (1981), personal comumication.

Taking $h = lm, a typical v a l for ~ the test area, we ccanputed the e s t h t e s shown in Table 3. For convenience these results a r e expressed i n term of the m s of N a B r to be injected i n w e l l A-7, The greater distance of w e l l A-8 to well A-2 results in a practically negligible 15%increase of the m s of traer to be injected ou3r the valcles corresponding to A-7. The background concentrations of the tracers haw? a mre pronounced effect in this particular case. The small munts of thiocyanates e s t i m t e d indicate the

H i r i a r t , G. (1980) "P?a&a de i n t e r f e r n c i a sector lkjarmniles" , oficina de ~valuacidny Estudios, mmisi6n Federal de Electricidad, Septenber 1980.

-

Home, R. (1981) 'Teotherrral &injection Eqerience in Japan", Society of PetrPleum Engineers of AIM3, paper SPE 9925, pp 423-433. McCabe, W.

-45-

, Manning,

M.,

and Barry, B. (1980)

Tracers Tests, Wairakei", G e o t k n m l Circular W J M d : 2, I n s t i t u t e of Nwlear Sciences INS-R-275, CGIR, New Zealand. w.8 Manning, MI and Barry B. (1981) "Tracer Tests, B r o a d l a n d s 1980", Geothermal Circular W m C 3, Institute of Nuclear Sciences INS-R-288, S I R , New Zealand.

-8

. Lends A., and Zuber,A. (1970) Y!racer Dispersion in W u n d w a t e r WmtS., in Is~topeHyWlogy 1970,

-8

V i e n n a pp 619-641.

. mpz, J. M. and Faplos, L. (1980) "Estudios QuLRiax del z!gul separada de 10s pzos produr tores del canpo geOt&mim de IBS Azufres, Michoacdn", -ria h. h m i 6 n ~acionalde Ceotecnia y Geotermia.

. !knplos, L. and Garcfa, G. (1981) "Pruebas de inyecci6n de agwi separada en el M d d o de Teja-

canpo GeoGnnia~de IDS a s , M i c b a C h 8 bExico", WE Divisidn del E j e ~ e o m l cdnico, A p r i l 1981. mniles,

. Tester, J. B i v i n s , R., and Potter, R (1979) "Intemll Traar Analysis of a Hydraulically F'ractured Granitic G e o t h d €?se.rmir", Society of Petroleun Engineers of ADE, paper SPE 8270.

. Wagner, 0. (1977) *'The use of Tracers in Diagnosing Intenel1 W s e r w i r Heterogsreitiesf i e l d Fesults", Journal of petmlem Technology, NOV. 1977, pp 1410-1416.

-46-


ANALYSIS OF FLOW DATA FROM SEVERAL BACA WELLS

T. D. Riney and S. K. Garg

Systems, Science and Software (S3) P. 0. Box 1620 La J o l l a , C a l i f o r n i a 92038

.

Abstract Analyses a r e presented of t h e downhole pressure b u i l d u p d a t a f o r w e l l s l o c a t e d i n t h e Redondo Creek area o f t h e Baca Geothermal F i e l d . The downhole d r i l l i n g i n f o r m a t i o n and p r e s s u r e / temperature surveys a r e f i r s t i n t e r p r e t e d t o l o c a t e zones a t which f l u i d e n t e r s t h e w e l l b o r e f r o m t h e f r a c t u r e d f o r m a t i o n and t o e s t i m a t e t h e i n i t i a l r e s e r v o i r temperature and pressure i n t h e s e zones. Interpretation o f the b u i l d u p d a t a f o r each w e l l considers w e l l bore e f f e c t s , t h e CO2 c o n t e n t o f t h e f l u i d and d i f f e r e n t i a t e s between t h e single- phase and two-phase p o r t i o n s o f t h e data. D i f f e r e n t s t r a i g h t - l i n e approximations t o t h e two p o r t i o n s o f t h e d a t a on t h e Horner p l o t f o r a f l o w t e s t y i e l d corresponding estimates f o r t h e s i n g l e and two-phase m o b i l i t i e s . Estimates f o r t h e f o r m a t i o n k h a r e made f o r the wells.

INGLE - PHASE

Temperature,

F i g u r e 1.

I n t r o d u c t i o n The Baca Geothermal F i e l d i s l o c a t e d i n t h e V a l l e s Caldera i n n o r t h central New Mexico, 60 m i l e s north of Albuquerque, and about 35 m i l e s northwest o f Santa Fe. A n a l y s i s of t h e downhole d a t a from w e l l s d r i l l e d i n t h e Redondo Creek area o f tne f i e l d indicates t h a t the bulk o f the formation p e r m e a b i l i t y i s i n a f r a c t u r e n e t work. Consequently, t h e performance o f a w e l l depends l a r g e l y upon whether i t i n t e r s e c t s one o r more f r a c t u r e s , how l a r g e each i n t e r s e c t e d f r a c t u r e i s and t h e degree t o which i t i s connected t o t h e r e s t o f t h e network. The r e s e r v o i r pressures i d e n t i f i e d from t h e downhole a n a l y s i s d e f i n e a v e r t i c a l g r a d i e n t o f 0.348 p s i / f t .

GAS

'C

Phase diagram f o r water w i t h CO2 mass fraction a=0.01. S a t u r a t i o n curve f o r pure water (a=O) i s dashed.

f l a s h i n g i n t h e f o r m a t i o n upon p r o d u c t i o n a r e v e r y s e n s i t i v e t o t h e CO2 c o n t e n t o f the f l u i d . The S3 r e s e r v o i r s i m u l a t o r CHARGR was r e c e n t l y employed i n a s e r i e s o f r a d i a l f l o w c a l c u l a t i o n s t o i n v e s t i g a t e t h e response of i n i t i a l l y single- phase and i n i t i a l l y twopnase r e s e r v o i r s undergoing p r o d u c t i o n o r i n j e c t i o n f r o m a s i n g l e w e l l (Garg and F i g u r e 2 shows t h e P r i t c h e t t [1980]). Horner p l o t f o r t h e simulated b u i l d u p h i s t o r y o f an i n i t i a l l y single- phase r e s e r v o i r t h a t undergoes f l a s h i n g upon p r o d u c t i o n . The i n i t i a l b u i l d u p behavior ( w e l l b o r e s t o r age e f f e c t s were n o t s i m u l a t e d ) i s governed by t h e two-phase r e g i o n o f t h e r e s e r v o i r ; t h e s l o p e on t h e Horner p l o t o f t h e c u r v e y i e l d s a value f o r t h e t o t a l kinematic visc o s i t y v t which i s c h a r a c t e r i s t i c o f t h e two-phase r e g i o n c r e a t e d d u r i n g drawdown. The d e t a i l e d c a l c u l a t i o n s show t h a t t h e pressure b u i l d u p i s accompanied by t h e propagation o f a condensation f r o n t , o r i g i n a t i n g a t the well, i n t o the formation; the condensation f r o n t e v e n t u a l l y e n g u l f s t h e

Chemical analyses o f t h e discharge f l u i d s from Baca w e l l s i n d i c a t e t h a t t h e r e s e r v o i r f l u i d i s o f low s a l i n i t y (5 8,000 ppm) w i t h an incondensible gas c o n t e n t ( p r i n c i p a l l y COz) of about 0.4 t o 1.5 p e r c e n t by mass ( a = 0.004 t o 0.015). The e f f e c t o f t h e incondensible gas c o n t e n t on t h e f l u i d s t a t e may be examined u s i n g an equation o f s t a t e f o r a m i x t u r e o f pure water and carbon d i o x i d e ( P r i t c h e t t , e t a l . C1981-J). Figure 1 i l l u s t r a t e s t h e e f E c t T n p-T space f o r a = 0.01. The probable e x t e n t o f t h e i n i t i a l two-phase r e g i o n i n t h e Baca r e s e r v o i r and t h e e x t e n t t o which t h e w e l l s w i l l induce

- 4 7-

I 8.8

1

'

8.7

a" 2.

8.6

6 - 4 TEMPERATURE SURVEYS

?1 8.5

ie

a

8.4

8.3 8.2

2 294 I

IO

IO'

200

0

IO'

Temperature (OF)

(?+At)/A?

F i g u r e 3. F i g u r e 2.

.

Pressure and temperature b u i l d u p d a t a f o r Case 8-1 simulated w e l l t e s t (Garg and P r i t c h e t t [1980]).

e n t i r e two-phase r e g i o n a f t e r which t h e b u i l d u p behavior i s e s s e n t i a l l y t h a t o f a The two-phase b u i l d u p single- phase f l u i d . extends over a f u l l l o g c y c l e ; t h e slope o f t h i s s t r a i g h t l i n e i m p l i e s a kinematic v i s c o s i t y which i s i n f a i r agreement w i t h t h e The single- phase a c t u a l two-phase value. b u i l d u p i n t h i s case extends over l e s s than one- half cycle; t h e slope o f t h i s s t r a i g h t l i n e i m p l i e s a k i n e m a t i c v i s c o s i t y which i s about 40 p e r c e n t l a r g e r than t h e a c t u a l single- phase value. T h i s example, and o t h e r cases t r e a t e d , i l l u s t r a t e t h e importance o f selecting the correct s t r a i g h t line. We have used t h e r e s u l t s o f t h e numerical s i m u l a t i o n s t o p r o v i d e guidance i n o u r i n t e r p r e t a t i o n o f the f l o w data from several Baca w e l l s . The i n t e r p r e t a t i o n c o n s i d e r s t h e e f f e c t s o f t h e CO2 c o n t e n t and f r a c t u r e permeability o f the reservoir. The a n a l y s i s a l s o accounts f o r t h e f a c t t h a t t h e pressureltemperature gages are downhole u s u a l l y l o c a t e d s e v e r a l hundred f e e t f r o m t h e p r i m a r y p r o d u c t i o n zone. I n the followi n g we w i l l p r e s e n t r e p r e s e n t a t i v e b u i l d u p analyses o f d a t a f r o m two Baca w e l l s . Well Baca No. 4 F i g u r e s 3 and 4 p r e s e n t s e l e c t e d temperature and pressure p r o f i l e s Survey S8 was taken recorded i n w e l l 8-4. p r i o r t o any p r o d u c t i o n t e s t i n g , b u t t h e w e l l had been deepened f r o m 5048 f t (4301 ASL) t o 6378 f t (2987 ASL) f i v e weeks e a r l i e r . Temperature survey S 1 1 was made 17 days a f t e r a 9-day p r o d u c t i o n t e s t and i s i n c l o s e agreement w i t h 58 f o r depths l e s s t h a n For v e r y l o n g s h u t i n t i m e s t h e 5000 ft. measured temperatures ( p r o f i l e s S24 and S40) are much h i g h e r than f o r t h e s h o r t e r s h u t i n t i m e s ( p r o f i l e s S8 and S11) except f o r c l o s e agreement a t t h e bottom o f t h e hole. This i s a t t r i b u t e d t o the i n f l u x o f hot f l u i d from a minor e n t r y near t h e bottom o f t h e well. Survey S16 was recorded two days a f t e r s h u t i n f o l l o w i n g a 64-day drawdown

-4a-

e

=

8-4 temperature surveys before, d u r i n g and a f t e r Flow Test No. 2.

o

8 .D -

-

s2ooo

M

S8 6 / 8 / 7 3

SI6 11/15/73 '-1S24 10/17/74 S32 9/15/75

m O

ar

Deepened 5 weeks Shut 2 days Shut Ilmonths shut 22 months

z0"

oltcr Flow Test

U

e

I6000

r"

m

0

Pressure

F i g u r e 4.

2Ooo ( psig)

8-4 pressure surveys before, d u r i n g and a f t e r Flow Test No. 2.

period. The p r i m a r y p r o d u c t i o n i s appare n t l y located a t 4800 f t (4546 ASL); t h e r e s e r v o i r r o c k i s c o o l e d by t h e f l a s h i n g o f t h e f l u i d w i t h i n t h e p r o d u c t i o n zone. The (S16 in corresponding pressure p r o f i1e F i g u r e 4) shows t h a t t h e f l u i d i n t h e w e l l bore two days a f t e r s h u t i n i s l i q u i d below 2100 ft. The long- term s t a b l e p r o f i l e s (S24 and S32) c r o s s t h e e a r l y t i m e p r o f i l e (S8) 4500-5000 ft. Pressure e q u i l i b r i u m at between t h e w e l l b o r e and r e s e r v o i r f l u i d i s maintained a t t h e zone o f p r i m a r y permeability 4800 f t .

-

-

-

I n summary, t h e zone o f p r i m a r y p e r m e a b i l i t y 4800 f t (4546 ASL) i n 8-4 i s l o c a t e d a t w i t h minor e n t r i e s l o c a t e d near t h e w e l l bottom and a t shallower depths noted d u r i n g d r i l l i n g . The i n i t i a l temperature and pressure a t 4800 f t a r e estimated t o be 1170 p s i g (- p ( a ) = 81.4 b a r s ) * and 500'F (260'C).

-

-

Although 8-4 has been f l o w t e s t e d t h r e e times, downhole measurements are avai 1a b l e o n l y f o r t h e b u i l d u p p e r i o d f o l l o w i n g Flow During t h e Test No. 2 (9/10/73-11/13/73). 64-day drawdown p o r t i o n o f t h i s t e s t t h e f l u i d flowed t o a separator vessel and t h e

steam and water phases were measured individually. A f t e r 50 days, t h e f l o w r a t e s t a b i l i z e d w i t h a wellhead pressure o f 120 p s i g and a separator pressure o f 113 p s i g A t separator c o n d i t i o n s t h e ( H a r t z [1976]). steam f r a c t i o n and t o t a l f l u i d e n t h a l p y were r e p o r t e d t o be 27.5 p e r c e n t and h t = 556.1 BTU/lbm. The mass f l o w r a t e and t h e d u r a t i o n o f Flow Test No. 2 are taken as M = 172,500 l b m / h r t = 1538 hours.

- 21.73

t h e p r e s s u r e r e q u i r e d t o ensure t h a t t h e 464°F (240'C) w e l l b o r e f l u i d be single- phase Corl i q u i d i s Pmax = 812 p s i a (56 b a r s ) . respondingly, t h e recorded s h u t i n pressure 6350 f t (3000 ASL) must s a t i s f y t h e at i n e q u a l i t y pws > 812 + 0.35 (4546-3000) = 1353 p s i a = 1343 p s i g

-

-

43 minutes a f t e r T h i s value i s a t t a i n e d a t s h u t i n ; h e r e a f t e r t h e r e i s a c o n s t a n t hydros t a t i c head o f 541 p s i between t h e gage pressure and t h e corresponding pressures i n t h e p r o d u c t i o n zone. Only t h e single- phase p o r t i o n o f t h e Horner p l o t ( F i g u r e 5 ) may be used i n t h i s case ( a = 0.01) i n e v a l u a t i n g f o r m a t i o n p r o p e r t i e s o f t h e p r o d u c t i o n zone.

kg/s

F i g u r e 5 shows a semi- log p l o t of t h e b u i l d u p d a t a recorded by Union a t a depth o f 6350 f t (3000 ASL). Since t h e measurements

-

i f t h e C02 c o n t e n t were 1 p e r c e n t then f r o m F i g u r e 1 we see t h a t t h e i n j t i a l r e s e r v o i r c o n d i t i o n s (81.4 bars, 260 C ) would correspond t o single- phase liquid. During drawdown t h e f o r m a t i o n f l u i d f l a s h e s i n t h e v i c i n i t y o f t h e w e l l b o r e and t h e f l a s h f r o n t propagates l a t e r a l l y i n t o t h e producing zone. A f t e r s h u t i n , condensat i o n commences b u t a two-phase r e g i o n created d u r i n g drawdown would n o t r e t u r n t o the pressure single- phase l i q u i d u n t i l b u i l d s up t o t h e value o f Pma associated 260'C, i.e., Pmax = 61 bars (972 with psia). The corresponding pressure a t t h e r e c o r d i n g depth o f 6350 f t (3000 ASL) i s given by pws = 972 + 0.35 (4546-3000) = 1513 p s i a = 1503 p s i g . T h i s value i s c l o s e t o t h e t r a n s i t i o n pressure (- 1575 p s i g ) between t h e two-phase and single- phase beh a v i o r shown i n F i g u r e 5.

Further, (a =

k

BACA NO PRESSURE BUILDUP AFTER FLOW TEST NO 2 9/10/73 - 11/13/73

1400

-

PRESSURE

m, = 1340 pIi/cycIc

450

600 IO0

IO'

IO'

IO'

IO'

-

d

( t +At)/At

F i g u r e 5.

8-4 pressure and temperature b u i l d u p f o l l o w i n g Flow Test No. 2.

were made so f a r below t h e p r i m a r y product i o n zone a t 4800 f t (4546 ASL), we must account f o r t h e pressure d i f f e r e n c e i n t h e d a t a in t e r p r e t a t ion.

We n o t e i n passing t h a t i f we had ignored t n e presence o f C02 and assumed t h e f l u i d t o be pure water ( a o = 0), we would comp u t e t h a t t h e producing zone a t 4800 f t would recover f r o m two-phase t o single- phase water when t h e pressure b u i l d s up t o pws = 1212 psig. T h i s v a l u e i s c o n s i d e r a b l y below t h e observed t r a n s i t i o n pressure (- 1575 psig).

During drawdown t h e w e l l b o r e i s f i l l e d w i t h two-phase f l u i d which e n t e r s from t h e p r i mary p r o d u c t i o n zone. The r e s e r v o i r r o c k i s cooled by t h e f l a s h i n g o f t h e f l u i d w i t h i n t h e p r o d u c t i o n zone. A f t e r s h u t i n t h e tempe r a t u r e w i t h i n t h i s zone recovers along w i t h u n t i ' l condensation occurs; it pressure subsequently recovers slowly, p r i m a r i l y by conduction. From t h e temperature survey made on 11/15/73 (S-16) we e s t i m a t e t h e temperature i n t h e w e l l b o r e a t 4800 f t (4546 ASL] a f t e r condensation i s i n i t i a t e d t o be 464 F (240'C).

Because o f t h e u n c e r t a i n t y i n t h e values o f and t h e i n i t i a l temperature i n t h e it i s of interest t o p r o d u c t i o n zone, determine t h e values of these parameters to which correspond to the two-phase single- phase t r a n s i t i o n pressure i n d i c a t e d by t h e Horner p l o t . The equation- of- state f o r COz/water m i x t u r e s ( P r i t c h e t t , e t a l . [19811) has been used t o determine-those p o i n t s ( a o , To) which correspond t o a pressure i n t h e p r o d u c t i o n zone o f Pmax = 1575 - 541 = 1034 p s i g = 1044 p s i a (72 bars). The estimated i n i t i a l temperature (TO 500'F) corresponds t o t h e h i g h e r measured value o f CO2 i n t h e discharge (ao = 0.011) whereas t h e lower fluid measured v a l u e (ao = 0.0085) c o r r e y o n d s t o an i n t i a l temperature o f To 512 F (267'C). These values are w i t h i n the u n c e r t a i n t i e s o f t h e measurements.

a

-

The CO2 c o n t e n t o f t h e d i s c h a r g e f l u i d from 8-4 has been measured t o be 0.85-1.1 p e r c e n t ( a = 0.0085-0.011). Although t h e i n - s i t u v a l u e may be d i f f e r e n t ( P r i t c h e t t , et a l . [1981]), we assume f o r t h e moment t h a t a = 0.01. Then according t o F i g u r e 1,

*

0.01),

-

-

Here and i n t h e sequel we add 10 p s i (0.7 b a r s ) t o t h e gage pressure t o c o r r e c t f o r t n e atmospheric pressure a t 4 0 0 0 ASL.

-49-

-

E

The slope m2 = 5 0 ' p s i l c y c l e = 3.44 x l o 5 P a / c y c l e i n t h e single- phase p o r t i o n o f t h e Horner p l o t r e f l e c t s t h e b u i l d u p behavior i n t h e p r o d u c t i o n zone and extends over two f u l l l o g c y c l e s . I t can, t h e r e f o r e , be used t o estimate the kinematic mobility- thickness product f r o m t h e r e l a t i o n

$2000 0-

m3

=

e

r:-

5050 md- ft.

s

m

-6

a 5000 f

-a 0

-

8

I

8 - 2 0 TEMPERATURE SURVEYS AFTER FLOW TEST NO. 4

-

S3 S5 SI0 SI2

1/6/81 1/8/81 2/6/81 5/7/81

6-20 pressure surveys Flow Test No. 4.

-

Time t i s c a l c u l a t e d by d i v i d i n g the cumulat i v e f l u i d mass produced d u r i n g t h e 104-day lbm) by t h e Flow Test No. 4 (148,830,000 t o t a l mass f l o w r a t e averaged over t h e l a s t t e n days o f t h e p r o d u c t i o n . F i g u r e 8 p r e s e n t s a semi- log p l o t o f t h e pressure b u i l d u p a t a depth o f 4500 f t (4700 ASL) where most o f t h e downhole r e c o r d i n g s were made. Since t h e p r i m a r y p r o d u c t i o n i s

8,

BACA NO 20 PRESSURE BUILDUP AFTER FLOW TEST NQ 4 Recorded (Gage at 4500 It ) Corrected (Gage 01 4ooo It 1 ---Cwreclad (Gage ot 5OoOlt) MPTH= 4500 11 m2=200psilcycle o

pdcycle

F i g u r e 6.

7.06 k g / s

t = 2,653 hours ( e q u i v a l e n t p r o d u c t i o n t i m e )

Shut-3hours Shut-2days Shut-l month Shut-4months

Temperoture

following

M = 56,100 l b m / h r (averaged over l a s t 10 days)

\

1000-

-

Four f l o w t e s t s have been performed on 6-20 b u t o n l y Flow Test No. 4 (9/24/80-1/6/81) was o f s u f f i c i e n t d u r a t i o n (104 days) and w i t h s u f f i c i e n t pressure t r a n s i e n t measurements t o warrant a n a l y s i s . During t h e f l o w t e s t t h e w e l l was flowed through a v e r t i c a l separator. The wellhead pressure and t o t a l f l o w r a t e s v a r i e d over t h e t e s t p e r i o d b u t s t a b i l i z e d over t h e l a s t two months. For t h e purposes o f a n a l y s i s we use

4c

57 1/13/81 Shut - 7 doys SI0 2/6/81 Shut Imonth SI2 5/7/81 Shut 4 months

Pressure (psig)

F i g u r e 7.

Well Baca No. 20 The temperature surveys, i n F i g u r e 6 e s p e c i a l l y S5 and S10, i n d i c a t e t h a t r e l a t i v e l y low temperature f l u i d e n t e r s t h e we1 1bore a t approximately 4000 f t (5165 ASL). This implies that the primary p r o d u c t i o n zone i s l o c a t e d a t 4000 f t and the f l u i d entering the wellbore a t t h i s , depth d u r i n g b u i l d u p subsequent t o Flow Test No. 4 has been c o o l e d by f l a s h i n g w i t h i n t h e formation. Below 5000 f t (4237 ASL) t h e r e appears t o be c o n d u c t i v e h e a t i n g o f t h e w e l l b o r e f l u i d (see S5 and S10). Y

-1

f

To e s t i m a t e t h e f o r m a t i o n kh p r o d u c t we need an approximate v a l u e f o r t h e t o t a l k i n e m a t i c v i s c o s i t y o f the reservoir f l u i d during the single- phase p o r t i o n o f t h e b u i l d u p r e sponse. We use t h e v i s c o s i t y o f l i q u i d water a t t h e i n i t i a l c o n d i t i o n s o f t h e producing zone (81.4 bars, 26OoC), v t = m 2 / s and compute vP = 1.315 x k h = 1.52 x

o

g

(OF)

8-20 temperature surveys f o l l o w i n g Flow Test No. 4.

200 IO0

The i n i t i a l pressure a t 4000 f t i s e s t i m a t e d f r o m S2 ( n o t shown) and S12 ( F i g u r e 7) t o be 975 p s i g ( p ( a ) = 67.9 bars). Since no s t a b l e temperature p r o f i l e i s a v a i l a b l e , we e x t r a p o l a t e S12 from 3500 f t t o e s t i m a t e t h e i n i t i a l temperature a t 4000 f t t o be 485-515'F (252-268'C). From S3 t h e temperat u r e i n t h e p r i m a r y p r o d u c t i o n zone a t 4000 f t (5165 ASL) i s estimated t o be reduced t o 432'F by t h e i n - f o r m a t i o n f l a s h i n g .

-

F i g u r e 8.

-

TEMRRATURE

IO'

---___ IO2 IO' ( t +AI )/At

10'

10'

6-20 pressure and temperature b u i l d u p f o l l o w i n g Flow Test No. 4.

a t 4000 f t and t h e b u l k o f t h e pressure d a t a were recorded a t 4500 ft, i t i s necessary t o account f o r t h e pressure d i f f e r e n c e i n i n t e r p r e t i ng t h e data. The pressure p r o f i l e s i n F i g u r e 7 show t h a t t h e w e l l b o r e f l u i d i s

-

-50-

two-phase d u r i n g t h e b u i l d u p period. The f l u i d below 4500 f t i s two-phase a t t h r e e hours ( S 3 ) and two days ( S 5 ) a f t e r s h u t i n . A f t e r seven days (57) t h e f l u i d below 4500 f t i s single- phase l i q u i d , t h e f l u i d above 4000 f t i s single- phase gas and t h e r e appears t o be a two-phase s e c t i o n a t 4000-4500 f t . A f t e r one month t h e r e i s l i q u i d below 3000 f t w i t h a gas cap above t h i s depth.

On t h e o t h e r hand, i f we assume t h e change i n slope corresponds t o a t r a n s i t i o n f r o m two-phase t o single- phase behavior d u r i n g buildup, then t h e equation- of- state f o r C02lwater m i x t u r e s may be used t o determine t h e s e t o f p e r m i s s i b l e p o i n t s (To, ao) f o r which Pmax = 62.8 b a r s (910 p s i a = 900 p s i g ) . A value o f To = 252'C would i m p l y a. = 0.01 whereas a v a l u e o f To = 268'C would i m p l y a0 = 0.003.

Since t h e w e l l b o r e pressure a t t h e gage depth must be c o r r e c t e d by d i f f e r e n t amounts at different buildup times, we have r e p l o t t e d t h e Horner p l o t i n F i g u r e 9. The d a t a p o i n t s a t 4000 f t (5165 ASL) recorded d u r i n g p r o f i l e surveys a r e i n d i c a t e d as are t h e e s t i m a t e s made f r o m surveys i n which r e c o r d i n g s were o n l y made a t 4500 f t (4700 ASL). In t h i s case t h e estimates f o r t h e p r i m a r y p r o d u c t i o n depth (4000 f t ) c o n t a i n c o r r e c t i o n s which account f o r changes i n t h e w e l l b o r e s t a t e w i t h changes i n b u i l d u p time. From t h e c o r r e c t e d p l o t ( F i g u r e 9) we see t h a t t h e t r a n s i t i o n f r o m slope mi t o m2 i s a c t u a l l y completed a t pws > 900 p s i g r a t h e r than t h e much l a r g e r apparent v a l u e i n f e r r e d f r o m F i g u r e 8.

Because o f t h e u n c e r t a i n t y i n a. and To we cannot e s t a b l i s h i f t h e p r i m a r y prod u c t i o n zone is initially single- phase P = 67.9 bars, To = l i q u i d (e.g., 252'C, a. = 0.017 o r i n i t i a l l y two-phase (e.g., P = 67.9 bars, To = 252'C, . a = 0.0147f. In e i t h e r case, however, t h e slope m i i n F i g u r e 9 approximates t h e two-phase p o r t i o n o f t h e pressure buildup f o r a f u l l l o g c y c l e and t h i s p o r t i o n o f t h e d a t a can be used t o i n f e r formation properties.

-

From t h e v a l u e o f mi = x l o 5 P a l c y c l e ) we corresponding v a l u e o f m a t i c mobil i t y - t h i c k n e s s kh _

BACA NO POPRESSURE BUILDUP AFTER FLOW TEST NO 4

F i g u r e 9.

't

-1

1.15

265 p s i l c y c l e (18.3 can c a l c u l a t e t h e t h e two-phase k i n e product

1.15 (7.06)

-

= 2n(18.3

=

7.06

10-7 ms

x lo5)

To e s t i m a t e t h e f o r m a t i o n kh p r o d u c t we f i r s t n o t e t h a t d u r i n g t h e l a s t t e n days o f Flow Test No. 4 t h e wellhead pressure and e f f e c t i v e ( t o t a l ) e n t h a l p y o f t h e produced WHP = 116 f l u i d were e s s e n t i a l l y constant: We have used psig; h t = 794.3 BTUllbm. these values, and an i s e n t h a l p i c model f o r two-phase f l o w i n t h e w e l l b o r e t h a t assumes no s l i p p a g e between t h e l i q u i d and gas components, t o e s t i m a t e t h e downhole f l o w i n g c o n d i t i o n s d u r i n g t h e f i n a l t e n days o f drawdown. The c a l c u l a t e d pressure a t 4000 f t (5165 ASL) i s pwf = 180 p s i g ( p ( a ) = 13.1 b a r s ) and t h e corresponding downhole flowing kinematic v i s c o s i t i e s o f the l i q u i d and gas components and t h e mass r a t i o o f t h e m i x t u r e e n t e r i n g t h e w e l l b o r e are as f o l l o w s :

Corrected 6-20 pressure b u i l d u p f o l l o w i n g Flow Test No. 4.

C02 mass f r a c t i o n o f 1.47 p e r c e n t has been measured i n t h e discharge f l u i d f r o m w e l l 6-20. T h i s i s h i g h e r than t h a t measured i n a l l b u t one o t h e r w e l l and may be h i g n e r o r lower than t h e i n - s i t u v a l u e I f we assume ( P r i t c h e t t , e t a l . [1981]). 2 c o n t e n t i n the primary t h a t the m p r o d u c t i o n zone i s i n f a c t t h e same as measured i n t h e discharge f l u i d , then t h e zone a t 4000 f t would be i n i t i a l l y two-phase f o r any temperature w i t h i n our range of According t o u n c e r t a i n i t y (- 252-268'C). the equation- of- state for C02lwater mixtures, a t 252'C t h e i n i t i a l i n s i t u gas s a t u r a t i o n would be S = 0.077 whereas t h e would i m p l y temperature o f 268'? 0.409. I n t h i s case, t h e transitions%; slope m i t o slope m2 i n F i g u r e 9 would merely reflect the recovery of the p r o d u c t i o n zone t o i t s i n i t i a l c o n d i t i o n s .

A

VQ

= 1.59 x

m2/s

v

= 2.29 x

m2/s

9

m lm 9

Q

= 1.12

The l i n e a r i z e d equations i n t h e Appendix can From Eq. now be used t o e s t i m a t e v t . (A-5) we compute k = 1.12

122.599 = 16.2

rQ I f we assume t h a t t h e f l o w w i t h i n t h e f o r mation i s p r i m a r i l y through a f r a c t u r e

-51-

network then f r o m Eq. (A-6) we can c a l c u l a t e the individual r e l a t i v e permeabilities k r = 0.942 and krp= 0.058. From Eq. (A-2) w% estimate the t o t a l kinematic v i s c o s i t y d u r i n g t h e l a s t t e n days o f drawdown.

"t =

(

0.058

Grant, M. A. and S . K. Garg (1981), " I n t e r p r e t a t i o n o f Downhole Data f r o m t h e Baca Geothermal Field," Topical Report No. DOE/ET/27163-12 Prepared by Systems, Science and Software Under Baca Geothermal Demonstration P r o j e c t , May. Hartz, J. D. (1976), "Geothermal Reservoir E v a l u a t i o n o f t h e Redondo Creek Area, Union O i l Sandoval County, New Mexico," Company of California, Santa Rosa, C a l i f o r n i a , September 9 and November 23.

+

1.59 x lo-'

2.29 x 10-

P r i t c h e t t , J. W., M. H. Rice and T. D. Riney (1980), "Equation-of-State f o r Water-Carbon D i o x i d e Mixtures: I m p l i c a t i o n s f o r Baca Topical Report No. Reservoir, I' DOE/ET/27163-8, Prepared by Systems, Science and Software Under Baca Geothermal Demonstration P r o j e c t , October.

We assume t h a t t h e k i n e m a t i c v i s c o s i t y d u r i n g t h e two-phase p o r t i o n o f t h e b u i l d u p i s approximated by t h i s value. An e s t i m a t e o f t h e f o r m a t i o n kh product f r o m t h e two-phase p o r t i o n o f t h e b u i l d u p d a t a can now be made. kh = (1.29 x

(7.06 x

A

endix

Several

authors (e.g., Garg and have obtained approximate analytical solutions for a geothermal r e s e r v o i r undergoing two-phase p r o d u c t i o n a t a constant rate. I n these s t u d i e s t h e balance equations f o r r a d i a l two-phase f l o w i n porous media are l i n e a r i z e d by assuming t h a t t h e t o t a l and t h e component k i n e m a t i c m o b i l i t i e s are r e l a t e d as f o l l o w s :

? rJ i t= c i ei-t€[1980]) = 90.8 x

m3

=

3030 md- ft

Discussion The o b j e c t i v e o f t h i s paper was t o i l l u s t r a t e an a n a l y s i s procedure which i s based on t h e s y n t h e s i s o f f i e l d measurements and t h e o r e t i c a l r e s u l t s i n i n t e r p r e t i n g geothermal w e l l f l o w data. The i n t e r p r e t a t i o n g i v e n here f o r w e l l 6-4 i s based on t h e single- phase p o r t i o n o f t h e b u i l d u p d a t a and y i e l d s a v a l u e o f kh which i s i n reasonable agreement w i t h t h e v a l u e o f 4207 md- ft estimated by H a r t z [1976] f r o m a convent i o n a l Horner p l o t a n a l y s i s . The i n t e r p r e t a t i o n g i v e n f o r w e l l 6-20, however, i s based on t h e two-phase p o r t i o n o f t h e b u i l d up d a t a and a c o r r e c t i o n o f t h e Horner p l o t t o account f o r t h e f a c t t h a t gage i s l o c a t e d s e v e r a l hundred f e e t f r o m t h e p r i m a r y prod u c t i o n zone. A c o n v e n t i o n a l Horner analys i s would n o t be a p p l i c a b l e .

k

(7)

t

1 't

=

krt +

k k

'Q

rg

vg k

=krL.rg "Q

(A;2)

'g

Given t h e f l o w i n g e n t h a l p y h t o f t h e two-phase m i x t u r e , i t i s a l s o p o s s i b l e t o o b t a i n t h e separate k i n e m a t i c m o b i l i t i e s .

Acknowledgements T h i s work was performed under C o n t r a c t 4514510 w i t h t h e Lawrence Berkeley L a b o r a t o r y as p a r t o f t h e evaluat i o n o f t h e Baca r e s e r v o i r being conducted under a c o o p e r a t i v e agreement between Union O i l Company and t h e Department o f Energy. The a s s i s t a n c e o f Ms. Mary Margaret P i e r c e i s g r a t e f u l l y acknowledged.

(A-3)

The expression

h - m h + (1-m ) hp (A-4) t - 9 9 g r e l a t i n g t h e t o t a l and f l u i d component e n t h a l p i e s may be used w i t h Eqs (A-3) t o write

References Garg, S . K. and J. W. P r i t c h e t t (1980), " Pressure T r a n s i e n t A n a l y s i s f o r Hot Water We1 1s: Some and Two-Phase Geothermal No. Numerical Results,'' Topical Report DOE/ET/27163-5, Prepared by Systems, Science and Software Under Baca Geothermal Demonstration P r o j e c t , October.

Here mg and l-mg a r e t h e gas components o f t h e f l u i d f l o w .

and

liquid

I f we assume t h a t t h e f l o w w i t h i n t h e formation i s p r i m a r i l y through a f r a c t u r e network, t h e n krQ+ krg = 1

-52-


FRACTURE STIMULATION EXPERIMENTS AT THE BACA PROJECT AREA

C. W. Morris

& M. J. Bunyak

Republic Geothermal, Inc. 11823 E. Slauson Avenue Santa Fe Springs, CA 90670 Abstract

Inc. (RGI) and i t s principal subcontractors (Maurer Engineering, Inc. and Vetter Research) have completed seven field experiments. Two experiments have been performed in the low temperature reservoir at Raft River, Idaho (Morris, et al., 1980); two experiments in the moderate temperature reservoir at East Mesa, California; one experiment in the high temperature, vapor-dominated reservoir at The Geysers; and two experiments, reported herein, in the high temperature reservoir at Baca, New Mexico.

The DOE-sponsored Geothermal Reservoir We1 1 Stimulation Program group performed hydraulic fracture treatments on two wells located in Union's Baca Project Area in north-central New Mexico. The treatment in Baca 23 was conducted on March 22, 1981, utilizing a cooling water pre-pad followed by a high viscosity frac fluid carrying a mixture of sintered bauxite and resin-coated sand as the proppant. A nonproductive 231-foot interval from 3,300 feet to 3,531 feet was isolated for the treatment. Post-stimulation surveys and production tests indicated a fracture had been successfully .created; however, the production rates declined to noncommercial levels because of the low formation temperature in the open interval and reduced relative permeability caused by two-phase flow effects in the formation. The second treatment was conducted in Baca 20 on October 5, 1981, again utilizing a cooling water pre-pad followed by a high viscosity frac fluid carrying only sintered bauxite as the proppant. A 240-foot interval from 4,880 feet to 5,120 feet, which was indicated to have produced only a small portion of the well's 56,000 lb/hr total mass flow, was isolated for the job. The temperature in this interval (540°F) gave Baca 20 the distinction of being the hottest well to be fractured in the United States to date. Post-stimulation tests and analyses indicate a fracture was created with a vertical height o f only about 100 feet at the bottom of the open interval. The productivity o f the well is poor, probably because of the low permeability formation surrounding the artificially created fracture. An acid cleanout operation is planned to remove possible damage to the fracture conductivity caused by the calcium carbonate fluid-loss additive. Introduction The U.S. Department of Energy-sponsored Geothermal Reservoir We1 1 Stimulation Program (GRWSP) was initiated in February 1979 to pursue industry interest in geothermal well stimulation work and to develop technical expertise in areas directly related to geothermal well stimulation activities. Republic Geothermal,

-53-

The Baca reservoir lies within the Jenez Crater, Valles Caldera, and is defined by more than twenty wells completed to date in the Redondo Creek area by Union Geothermal Company of New Mexico (Union). The main reservoir, 4,000 to 6,000 feet in thickness, is composed of volcanic tuffs with low permeability and a primary flow system of open fracture channels. In the Redondo Creek area, wells have encountered a high temperature (5OO0F+) liquiddominated reservoir, but several we1 1s have not been of commercial capacity, primarily because of the absence of productive natural fractures at the wellbore. It is believed that hydraulic fracture treatments can create the fractures required to make these wells commercial and that such a well stimulation may be an attractive alternative to redrilling. The relatively large amount of reservoir data available and the high reservoir temperature made this field a good candidate for field experiments in the evaluation of geothermal stimulation techniques, fracture fluids, proppants, and mechanical equipment. After considering several candidate wells, RGI and Union agreed that Baca 23 and subsequently Baca 20 were the best sites for the fracture treatments. These wells, shown in Figures 1 and 5, were selected because: (1) they were noncommercial or a poor producer; (2) the fracture system is present in the area as proven by the surrounding wells; (3) the wells could be recompleted to isolate the stimulation interval; (4) observation wells were available within 1,500 feet; (5) the wellsite was large enough for the frac equipment; and (6) in the case of Baca 23 the rig was already on location.

ALORIQIYAL WPLETION

0. RECWTLETEO FOR FRAC JOB

TABLE 1

C. FINAL

WPLEllON

13 W

BACA 23 TREATINQ SCHEDULE PLANNED

ACTUAL

STAGE NO.

SIZE (bbl)

SIZE (bbl)

1

4,OW

3,582

0

-

-

-----

14W

S.31

CYT

38011

-

WIOPPANT (LBIGAL) (SIZE)

FLUlO

PROOUCEOWATERWITH FLUlO LOSS ADOlTlVE (FLA)

2

500

502

0

3

500

502

2 100-MESH POLYMER GEL WITH FLA

4

500

526

0

5

900

905

1

6

1.000

1.000

2

7 8

600

582

3

58

62

0

SAND UGV

IBPl

-

-

8,058

7,641

-

&# ,$kt -

POLYMERGELWITHFLA POLYMERGELWITHFLA POLYMERGEL POLYMERGEL POLYMERGEL PRODUCE0 WATER

U FIGURE 1 BACA 23 COMPLETION DETAILS.

The experiments were cost-shared by Union and the GRWSP. Union paid the cost of rig mob lization and demobilization plus the cost of recompleting the wells for the treatments. The GRWSP paid for the stimulation treatment and other directly related costs totaling about $450,000 for Baca 23 and about $581,000 for Baca 20.

I 2mo-

Baca 23

Well Recompletion - Baca 23 was originally completed as shown in Figure 1A with a 9-5/8" liner cemented at 3,057 feet and 8-3/4" open hole to 5,700 feet. The well was flow tested and at that time would not sustain flow. An interval from 3,300 feet to 3 , 5 0 0 ~feet in the well was selected for fracture stimulation. Good production had previously been encountered near this depth approximately 200 feet away in Baca 10. The interval is now cemented off behind casing in Baca 10. Fracturing a more shallow interval, immediately below the shoe of the 9-5/8" casing, was considered to have a substantial risk of comnunication with lower temperature formations above. The temperature in the zone selected was approximately 450OF.

"

I

I

,

RATE

TIME IYINI

FIGURE 2 BACA 23 FRACTURE STIMULATION PRESSURE PRESSURElRATE HISTORY.

The treatment was pumped through a 4-1/2" tubing frac string with a packer set near the top of the 7" liner as shown in Figure 1B. The frac string was necessary to isolate liner laps in the well\from the treating pressure. Although the job was basically a conventional hydraulic fracture treatment, the high formation temperature (45OOF) dictated special design and materials selection requirements. Therefore, 50 percent of the frac fluid was dedicated to wellbore and fracture pre-cool ing with the final 50 percent of the fluid used to place the proppant. While frac fluid properties are known to degrade rapidly at high temperature, these effects were minimized by pre-cooling, by pumping at high rates (up to 75 BPM), and by limiting the frac interval to 231 feet. Proppants were selected for their insensitivity to the high temperature (Sinclair, et al., 1980). Both resin-coated sand and sintered bauxite were used. Chemical work included compatibility studies of the frac materials with the formation fluids and the use of chemical tracers to monitor fluid returns.

Since the top of the selected interval was deeper than the existing 9-5/8" liner, a 7 " liner was cemented t o a depth of 3,300 feet to exclude the interval above. The lower portion of the hole was sanded back to 3,800 feet and plugged with cement to 3,531 feet to contain the treatment in the desired interval. This recompletion is shown in Figure 1B. The treatment interval was totally nonproductive after being isolated for the stimulation treatmen t

.

Treatment Summary - A hydraulic fracture treatment was performed on the well consisting of 7,641 bbl of fluid and 180,000 lb of 20/40-mesh proppant pumped in eight stages. The stages are detailed in Table 1 and the pressure/rate history is shown in Figure 2.

The job was separated into eight stages. In Table 1 the eight stages are shown with the -54-

ation in planning and evaluating future fracture treatments.

planned and actual stage sizes. The fluid used for pre-cooling the formation (Stage 1) was produced geothermal water stored in a pit near the location. The job ran short by 418 bbl of pre-pad water because the usable volume of the pit was underestimated. No harmful effects resulted from this short fall, however. Otherwise, the schedule was followed closely. The fluid for Stages 2-7 was a 60 lb/1,000 gal hydroxypropyl guar polymer gel pre-mixed using fresh water. The gel was crosslinked as it was pumped.

Test Results and Analyses - During the fracture treatment. Los Alamos National Laboratory performed a fr-acture mapping experiment using Baca 6 as an observation well. A triaxial geophone system was placed in the well; and using techniques developed for the Hot Dry Rock Project, microseismic activity caused by the fracture job was mapped. The 14 discrete seismic events indicate northeast trending activity in a zone roughly 2,300 feet long, 650 feet wide, and 1,300 feet high. The rock failure, therefore, occurred in a broad zone and suggests the stimulation did not result in the creation of a singular monolithic fracture. These microseismic events would be expected to proceed in advance of any significantly widened, artificially created fracture and would not necessarily define a final propped flow path to the wellbore at Baca 23. Calculations of the theoretical fracture length were made assuming a 300-foot high fracture. The results suggest a fracture wing of 430 to 580 feet in length may have been created, depending on the assumptions utilized for the frac fluid, fluid efficiency, and fracture width.

Finely ground calcium carbonate was selected as a fluid-loss additive (FLA) for Stages 1-4. About 5,700 lb of fine fluid-loss additive were used during the job. A larger fluid-loss additive consisting of 42,000 lb of 100-mesh sand was pumped in Stage 3 to slow leaks into the natural fractures of the formation. Total proppant placed in the formation during the job was 180,000 lb. The original plan was to use a 50/50 mixture of sintered bauxite and resin-coated sand, both 20/40-mesh. The actual proportion of the proppants was 54 percent sintered bauxite and 46 percent resin-coated sand by weight. Actual horsepower required for the job was 6,400 hhp versus the 5,880 hhp estimated by assuming an 80 BPM pump rate and 3,000 psi we1 1 head pressure. Higher than expected frac gradients were responsible for the increase. Frac gradients measured at the beginning, middle, and end of the job were 0.83, 0.92, and 1.175 psi/ft respectively. The buildup in frac gradient is difficult to interpret here, but nonetheless should be noted for considerlw0

I

I

I

1

As discussed above, the 231-foot interval iso-

lated for stimulation was nonproductive prior to the treatment. This indicated that no significant natural fractures intersected the wellbore. Twelve hours after the frac job, a static temperature survey (shown in Figure 3 with a pre-frac survey) was obtained by Denver Research Institute. This survey showed a zone cooled by the frac fluids estimated to be more than 300 feet in height at the wellbore. I

I’

I

I

I

1

9 518” CASING

2917’

3057’

5 112”

IPERF.

LINER

L3u(y TEMPERATURE. OF

FIGURE 3 BACA 23 TEMPERATURE SURVEYS.

-55-

After the post-frac temperature survey was obtained, the frac string was pulled and the well was circulated with aerated water and allowed to flow to be sure that production of proppant into the wellbore would not interfere with subsequent testing. No significant amount of proppant was produced into the wellbore after the frac job. At this time it was determined that the well was worthy of final completion and further testing. A 5-1/2" pre-perforated liner was installed in the treatment interval as shown in Figure 1C.

from other noncommercial wells in the area and with the 6,000 md-ft average reservoir value obtained by Union from interference we1 1 tests (Hartz, 1976). Although the linear flow indicators were weak, the length of the fracture was calculated to be about 300 feet from the pressure versus square root of time analysis. A skin factor of -3.9 was also calculated. The maximum recorded temperature was 342OF which indicated that the near wellbore area had not recovered from the injection of cold f1 uids.

On March 26, 1981, a six-hour production test through drillpipe was performed in which transient, downhole pressure and temperature measurements were obtained. A unique testing method was utilized to overcome the data gathering problems usually associated with flow testing a geothermal well. The procedure was a combination of conventional drillstem test (DST) methods (to eliminate large wellbore storage effects) and gas lift to maintain steady, single-phase flow to the wellbore. The gas lift was provided by injecting nitrogen gas at depth through coil tubing inside the drillpipe. As a result of this procedure, the well flowed at a low, steady rate (about 21,000 lb/hr) and the transient pressure data obtained downhole provided an indication of wellbore storage effects, fracture flow effects, and reservoir transmissivity.

Following the modified DST, a 49-hour flow test was performed to determine the well's productive capacity. The results showed that the we1 1 could produce approximately 120,000 lb/hr total mass flow at a wellhead pressure of 45 psig, although the rate was continuing to decline. The chemical tracer data showed that the frac fluid stages were thoroughly mixed together in the return fluids and the frac polymer had thermally degraded by the end of this test. Union performed a long-term flow test on the well during April-May 1981. A static temperature profile of the well prior to this test showed that the bottom-hole temperature still remained low (401°F). Temperature and pressure surveys run on April 21 (Figure 3) recorded a maximum temperature of 344OF and a maximum pressure of 120 psig at 3,500 feet. Therefore, two-phase flow was occurring in the formation, with the steam fraction estimated at more than 50 percent. This two-phase flow condition, has been observed in other wells in the field.

A conventional Horner analysis (Figure 4) of the pressure buildup data yielded an average reservoir permeability-thickness of 2,500 md-ft. This compares closely with results

kh - 162.6a M

~p

kh = 2,500 MD-FT AVG.

e

10

100

At FIGURE 4 BACA 23 PRESSURE BUILDUP DATA-MARCH 26,1981

-56-

p r o d u c t i o n i n t h e area has been found near t h e bottom o f t h e Bandelier T u f f and because o f i t s h i g h r e s e r v o i r temperature (540°F).

The f o r m a t i o n c o o l i n g seen i n t h e A p r i l 13 temperature survey i s a p p a r e n t l y a r e s u l t o f t h e temperature drop associated w i t h f l a s h i n g i n t h e formation.

-

The h y d r a u l i c f r a c t u r e Treatment Sumnary treatment was accomplished i n t h e eleven stages d e f i n e d i n Table 2. The h i g h f o r m a t i o n temp e r a t u r e (540°F) once a g a i n d i c t a t e d s p e c i a l design and m a t e r i a l s s e l e c t i o n .

Of g r e a t e r concern i s t h e low p r o d u c t i v i t y observed d u r i n g t h i s l a s t t e s t . The mass f l o w ' r a t e had dropped t o 73,000 l b / h r (about 50 p e r c e n t steam) w i t h a wellhead pressure o f 37 p s i g i n May 1981. Since t h e w e l l recovers p r o d u c t i v i t y f o l l o w i n g each s h u t - i n p e r i o d and t h e n e x h i b i t s t h e same d e c l i n e again, t h e cause o f t h e r a t e d e c l i n e i s probably n o t due t o s c a l i n g i n t h e formation. Partial closing o f the f r a c t u r e i s possible, b u t the p r o d u c t i v i t y l o s s i s probably t h e r e s u l t o f r e l a t i v e permea b i l i t y r e d u c t i o n associated w i t h two-phase f l o w e f f e c t s i n t h e formation. The r e l a t i v e l y low f o r m a t i o n temperature i n t h e completion i n t e r v a l a l s o c o n t r i b u t e s t o t h e w e l l ' s poor productivity

TABLE 2 BACA 20 TREATING SCHEDULE Actual Size (bbl)

1.

ZOO0

ZOO0

2.

500

639

3.

5M)

350

FRESH WATER WITH FLA

4.

1500

1400

POLYMER GEL WITH FLA

5.

500

566

6.

500

500

7.

1150

1168

850

-- -

.

Baca 20

-

Baca 20 was o r i g i n a l l y Well Recompletion completed as shown i n F i g u r e 5A w i t h a 9-5/8" l i n e r cemented a t 2,505 f e e t and a 7" s l o t t e d l i n e r hung a t 2,390 f e e t w i t h t h e shoe a t 5,812 f e e t . The 7" s l o t t e d l i n e r was p u l l e d , A.ORIGINAL COMPLETION

8. RECOMPLETED FOR FRACJOE

Proppant

stags No.

Planned Size bbl)

C. FINAL

(Ib/gal)

Size

b

Fluid

FRESH WATER WITH FLUID LOSS ADDITIVE (FLA) 0.39

1.33

100-MESH CaCO (10,530 LB)

100-MESH CaCO (31.530 LB)

FRESH WATER WITH FLA

POLYMER GEL WITH FLA POLYMER GEL WITH FLA

COMPLETION

8. a

-

0.46

16/20-MESH BAUXITE

682

1.85

378

2.77

16/20-MESH BAUXITE 16/20-MESH BAUXITE

12u'

POLYMER GEL POLYMER GEL POLYMER GEL

1416

2

9.

300

450

2.11

12/20-MESH BAUXITE

POLYMER GEL

10.

750

451

4.21

12/20-MESH BAUXITE

POLYMER GEL

11.

-

-

150

8700

w

l8w i l / Y FJ 17 LB NlTH 11YDRILLED 'ERFORATIONS 131' EMENT SAND 5827'

FIGURE 5 BACA 20 COMPLETION DETAILS.

l o s t c i r c u l a t i o n zones cured u s i n g cement plugs, and then a 7" b l a n k l i n e r was cemented i n p l a c e a t 4,880 f e e t i n o r d e r t o i s o l a t e t h e d e s i r e d treatment i n t e r v a l . Since t h e f r a c i n t e r v a l was t o b e f r o m 4,880 f e e t t o 5,120 f e e t , a sand p l u g was p l a c e d f r o m 5,827 f e e t t o t a l depth t o 5,400 f e e t and t h e n capped w i t h cement t o 5,120 f e e t . The recompletion i s T h i s p a r t i c u l a r 240- foot shown i n F i g u r e 58. i n t e r v a l was chosen p r i m a r i l y because t h e b e s t

151

FRESH WATER

8735

The treatment was pumped through a 4-1/2" t u b i n g s t r i n g w i t h a packer s e t a t 2,412 f e e t , j u s t below t h e 7" l i n e r hanger. A 3,000 b b l f r e s h water pre-pad was used t o c o o l t h e w e l l b o r e and f r a c t u r e . The proppant s e l e c t e d was 119,700 l b o f 16/20-mesh s i n t e r e d b a u x i t e , f o l l o w e d b y 119,700 l b o f 12/20-mesh s i n t e r e d bauxite. The proppant was c a r r i e d b y a 60 lb/1,000 g a l hydroxypropyl guar polymer g e l mixed i n f r e s h water. T h i s f l u i d was a new high-pH c r o s s l i n k e d HP guar system having b e t t e r s t a b i l i t y a t h i g h temperature. The g e l was c r o s s l i n k e d as i t was b e i n g pumped. Chemical t r a c e r s were added t o t h e i n j e c t e d f l u i d t o monitor f l u i d r e t u r n s . Approximately 4,200 l b o f 200-mesh calcium carbonate was added i n Stages 1-6 t o a c t as a I n an e f f o r t t o s t o p fluid- loss additive. leakage i n t o t h e smal 1 n a t u r a l f r a c t u r e s , 42,000 l b o f 100-mesh c a l c i u m carbonate was pumped i n Stages 2 and 5 i n c o n c e n t r a t i o n s o f 0.39 ppg and 1.33 ppg, r e s p e c t i v e l y . The 100-mesh m a t e r i a l was i n j e c t e d i n " s l u g s " t o enhance i t s chances o f b r i d g i n g on t h e f r a c tures.

-57-

Test Results and Analysis - During the fracture treatment Los Alamos National Laboratory again performed a fracture mapping experiment using Baca 22 as an observation well. A triaxial geophone system was placed in the well at a depth o f approximately 3,000 feet and the microseismic activity caused by the fracturing job was mapped. A large number of discrete events (45) were recorded during the job, however, the orientation measurement of the tool was lost. Again the activity occurred in a broad zone which was roughly 2,000 feet long, 1,600 feet wide, and 1,700 feet high. Theoretical calculations of the artificially created fracture length would be 340-800 feet in a homogeneous matrix material, depending on the assumptions utilized for the frac fluid, fluid efficiency, and fracture height. These calculations were based primarily on the injected fluid and proppant volumes in a single, vertical fracture.

The majority of the treatment fluid was pumped at approximately 80 BPM. The rate was slowed to 40 BPM in Stage 10 when the proppant concentration was increased to 4.2 lb/gal. In anticipation of frac gradients of 0.9 psi/ft and higher, as seen in Baca 23, a total capacity of 11,000 hhp was made available and connected in the system. However, the actual peak hydraulic horsepower used was only 7,450 hhp because of lower frac gradients. An instantaneous shut-in pressure was measured soon after the treatment was initiated (1,000 psig) and again near the end of the job (1,300 psig), giving frac gradients of 0.63 psi/ft and 0.69 psi/ft, respectively. The pressure/ rate history is shown in Figure 6. Minor variations in the planned pumping schedule occurred during the treatment (Table 2), but all fluids and proppants were injected into the formation and the desired goal of ending the treatment at a relatively high proppant concentration was achieved. The variations occurred: (1) in Stage 7 when only 1/2 lb/gal of proppant was inadvertently added instead of the planned 1 lo/gal; ( 2 ) in Stage 8 when a higher proppant concentration was used to make up for the smaller amount used in Stage 7; and ( 3 ) in Stage 9 where the proppant concentration was increased to 3 lb/gal of the larger proppant instead of the planned 2 lb/gal. In Stage 10 the rate was slowed and the proppant concentration increased to 4.2 lb/gal to achieve a more widely propped fracture. The wellhead pressure and frac gradient were lower than expected, offering reasonable assurance that the proppant would not screen out at the lower rate and higher concentration. 4Ooo

I

1

0

As discussed above, the 240-foot interval was nonproductive prior to the treatment, although there was a small rate of fluid loss during the well completion operations. This indicated that at least one lost circulation zone existed in the wellbore. Approximately 12 hours after the frac job the first of several temperature surveys, as shown in Figure 7, was obtained in the well. These temperature surveys showed a zone cooled by the frac fluids, estimated to be less than 100 feet in height, near the bottom of the open interval. In addition, the zone located behind the 7" liner casing at approximately 4,720 feet also indicated some cooling. This zone was apparently cooled by the workover fluids and possibly by I

I

1

I

I

I

I

I

I

I

I

70

I 80

I

50

1 60

I

40

90

100

110

120

1

I

I

I

I

10

20

I 30

I

TIME, MINUTES FlGURE 6 BACA 20 FRACTURE STIMULATION PRESSURE/RATE HISTORY.

-58-

I

the fracturing fluids; however, the communication between this zone and the open interval (if it exists) appears to be at some distance away from the wellbore. Electric log surveys were run in the open interval following the frac job. No significant new fracture zones (or high porosity) were observed, although several zones did show increased neutron porosity values.

skin factor of -4.9 was also calculated. Numerical simulation of a high conductivity fracture in a low permeability formation supports this interpretation, although the solution is not unique. The maximum recorded temperature during the test was 32OOF and indicated that the near wellbore area had not recovered from the injection of cold fluids. Additional temperature surveys were run in the well following the DST, as shown in Figure 7, which again indicated the fluid was entering (leaving) the wellbore in the lower part of the open interval.

At this time it was determined that the well was worthy of final completion and further testing. A 5-1/2” pre-perforated liner was installed in the treatment interval as shown in Figure 5C. On October 10-11, 1981, a 6-hour production test through drillpipe was performed in the same manner as the orillstem test at Baca 23. A steady rate of about 21,000 lb/hr single-phase flow was maintained to. the wellbore. Transient pressure and temperature data were obtained downhole during the DST. A conventional Horner analyis of the pressure buildup data (Figure 8) yielded an average reservoir permeability-thickness of 1,000 md-ft. Evaluation of these data also indicated small wellbore storage effects and fracture (1 inear) flow near the we1 lbore. Although the indicators o f linear flow were weak, the length o f the fracture was calculated to be about 280 feet from the pressure data (pressure vs square root of time). A

Following the modified DST, a 14-day flow test was performed to determine the well’s productive capacity. The well produced approximately 120,000 lb/hr total mass flow initially, but declined rapidly to a final stabilized rate of approximately 50,000 lb/hr (we1 lhead pressure of 25 psig) under two-phase flow conditions in the formation. Because of the poor performance of the well, it was decided to perform an acid cleanout o f the fracture. As indicated above, calcium carbonate was used as the fluid-loss additive during the hydraulic fracture treatment. This material was used with the expressed intent of performing an acid cleanout should the fracture conductivity show damage. The possibility of

2000

3000

G Y 4000 0

w 0

5ooa

i. i m a 150

200

250

300

350

400

450

500

TEMPERATURE, O F FIGURE 7 BACA 20 TEMPERATURE PROFILES-OCTOBER 1981.

-59-

550

such damage with insoluble fluid-loss additives (e:g., 100-mesh sand) has been a concern in prlor stimulation experiments. Although the pressure data does not indicate that the fracture conductivity has been damaged, it does not preclude the possibility that the calcium carbonate has plugged the natural fractures and flow paths in the formation which intersect the artificial fracture.

low formation temperature in the shallow treatment interval. The productivity of Baca 20 is severely restricted because of the low permeability formation surrounding the artificially created fracture.

4. Although the stimulation treatments did not result in comnercial wells at Baca, the hydraulic fracturing techniques show promise for future stimulation operations and for being a valid alternative to redrilling in other reservoirs.

Conclusions 1.

Large hydraulic fracture treatments were successfully performed on both Baca 23 and Baca 20. Production tests indicated that high conductivity fractures were propped near the wellbore and communication with the reservoir system was established.

References Hartz, J. O., Geothermal Reservoir Evaluation of the Redondo Creek Area, Sandoval County, NM, Union Oil Company Report, September 1976. Morris, C. W . , Verity, R. V., and Sinclair, A. R., "Raft River Well Stimulation Experiments," Geothermal Resources Council, Transactions, Vol. 4, September 1980.

2. The productivities of Baca 23 and Baca 20

have declined to noncomnercial levels since the fracture treatments. The probable cause is relative permeability reduction associated with two-phase flow effects in the formation. 3.

Sinclair, A. R., Pittard, F. J., and Hanold, R. J., "Geothermal We1 1 Stimulation," Geothermal Resources Counci 1, Transactions, Vol. 4, September 1980.

The ability of Baca 23 to produce substantial quantities o f fluid at a high wellhead pressure is limited because o f the

480

I

I t*

At FIGURE 8 BACA 20 DST NO. 2, PRESSURE&-HORNER

-60-

PLOT-OCTOBER 11,1981.

Proceedings Seventh Workshop Geothermal Reservoir Engineering Stanford, December 1981. ' SGP-TR-55.

THE MYKJANES GEOTHERMAL FIELD IN ICELAND: SUBSURFACE EXPLORATION AND WELL DISCHARGE CHARACTERISTICS J. S . Gudmundsson T. Hauksson J. Tomasson

Geothermal Division, Orkustofnun (Iceland Energy Authority) Reykjavik Introduction The exploration and development of the Reykjanes geothermal field dates back .to about 1956 when the first well was drilled. This well was 162 m deep and had a maximum temperature of 185OC. The chloride concentration of the deep brine was about 25% higher than that of ordinary seawater. The well produced 3-4 kg/s of a steam-brine mixture for over 10 years without a noticeable change in chemical composition (Bjornsson et al. 1971). The fact that the fluid produced was of brine origin but not rainwater affected greatly the course of exploration and development of the Reykjanes field. Most high-temperature geothermal fields in Iceland produce fluids of rainwater origin.

extensive output measurements were done on this well, showing both flowrate and enthalpy at various wellhead conditions. The main purpose of the present paper is to report on the long-term discharge test and the more recent output measurements. An important feature of the salt-making in Reykjanes is that the geothermal brine itself will be the source of chemicals, so that the steam-brine mixture produced by the wells will supply both the energy and raw material to the process. The long-term chemical characteristics of the production wells is therefore of great importance. The emphasis of this paper will therefore be on reporting the geochemical nature of the Reykjanes field and the one production well operated for nearly a decade. To provide some background information, the reported subsurface exploration work will be discussed. The extensive surface exploration work of Bjornsson et al. (1970, 1971, 1972) will, however, not be discussed. It is hoped that the present paper may contribute something to the near future drilling and development activities in the Reykjanes field. At the same time it may provide reservoir engineering and other information for those not familiar with geothermal resource developments in Iceland.

The Reykjanes field was investigated extensively in the years 1968-1970 (Bjornsson et al. 1970, 1971, 1972). This was done in relation to plans to produce 250,000 tonneslyear of not only common salt but various other sea-chemicals for export (Linda1 1975). The investigation showed that the field would be suitable for development. However, the proDosed sea-chemicals scheme did not materialize at that time and further geothermal work in Reykjanes was terminated. Four years ago a company was formed to consider again the production of common salt and other sea-chemicals in Reykjanes. Since that time it has conducted pilot plant and other studies to investigate the feasibility of salt production, mainly for the Icelandic market, which is currently about 60,000 tonneslyear. The building of a demonstration plant has now been decided. It is therefore anticipated that the Reykjanes field will come under development in the next few years.

Subsurface Exploration In 1968 and 1969 seven boreholes were drilled in the Reykjanes geothermal area. Four of the wells (2, 3 , 4 and 8) were deep (301 m, 1165 m, 1036 m and 1754 m, respectively) and encountered both high temperatures and good aquifers, while three wells (5, 6 and 7) were relatively shallow (112 m, 572 m and 73 m) and did not penetrate the hot reservoir. The locations of all the boreholes are shown on Figure 1, which is the resistivity survey map of the area (Bjornsson et al. 1970, 1971, 1972) The surface area of the Reykjanes field has been estimated to be 1-2 km2, which makes it one of the smallest in Iceland.

From the time geothermal work was terminated in Reykjanes 10 years ago, two other hightemrerature fields have been explored and developed in Iceland (Gudmundsson et al. 1981). These are the Krafla field (Stefansson 1981) in the northeast and the Svartsengi field (Thorhallsson 1979) in the southwest. These two fields are still being developed. While the Reykjanes field waited for development the one production well drilled was kept on discharge. The purpose of this test was to learn about the long-term discharge characteristics of the welllreservoir system. In 1980

Drilling in the Reykjanes field proved to be difficult. Well 2 was never completed because it could not be controlled during drilling once the casing had been run to bottom-hole. Borehole 3 collapsed during initial discharqe while well 4 produced for a few weeks before doins the same. This latter well was later

-61-

Figure 1: Resistivity survey map of the Reykjanes geothermal area, also showing the location of boreholes (Bjornsson et al. 1970, 1971, 1972).

abandoned after unsuccessful attempts to drill out the collapse. Because of the experiences with wells 3 and 4 it was decided to put a slotted liner to the bottom of well 8. It should be stated here that prior to 1968 it was not the practice in Iceland to put slotted liners in boreholes drilled in high-temperature areas. Until then open hole completion (without slotted liner) had been used with success in several fields. Well hole 8,turned out to be one of the best drilled in Iceland at that time. On initial discharge it delivered about 80 kg/s of a steam-brine mixture.with an enthalpy of 1200 kJ/kg. Wells 2 and 4 delivered much less or about 30 kg/s and 20 kg/s, respectively. The average concentrations of major elements in the deep brines feeding the geothermal wells in Reykjanes are shown in Table 1 (Hauksson 1981). The table shows that the brines are similar in total dissolved solids (TDS) to that of ordinary seawater. Also shown in Table 1 are the well depths and estimated in-

-62-

flow brine temperatures as derived from silica geothermometry. The chloride ion (C1) concentration expresses salinity and is independent of water-rock thermal equilibria (Arnorsson 1978). It is clear from the table that salinity increases with decreasing well depth and therefore with decreasing brine temperature also. This increase is considered to be due to boiling of the geothermal brine as it rises toward the surface in the reservoir. It should be noted that while the total dissolved solids of the geothermal brines are similar to that of seawater, there are some differences in individual components. These differences are mainly due to ion exchange equilibria between rock and water. The geothermal brines are deficient in magnesium (Mg) and sulphate (SOL,) but enriched in potassium (K) and calcium (Ca). The change in sodium(Na) is not as marked, being less in well 8 than in seawater. The amount of silica (Si02) in the geothermal brines is an order of magnitude higher than in . seawater, as would be expected from its increased solubility with temperature.

Table 1 : Averaqe concentration (mg/kg) of chemical components in geothermal brine in Reykjanes and standard seawater. A l s o shorn are well depth and estimated brine temperature.

Component

Geyser

Well 1

Well 2

Well 4

Si02

569 14,300 2,020 2,400 56 155 28,900 0.25

4 14 12,400 1,510 1,980 1.13 82.2 23,700 0.15

355 10,700 1 ,400 1,790

452 10 ,100 1,340 1,640

75 -6 20,500

74.5 19,800

Na K

Ca Mg

so4 c1 F

-

-

Subsurface stratigraphy of the Reykjanes field has been constructed by investigation of drillcuttings and a few cores taken during the main exploration work from 1968-1970 (Tomasson and Kristmannsdottir 1972). It was found that hyaloclastic tuffs and breccias and tuffaceous sediments dominate in the uppermost 1000 m. At greater depths about half the rock is basalt and the rest tuffaceous rocks, mainly sediments. Although the hyaloclastite formation is highly porous, few good aquifers were encountered in the wells drilled. Numerous aquifers were, however, found in the interbeds of the deeper basalt formation. Contacts between lava flows and interbeds are expected to be porous and highly permeable. Faults and fissures seem also to form channels of substantial permeability. The examination of the drill-cuttings showed that calcite (CaC03) was found throughout the rock. Although the amount of calcite varied with depth and location it was found to be most abundant in the uppermost 500- 700 m. Calcite tends to precipitate out due to boiling when geothermal fluids flow toward the surface in boreholes and reservoirs. It seems likely that calcium rich rocks represent a region of initial boiling in the Reykjanes field. A further evidence for boiling in the upflow zone of the reservoir is the chloride concentration of the brine feeding the geyser and boreholes as shown in Table 1. The chloride concentration shows an increase between wells 8 , 4 and 2 from 2% to 5% to 9% higher than that in seawater, the wells being 1 7 5 4 m, 1036 m and 301 m deep, respectively. The corresponding values for well 1 are 26% and 162 m.

-

-

Well 8 588 9 520

1 380 1 580 1.43 40.8 19 200 0.15

Seawater 6.0 10,470 380 398 1,250 2,630 18,800 1.26

Tne Reykjanes Peninsula is rather flat. The Reykjanes geothermal field is about 20 m above sea-level while the Svartsengi field 15 km to the northeast is about 40 m above sea-level. The hydrostatic pressure on the Peninsula should therefore be rather uniform although there must be some regional gradient away from the hills and mountains to the east or northeast. The volcanic rocks that make up the bulk of the Reykjanes Peninsula are considered highly porous throughout. It is therefore not surprising to find that the ground is saturated with seawater even 30 km inland. Geohydrological studies have indicated that the bulk of the Peninsula is saturated with seawater which becomes more diluted as the hills and mountains are approached. On top of the seawater there is a freshwater lens which exhibits a classical freshwater-seawater interface of coastal aquifers. In the geothermal fields the freshwater lens does not exist because of the upflow of hot water and steam. It has been found that the water table within the Reykjanes field is similar to that of the groundwater table surrounding the system. It follows that there must be some boundary or separation that prevents the cold water from invading the field. It has been argued that accompanying the circulation of cold seawater toward the field and down into the ground, there must occur substantial precipitation of secondary minerals (mainly anhydrite) at the boundary of the thermal system, forming an impervious cap. This will lead to the separation of the hydrothermal system from the surrounding colder seawater. This separation is considered most advanced close to the surface and to decrease progressively downwards.

-63-

The amount of deuterium (D) in the deep brine in Reykjanes has been measured (Arnason 1976 and Olafsson and Riley 1978). It was found that the brine in well 8 contained about 23 o / o o less (6D=-23 o / o o ) than standard seawater. Local rainwater contains about 48 o / o o less deuterium than seawater. The usual explanation for the low deuterium concentration of the Reykjanes brine has been that the reservoir fluid was a mixture of seawater and rainwater. The two would mix in near equal proportions (48% and 52%) to result in the measured deuterium value. But the chloride concentration of the geothermal brine is approximately the same as that of seawater as shown in Table 1. This "coincidence" has been attributed to continuous boiling and evaporation of the seawaterIrainwater mixture. In the Svartsengi high-temperature field about 15 km away from Reykjanes, the deuterium concentration in the deep brine has been measured (Arnason 1976). Its value is also 23 o / o o less than that of seawater. Typical salinity (Cl) in the Svartsengi field is about 12,600 mg/kg (Thorhallsson 1979), which concentration has been explained as resulting from the mixture of 67% seawater and 33% rainwater (Kjaran et al. 1979). As in the Reykjanes field the 6 D = -23 o f 0 0 value has been explained by continuous boiling and evaporation in the reservoir. However, the "coincidence" of both fields having 6 D = -23 o/oo but different salinities may require an explanation that also satisfies the "coincidence" previously mentioned. It is postulated here that the deuterium concentration in deep geothermal brines is mainly controlled by water-rockinteractions. Deep brines are understood here to be geothermal fluids at depths greater than 1 km and which have not experienced evaporation in the main upflow zone of a geothermal field. It should be noted that hydrogen isotope fractionation between OH-bearing minerals and water have been demonstrated in the laboratory (O'Neil and Kharaka 1976, Suzuoki and Epstein 1976). The Reykjanes and Svartsengi fields are in similar geologic environments associated with interactions of seawater, basaltic rock and recent or active volcanism. The alteration minerals that form due to geothermal activity contain numerous hydroxyl (OH) groups that may take part in the proposed isotope interactions with circulating brine. This assumes that hydrothermal alteration progresses continuously during the lifetime of geothermal fields and that seawater behaves differently than freshwater. Seawater has higher ionic concentrations than freshwater so that chemical interaction processes are enhanced. This means that for the above postulate to hold the fractionation of deuterium between hydrothermally altered basaltic rock and geothermal brine could be chemically controlled. This tentative postulate could perhaps be examined by comparing the deuterium content of alteration minerals in Reykjanes with that found in high-temperature fields having water of rainwater origin.

Production Well The feasibility of most geothermal projects depends heavily on the success of drilling. The cost, deliverability and longevity of wells is therefore of major importance. Well 8 in Reykjanes is the only borehole there that has discharged for any length of time. An examination of the experience gained from the 12 years of its operation should therefore be most relevant to the future development of the Reykjanes geothermal field. Also to be considered is what information borehole measurements can contribute to our understanding of the reservoir/ well system. Because the geothermal brine in Reykjanes will be put to direct process use as well as for energy purposes, in the production of salt, its chemical as well as other characteristics are of concern. Well 8 was drilled to a depth of 1754 m. It has a 13 318" anchor casing down to 89 m and a 9 5/8" production casing to 297 m, both cemented. It has a 7 518" liner from 260 m to a depth of 1685. This liner was placed in the well about one year after drilling. The liner is slotted at four intervals 984-1037 m (49 m), 1122-1308 m (186 m), 1484-1535 m (50 m) and 1624-1685 m (61 m). These intervals were selected because circulation losses had been encountered there during drilling. The drilling of well 8 was completed on November 28, 1969. Its temperature was measured both during and after drilling. Figure 2 shows a few of these measurements. The temperature log from November 9, 1969, indicated aquifers at depths of about 360 m (300-450 m) and 820 m (750-900 m). The well was first discharged about one year after drilling on October 24, 1970. At that time the bottom-hole temperature was 250-260°C and about 23OoC at 750-850 m. On initial discharge the well was kept on a 10" critical lip-pressure nozzle for about two months until January 1971 when it was shut-down. On February 3, 1971, its temperature was measured as shown on Figure 2. The bottom-hole temperature had clearly increased to 280-290°C. The temperature log taken after the first 2 months o f production (February 3, 1971) is of particular interest as it may indicate where flashing starts in the well during discharge. If two straight lines are drawn through the temperature profile, they intersect at a depth of about 910 m. It must be remembered here that the first slots of the liner start at 984 m such that the total flowrate of the well had been developed at that point. The straight line below 910 m may represent the flow of liquid brine at about 29OoC from near bottom, being mixed with colder brine as it passes the liner slots. The point of flashing represents the saturation temperature of the brine mixture from the various aquifers. Figure 2 shows this temperature to be about 27OoC, the same as indicated in Table 1. Above 910 m the steambrine mixture flashes continuously and cools

-64-

6 bar-a.

Thorhallsson (1977) estimated that the long-term discharge of the well with the 4" nozzle may have been 45-55 kg/s during the first four years shown in Figure 3 . The well was shut-in for three years from October 1974 to October 1977.

TEMPERATURE ('C) 200 1

400

-z

t- '

000-

I c n W

n 1200 -

* a A

1600 -

0

11/9/1969 2/20/1970 2/3/1971 1/7/1977

Figure 2: Temperature measurements in well 8 in Reykjanes

eo

, 1970

1971

1972

1973

1974

1975

TIME (YEARS)

as it flows up the borehole. It should be noted that the temperature log of February 3, 1971, was not a flowing survey, but measured shortly after the well had been shut-in. Because the well had produced for a long time and reached stable thermal conditions, the temperature of its immediate surroundings may have approached the temperature profile existing in the well during discharge. It would be of great interest to compare the flashing point estimated above to theoretical borehole calculations for pressure drop and other characteristics of steam-water flows in geothermal wells. The discharge history of well 8 has been presented by Thorhallsson (1977). Figure 3 shows the well discharge in the first four years of production, from October 1970 to October 1974. When the well was shut-down after the first two months the 10" nozzle was removed and replaced by an 8" one to carry out a deliverability test as discussed below. When this test was over, the well was again shht-down and fitted with a 4" nozzle. The well discharged through this nozzle for long periods but was temporarily (few hours) shut-domi and fitted with the 8" nozzle (critical lip-pressure) for all subsequent (instantaneous) flowrate measurement s . The initial discharge of well 8 was about 85 kg/s through the 10" nozzle to the atmosphere. When it was fitted with the 8" nozzle, two months later, its maximum flowrate was about 70 kg/s (deliverability test January 1971) whereas by 1974 this had decreased just below 60 kg/s, the well-head pressure being about

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Figure 3: Discharge history of well 8 in Reykjanes

BOO

. 0 O

0

r

400 40( t

I

0

lO.000

CHLORIDE (CI)

I970

1972

1974

1976

1978

198(

TIYE ( Y E A R S )

Figure 4: Silica and chloride concentiation with time in the deep brine of well 8 in Reykjanes The discharge history curve of well 8 in Figure 3 shows several features of interest. Before considering these the corresponding changes in deep brine chemistry will be examined. Figure 4 shows the silica (SiO2) and chloride (Cl) concentrations with time, not only the first four years, but also later measurements (Hauksson 1981). The silica concentration changed from about 470 mg/kg in October 1970 to about 730 mg/kg in February 1971.

This corresponds to an apparent increase in deep brine temperature from about 250'C to over 280'C according to silica geothermometry. During the same period the chloride concentration increased from about 18,000 mg/kg to 20,000 mg/kg. From the time of initial discharge the concentration of most major elements in the deep brine have however been found to remain fairly constant (Hauksson 1981). It is perhaps of interest to note that the same conclusion had been arrived at from the monitoring of well 1 from 1956 to 1966. During the first two months of full discharge the flowrate of well 8 decreased from about 85 kg/s to 70 kg/s. At the same time the silica concentration of the deep brine increased, indicating a temperature increase from about 25OoC to 280'C. There appear to be two main explanations why the flowrate decreased and the indicated temperature increased during initial discharge. The first is that the well initially may have produced fluids that were influenced by cold water injected when the slotted liner was placed in the well. It must be remembered, however, that the slotted liner was run into the well about one year after the drilling finished. The second explanation is that the deeper feed zones (aquifers) of the well are more permeable than the upper feed zones. Therefore, when the well starts discharging at high flowrates (70-80 kg/s), there occurs greater drawdown in the upper feed zones compared to the deeper zones. The result of this would be that the contribution of the upper feed zones to the total well flowrate would decrease more rapidly with time than that of the deeper and hotter feed zones. #en well 8 was put on long-term discharge through the 4" nozzle to the atmosphere in January 1971, the total long-term steam-brine flowrate decreased from the 70-85 kg/s in the initial period to the estimated 45-55 kg/s, as mentioned above. When the well was measured in March 1971 the total (instantaneous) flowrate through the 8" nozzle was 80 kg/s or about 10 kg/s greater than that measured with the 10" nozzle at the end of the initial two months' discharge period. The problem at hand is to explain why the flowrate increased. The silica concentration of the inflow brine seemed also to have decreased to a value at which it remained after that. A possible explanation is that at the lower long-term flowrate of 45-55 kg/s, as compared to 70-85 kg/s, there is less drawdown in the aquifers feeding the well. Each 8" nozzle test lasted a day or two during which time further drawdown presumably did not develop to affect the discharge flowrate. An instantaneous measurement using the 8" nozzle should therefore result in a higher flowrate than at conditions of greater drawdown when discharging for two months through the 10" nozzle.

-66-

From 1971 to 1974 the 8" nozzle (instantaneous) flowrate of well 8 decreased from over 80 kg/s to under 60 kg/s. The long-term flowrate (between each measurement) was much lower or 4555 kgls. An examination of Figure 3 shows that the total flowrate declines in a fashion where drawdown is increasing. It should be noted that the total flowrate declines smoothly with time, also, that the rate of decline 1971-1974 (flow 45-55 kg/s) is much lower than 1970 (flow 70-85 kg/s). There is some other evidence to support the above suggestion that drawdown in well 8 has increased with time. In 1970 the water level in the well was at about 40 m while in 1979 it was at 109 m (Bjornsson et al. 1971 and Gudmundsson 1980). In both instances the well was in near thermal equilibrium with the reservoir, i.e., not filled with cold water after drilling. In the autumn of 1977 the wellhead of borehole 8 was overhauled after three years of shutdown (Thorhallsson 1977). The temperature in the well was measured as shown in Figure 2. It was found that the well was blocked at a depth of about 1370 m and that the temperature was very different from that measured before. The temperature profile had assumed an s-type curve with an inflection point at about 820 in depth. Several simple caliper (wire frame) measurements were made in the 7 518" slotted liner. It was found that in addition to a blockage at 1369 m, there appeared to be a minor restriction at 704 m and a major one at 841-845 m. The well was discharged on October 3 , 1977, through an 8" nozzle. After two days the total flowrate was measured (critical lip-pressure method) to be 43 kg/s and the wellhead pressure 4 bar-a. On October 5, 1977, the well was shut-down temporarily and fitted with a 4" nozzle to produce 37 kg/s at a well-head pressure of 17 bar-a the following day. The discharge rate was therefore almost independent of wellhead pressure, indicating some restriction (choking) in the wellbore. #en the well was shut-in three years earlier the discharge through the 8" nozzle had been just under 60 kg/s so that the flowrate had decreased by about 1/3. It seems that this loss of production could be attributed to the wellbore blockages. Silica geothermometry of samples collected at the time of the above discharge measurements indicated deep brine temperatures just above 270'C as before. Because of the blockages found in well 8 in 1977, a difficulty arises with respect to the flowrate decline during 1971-1974 shown in Figure 3. It is possible that all or some of the decline was due to gradual wellbore blockage rather than drawdown in the main feed zones of the well. For the time being, however, it will be assumed that the two are separate because the flowrate decline is gradual while casingfliner failures are likely to occur suddenly, for example, when

w e l l s are p u t on d i s c h a r g e a f t e r l o n g s t a n d i n g p e r i o d s . !Jell 8 was a g a i n shut-down i n J u l y 1978 f o r f u r t h e r s i m p l e c a l i p e r measurements which confirmed e a r l i e r r e s u l t s . The w e l l w a s t h e n k e p t on p r o d u c t i o n u n t i l Novemb e r 1978 when i t w a s shut-down f o r r e p a i r i n t h e f o l l o w i n g December. T h i s r e p a i r o r workover w a s done w i t h a 6 3/8" d r i l l - b i t down t o about 750 m and t h e n a 6" d r i l l - b i t t o bottomhole. The d e t a i l e d r e s u l t s of t h i s work-over are n o t y e t w e l l u n d e r s t o o d . A f t e r t h e work-over t h e w e l l may have no l i n e r a t d e p t h s of 630-725 m and where t h e two main blockages were found as i n d i c a t e d above. The d e l i v e r a b i l i t y of geothermal w e l l s express e s t h e i r t o t a l f l o w r a t e a g a i n s t back p r e s s u r e Two main d e l i v e r a b i l i t y measurements have been c a r r i e d o u t on w e l l 8 , t h e f i r s t i n 1971 and t h e second i n 1980. The former was done u s i n g t h e c r i t i c a l l i p - p r e s s u r e method where t h e b r i n e e n t h a l p y w a s determined by s i l i c a geothermometry (Bjornsson e t a l . 1 9 7 1 ) . The l a t t e r w a s s i m i l a r l y based on t h e c r i t i c a l l i p p r e s s u r e method b u t involved a l s o t h e d e t e r m i n a t i o n of t h e b r i n e f l o w r a t e when t h e t o t a l m i x t u r e had f l a s h e d a t a t m o s p h e r i c c o n d i t i o n s . By t h i s improved method t h e e n t h a l p y of t h e steam- brine m i x t u r e could be determined indep e n d e n t l y (Gudmundsson 1 9 8 0 ) . The two d e l i v e r a b i l i t y t e s t s a r e shown i n F i g u r e 5 . The measurements i n 1971 were c a r r i e d o u t i n one dav a t t h e end of t h e i n i t i a l d i s c h a r g e p e r i o d ( s e e a b o v e ) , The w e l l was f i t t e d w i t h a n 8" n o z z l e and t h e f l o w r a t e w a s a d j u s t e d by e v a l v e , t h e r e a d i n g s b e i n g t a k e n one o r two h o u r s l a t e r . The measurements i n 1980 were t a k e n over a p e r i o d of f o u r d a y s . The w e l l had been d i s c h a r g i n g almost c o n t i n u o u s l y f o r one y e a r w i t h a wellhead p r e s s u r e of 20-30 b a r . The c o n d i t i o n s under which t h e two del i v e r a b i l i t y t e s t s were t a k e n were t h e r e f o r e q u i t e d i f f e r e n t . The 1980 t e s t w a s c a r r i e d o u t u s i n g 3", 5" and 7" n o z z l e s . When changi n g n o z z l e s t h e flow was d i r e c t e d t h r o u g h a by- pass l i n e t o minimize a l l p r e s s u r e and temperature t r a n s i e n t s i n t h e reservoir- well system. Each n o z z l e w a s allowed a t l e a s t one day t o a d j u s t t o s t a b l e wellhead p r e s s u r e and f l o w r a t e v a l u e s . Two s e t s of measurements were t a k e n f o r e a c h n o z z l e , t h e f i r s t a f t e r one day of a d j u s t m e n t and t h e second about 2 h o u r s a f t e r d e c r e a s i n g t h e f l o w r a t e by a v a l v e When u s i n g t h e 7" n o z z l e , however, two more s e t s of measurements w e r e t a k e n , a l s o two h o u r s a f t e r adjusting the flowrate. The e n t h a l p y o f t h e steam- brine m i x t u r e was measured i n t h e 1980 d e l i v e r a b i l i t y t e s t . A chemical sample was a l s o t a k e n ( J a n u a r y 8 , 1980) and t h e t e m p e r a t u r e of t h e deep b r i n e e s t i m a t e d from s i l i c a geothennometry. A l l t h e s e measurements a r e shown i n F i g u r e 6. The e n t h a l p v of t h e steam- brine m i x t u r e was found t o depend on t h e wellhead p r e s s u r e and d e c r e a s i n g f l o w r a t e . T h i s seems t o i n d i c a t e t h a t t h e upper a q u i f e r s f e e d i n s t h e b o r e h o l e a r e n o t a s

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permeable as t h e d e e p e r a q u i f e r s . Two measurements were t a k e n f o r each n o z z l e / v a l v e s e t t i n g . When t h e wellhead p r e s s u r e was i n c r e a s e d t h e f i r s t t i m e , f o r a l l t h e nozzles, t h e enthalpy d i d n o t change. However, a f t e r t h e second and t h i r d wellhead p r e s s u r e changes when u s i n g t h e 7" n o z z l e , t h e e n t h a l p y d e c r e a s e d . T h i s may i n d i c a t e t h a t t h e p r e s s u r e and t h e r m a l condit i o n s i n t h e r e s e r v o i r - w e l l system had n o t reached s t a b l e c o n d i t i o n s . The deep b r i n e t e m p e r a t u r e e s t i m a t e d from s i l i c a geothemometry i s shown i n F i g u r e 6. It i n d i c a t e s b a s i c a l l y t h e same e n t h a l p y as o b t a i n e d from t h e d i r e c t neasurements. The s a t u r a t i o n c u r v e f o r steamwater i s shown i n F i g u r e 6 f o r r e f e r e n c e .

@.'NOZZLE ,i/i0/1971 3'NOZZLE ,114/1980 A 5"NOZZLE .l/6/1980 + 0

-

\ . )

o 60-

L

I

w

s 5

9I&.4 0 -

-

20 -

t

OO 20 40 WELLHEAD PRESSURE (bar - a 1

F i g u r e 5: D e l i v e r a b i l i t y measurements of w e l l 8 i n Rerkjanes

Concluding Remarks Twenty-f i v e y e a r s have now passed s i n c e t h e f i r s t e x p l o r a t i o n was i n i t i a t e d i n t h e Reykjanes f i e l d . The main explor a t i o n work was, however, c a r r i e d o u t about 1 0 y e a r s ago so t h a t t h e e a r l y work (25 y e a r s ago) i s mainly of h i s t o r i c i n t e r e s t . Nevert h e l e s s a n i m p o r t a n t i s s u e must be how t h e example of Reykjanes r e f l e c t s on t h e f u t u r e development of geothermal e n e r g y . I n a d d i t i o n 'to what h a s a l r e a d y been s t a t e d above, t h e f o l lowing a s p e c t s may be c o n s i d e r e d : (1) The res u l t s of t h e f i r s t e x p l o r a t i o n work i n f l u e n c e d t h e way i n which t h e r e s o u r c e may be u s e d ; (2) The p i o n e e r i n g n a t u r e of t h e s e a c h e m i c a l s scheme, as proposed about 15 y e a r s ago, was an i m p o r t a n t r e a s o n f o r t h e l a c k of development o f t h e Reykjanes f i e l d ; (3) Although t h e main

- STEAM

The feasibility of geothermal projects depends greatly on the success of drilling. Factors of importance in this success are the cost of drilling and also the deliverability and lifetime of boreholes. These two latter issues are explored in this paper by way of well 8 in Reykjanes as an example. The lifetime of boreholes, as the term is used here, concerns their mechanical condition mainly. It is evident that geothermal wells will generally experience difficult temperature and pressure transients. These may lead to failures resulting in permanent damage. The experience gained from well 8 in Reykjanes shows that great care is called for in the operation of high-temperature boreholes. The cementing of casings appears to be one of the most important factors in prolonging the lifetime of geothermal wells. Avoiding severe temperature transients in wells will aid in their successful operation In addition to casing and liner failures, problems may arise due to the deposition of calcite (CaC03) and other materials in the wellbore. Such problems tend to be field specific, while their solution (cleaning) usually involves drilling. Such workover operations will inevitably put strain on boreholes and may lead to failures as already mentioned.

TABLES

o 3 ' N O Z Z L E , 1/4/1980 A S'NOZZLE , 1/6/1980 0

SO0

T'NOZZLE

, 1/8/1980

SILICA TEMPERATURE

t -

0

o

20

40

60

eo

I

PRESSURE (bar- a)

Figure 6: Enthalpy measurements of well 8 in Reykjanes exploration phase (ten years ago) lasted only 3 years the basic results are still considered valid. There have, however, been advances in several areas of geothermal exploration and evaluation that should be applied to the field now.

Measurements of the deliverability of geothermal wells (well characteristics) are used for three main purposes: To specify steam-water transmission lines/equipment and power plants; To aid in the exploration for good production fields in geothermal areas, and; To monitor the performance of wells with time once under production. Such tests do not require the shut-down of wells and should therefore not introduce great temperature transients downhole. Deliverability tests will continue to provide the reservoir engineer with firsthand measurements of some of the dynamic fluid and heat flow processes taking place underground.

The problems experienced in the drilling and discharging of the Reykjanes wells were new in Iceland. They had not been met in other fields and indicated that perhaps each geothermal area should be treated as unique. This view was, however, not arrived at until much later when further experience from other fields showed that experience gained in one geothermal field was not necessarily applicable to others. Drilling in geothermal fields provides direct access to the energy resource in the ground. In exploration it should give information about the nature of the reservoir with respect to the translation of thermal energy into power. An important consideration in this translation must be the natural circulation of fluids in the geothermal reservoir. Geochemistry, both that of the water and the rock, has the potential of providing such information. It is well established that the Reykjanes field produces a deep brine having salinity similar to that of seawater. The origin of the geothermal brine is, however, still a subject of discussion. The isotope chemistry of the brine has puzzled investigators and will probably continue to do so f o r some time yet. In this paper a hypothesis is set forth as to the origin of the deep brine. It is suggested that water-rock interactions influence the deuterium content of the deep brine. This is very much a tentative suggestion but should nevertheless warrant consideration. -68-

Acknowledgment The authors would like to thank V. Stefansson, B. Steingrimsson and E. Linda1 for reading the draft of this paper. References Arnason, B., 1976: Groundwater Systems in Iceland Traced by Deuterium, SOC. Sci. IS^., Pub. No. 4 2 , 236 pp. Arnorsson, S., 1978: Major Element Chemistry of the Geothermal Sea-water at Reykjanes and 209Svartsengi, Iceland, Mineral Mag., 9, 2?n.

Stefansson, V., 1981: The Krafla Geothermal Field, Northeast Iceland. Ryback, L., and Muffler, L.J.P. (eds.), Geothermal Systems, Wiley, 273-294.

Bjornsson, S., Arnorsson, S., and Tomasson, J., 1970: Exploration of the Reykjanes Thermal Brine Area, Proceedings, U.N. Symp. Development Utilization Geothermal Resources, 2, (2), 1640-1650. Bjornsson, S., Arnorsson, S., Tomasson, J., Olafsdottir, B., Jonsson, J., and Sigurmundsson, S. G . , 1971: Reykjanes--Final Report on Exploration, Iceland Energy Authority, 122 pp. (In Icelandic). Bjornsson, S., Arnorsson, S., and Tomasson, J., 1972: Economic Evaluation of Revkianes Thermal Brine Area, Iceland, Am. Assoc. Pet. Geol. Bull., 56, (121, 2380-2391.

_-

Suzuoki, T., and Epstein, S., 1976: Hydrogen Isotope Fractionation Between OH-bearing Minerals and Water, Geochimica et Cosmochimica Acta, 60, 1229-1240. Thorhallsson, S., 1977: Well 8 in Reykjanee, Iceland Energy Authority, Note OS-JHD-7730, 9 pp. (In Icelandic). Thorhallsson, S., 1979: Combined Generation of Heat and Electricity from a Geothermal Brine at Svartsengi in S.W. Iceland, Geoth. Resources Council Trans., 3, 733-736.

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Georgsson, L. S., 1981: A Resistivity Survey on the Plate Boundaries in the Western Revkjanes Peninsula, Iceland, Geoth. Resources Council Trans., 5, 75-78.

Tomasson, J., and Kristmannsdottir, H., 1972: High Temperature Alteration Minerals and Thermal Brines, Reykjanes, Iceland, Mineral. Petrol., 36, 123-134,

x.

Gudmundsson, J. S., 1980: Discharge Measurements of Well 8 in Reykjanes, Iceland Energy Authority, Note JSG-80/01, 16 pp. (In Icelandic). Gudmundsson, J. S., Thorhallsson, S., and Ragnars, . K., . 1981: Status of Geothermal Electric Power in Iceland 1980, Proc. Fifth E.P.R.I. Annual Geoth. Conf. Workshop, 7/52-7165. Hauksson, T., 1981: Reykjanes- - Concentration of Chemicals in Geothermal Brine, Iceland Energy Authority, Note TH-81/04, 53 pp. (In Icelandic). Kjaran, S. P., Halldorsson, G. K., Thorhallsson, S., and Eliasson, J., 1971: Reservoir Engineering Aspects of Svartsengi Geothermal Area, Geoth. Resources Council Trans., 2, 337-339. Lindal, B., 1975: Development of Industry Based on Geothermal Energy, Geothermal Brine and Sea Water in the Reykjanes Peninsula, Iceland, Proc. Second U. N. Symp. Development Use Geothermal Resources, 2, 2223-2228. Olafsson, J., and Riley, 3. P., 1978: Geochemical Studies on the Thermal Brines from Reykianes (Iceland), Chem. Geol., 2, 219-237. O'Neil, J. R., and Kharaka, Y. K., 1976: Hydroqen and Oxygen Isotope Exchange Reactions Between Clay Minerals and Water, Geochimica et iosnochi.nica Acta, 9, 241-246.

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Proceedings Seventh Workshop Geothermal Reservoir Engineering S t a n f o r d , December 1981. SGP-TR-55.

ANALYSIS OF WELL DATA FROM THE KRAFLA GEOTHERMAL FIELD I N ICELAND

G.S.

Bodvarssont, S. Bensont, 0. Sigurdssons H a l l d o r s s o n s , and V. S t e f a n s s o n s

G.K.

t E a r t h S c i e n c e s D i v i s i o n , Lawrence Berkeley Laboratory, Berkeley, C a l i f o r n i a 94720 §National Energy A u t h o r i t y , Reykjavik, I c e l a n d

I n t r o d u c t i o n As p a r t of an i n f o r m a l agreement between Lawrence Berkeley L a b o r a t o r y (LBL) and f o u r I c e l a n d i c I n s t i t u t i o n s r e s p o n s i b l e f o r t h e e x p l o r a t i o n and development of geothermal e n e r g y i n I c e l a n d , w e l l d a t a from t h e K r a f l a geothermal f i e l d i n I c e l a n d have been a n a l y s e d . The d a t a c o n s i s t of i n j e c t i o n t e s t d a t a and p r o d u c t i o n d a t a . The i n j e c t i o n t e s t d a t a were a n a l y z e d f o r t h e r e s e r v o i r t r a n s m i s s i v i t y and s t o r a t i v i t y . A n a l y s i s of t h e production d a t a t o determine t h e r e l a t i v e permeability parameters f o r t h e Krafla f i e l d i s i n progress. I n t h i s paper, t h e a n a l y s i s of i n j e c t i o n t e s t s a t t h e K r a f l a f i e l d w i l l be described.

S u r f a c e g e o p h y s i c a l e x p l o r a t i o n a t K r a f l a was i n i t i a t e d i n 1970. I n 1974, t w o e x p l o r a t i o n w e l l s w e r e d r i l l e d , and t h e s u b s u r f a c e d a t a i n d i c a t e d t h e p r e s e n c e of a h i g h t e m p e r a t u r e (>3OO0C) geothermal f i e l d . P r e s e n t l y , 18 w e l l s have been d r i l l e d a t K r a f l a : t h e l o c a t i o n s of t h e w e l l s a r e shown i n F i g u r e 2. S t e f a n s s o n (1981) h a s p r e s e n t e d a d e t a i l e d d e s c r i p t i o n of t h e r e s e r v o i r system a t K r a f l a ; In t h e o l d h i s model i s summarized below. w e l l f i e l d ( w e l l s 1-13, & 15) p r e s s u r e and t e m p e r a t u r e d a t a from t h e w e l l s have i n d i c a t e d t h e p r e s e n c e of two r e s e r v o i r s . The upper r e s e r v o i r c o n t a i n s s i n g l e phase l i q u i d w a t e r a t a mean t e m p e r a t u r e of 205OC. T h i s reserv o i r e x t e n d s from a d e p t h of 200 m t o a d e p t h The deeper r e s e r v o i r i s of a b o u t 1100 m. two-phase, w i t h t e m p e r a t u r e s and p r e s s u r e s following t h e s a t u r a t i o n curve with depth. This reservoir d i r e c t l y underlies a t h i n c o n f i n i n g l a y e r a t a d e p t h of 1100-1300 m and it e x t e n d s t o d e p t h s g r e a t e r t h a n 2200 ~n ( t h e d e p t h of t h e d e e p e s t w e l l ) . The two r e s e r v o i r s s e e m t o be connected n e a r t h e g u l l y , Hveragil. I n t h e new w e l l f i e l d ( s o u t h of M t . K r a f l a , wells 1 4 , 16- 18), t h e upper r e s e r v o i r h a s n o t been i d e n t i f i e d , and o n l y t h e twc-phase l i q u i d dominated r e s e r v o i r seems t o be p r e s e n t .

The K r a f l a geothermal f i e l d i s l o c a t e d on t h e n e o v o l c a n i c zone i n n o r t h - e a s t e r n I c e l a n d ( F i g u r e 1 ) . The n e o v o l c a n i c zone i s charact e r i z e d by f i s s u r e swarms and c e n t r a l volcanoes. The K r a f l a geothermal f i e l d i s l o c a t e d i n a c a l d e r a ( 8 x 10 km), w i t h a l a r g e c e n t r a l v o l c a n o , a l s o named K r a f l a .

XBL 793-6332

F i g u r e 1 The l o c a t i o n of t h e K r a f l a geothermal f i e l d i n I c e l a n d .

Figure 2

-71-

W e l l locations.

W e l l t e s t i n g a t Krafla

A common p r o c e d u r e a t K r a f l a i s t o perform an i n j e c t i o n test soon a f t e r t h e d r i l l i n g i s completed. T h i s procedure h a s been a p p l i e d t o the l a s t 13 w e l l s d r i l l e d a t K r a f l a ( w e l l s 6- 18). The purpose of t h e i n j e c t i o n t e s t s i s twofold: 1) t o a t t e m p t t o s t i m u l a t e t h e w e l l , i.e., i n c r e a s e t h e water l o s s e s , and 2) t o o b t a i n d a t a t h a t can be a n a l y z e d t o y i e l d t h e t r a n s m i s s i v i t y of t h e formation.

Experience o b t a i n e d from i n j e c t i o n t e s t i n g of w e l l s i n K r a f l a , a s w e l l a s i n s e v e r a l o t h e r geothermal f i e l d s i n I c e l a n d , h a s shown t h a t i n many c a s e s a p p a r e n t l y d r y w e l l s ( s m a l l water l o s s e s ) have been s u f f i c i e n t l y stimul a t e d t o become r e a s o n a b l y good p r o d u c e r s . The r e a s o n s f o r t h i s a r e n o t p r e s e n t l y known, but several possible explanations are: 1) c l e a n i n g of f r a c t u r e s , 2) opening up of f r a c t u r e s due t o i n c r e a s e s i n pore f l u i d p r e s s u r e , or 3) thermal c r a c k i n g c l o s e t o t h e w e l l , due t o t h e t e m p e r a t u r e d i f f e r e n c e between t h e i n j e c t e d water and t h e h o t r e s e r v o i r water.

L XBLBIZ-Z~OO

Figure 3 KG-13.

t r a n s d u c e r was l o c a t e d a t a d e p t h of approxi m a t e l y 220 m below ground s u r f a c e , and continuous r e a d i n g s were o b t a i n e d a t t h e s u r f a c e . The t e m p e r a t u r e of t h e i n j e c t e d water was approximately 2 O O C .

Conventional t y p e c u r v e a n a l y s i s of t h e i n j e c t i o n t e s t d a t a from K r a f l a h a s been r e p o r t e d by Sigurdsson and S t e f a n s s o n (1977) and Sigurdsson ( 1 9 7 8 ) . I n t h e p r e s e n t s t u d y t h e use of numerical s i m u l a t o r s f o r w e l l t e s t a n a l y s i s is i l l u s t r a t e d .

The i n j e c t i o n r a t e s a t t h e s u r f a c e a r e shown i n F i g u r e 4 a l o n g w i t h t h e water l e v e l d a t a f o r t h e second t e s t . A f t e r t h e f i r s t i n j e c t i o n t e s t w a s completed ( J u l y l o t h ) , i n j e c t i o n was c o n t i n u e d throughout t h e n i g h t a t a s t a b l e i n j e c t i o n r a t e of a p p r o x i m a t e l y 29 kg/s u n t i l i n i t i a t i o n of t h e second t e s t (see F i g u r e 4 ) . The second i n j e c t i o n t e s t c o n s i s t e d of an i n i t i a l f a l l o f f , followed by t h r e e i n j e c t i o n segments w i t h i n c r e a s i n g flow rates, and f i n a l l y , a second f a l l o f f . During t h e t e s t , a f r e e s u r f a c e w a t e r t a b l e was P r e s e n t i n t h e w e l l , and s i n c e t h e i n j e c t i o n t e s t s are s h o r t , s i g n i f i c a n t w e l l b o r e s t o r a g e Furthermore, a n a l y s i s e f f e c t s were p r e s e n t . of t h e i n j e c t i o n t e s t w a s p o s s i b l y complicated by t h e r m a l e f f e c t s , a s 2 0 ° C t e m p e r a t u r e water was i n j e c t e d i n t o a two-phase r e s e r v o i r of

A n a l y s i s of I n j e c t i o n T e s t Data The w e l l KG-13 (W13) a t K r a f l a w a s d r i l l e d i n June- July A s i m p l i f i e d c a s i n g diagram 1980 ( F i g u r e 2 ) . f o r t h e w e l l i s shown i n F i g u r e 3. The w e l l i s c a s e d ( 9 5/8 i n c a s i n g ) t o a d e p t h of 1021 m, below t h a t a 7 5/8 i n s l o t t e d l i n e r e x t e n d s Thus t h e w e l l i s comt o t h e w e l l bottom. p l e t e d o n l y i n t h e lower, two-phase r e s e r v o i r . The f i g u r e a l s o shows t h e l o c a t i o n of a major f r a c t u r e f e e d i n g t h e w e l l a t a d e p t h of 1600-1700 m. A few days a f t e r d r i l l i n g , two i n j e c t i o n t e s t s were performed ( 1 0 t h a n d - l l t h of J u l y , 1980, r e s p e c t i v e l y ) . During t h e t e s t s a p r e s s u r e

Time (minuter)

F i g u r e 4.

S i m p l i f i e d c a s i n g diagram f o r w e l l

xm. 112

- 2597

Flow r a t e and w a t e r - l e v e l d a t a f o r i n j e c t i o n t e s t of w e l l KG-13.

-72-

However, these attempts w e r e u n s u c c e s s f u l , as a r e a s o n a b l e match w i t h t h e water l e v e l data for e n t i r e test ( t h e i n i t i a l f a l l o f f , t h e t h r e e i n j e c t i o n s t e p s , and t h e second f a l l o f f ) Further attempts t o c o u l d n o t be o b t a i n e d . s i m u l a t e t h e i n j e c t i o n test d a t a were made u s i n g t h e v a r i a b l e f l o w r a t e Theis t y p e model ANALYZE (McEdwards and Benson, 1981) and t h e numerical s i m u l a t o r PT i n i t s i s o t h e r m a l mode. The b e s t match o b t a i n e d i s shown i n F i g u r e 5 .

much h i g h e r t e m p e r a t u r e . For t h e p r e s e n t a n a l y s i s , t h e f r a c t u r e zone a t 1600-1700 m depth w a s assumed t o be t h e primary a q u i f e r ; t h e thermodynamic c o n d i t i o n s a t t h i s depth c o r r e s p o n d t o a t e m p e r a t u r e of a p p r o x i m a t e l y 32OOC. The f i r s t s t e p i n t h e a n a l y s i s of t h i s w e l l t e s t w a s t o c o r r e c t f o r t h e wellbore s t o r a g e e f f e c t s . They w e r e e a s i l y accounted f o r by u s i n g v a r i a b l e flow rate a n a l y s i s , r a t h e r than t h e c o n s t a n t s t e p - r a t e s u r f a c e f l o w rates shown i n F i g u r e 4 . S i n c e t h e w e l l head f l o w r a t e and t h e water l e v e l i n t h e w e l l were known, t h e s a n d f a c e f l o w r a t e c o u l d be c a l c u l a t e d on basis of simple mass b a l a n c e as follows:

The match i s v e r y good a t a l l t i m e s , e x c e p t f o r t h e t h i r d i n j e c t i o n s t e p , where t h e c a l c u l a t e d water l e v e l v a l u e s are s l i g h t l y h i g h e r t h a n t h e observed v a l u e s . F i g u r e 5 a l s o shows t h e c a l c u l a t e d s a n d f a c e f l o w rates used i n t h e s i m u l a t i o n , as w e l l a s t h e w e l l head f l o w rates. The p a r a m e t e r s o b t a i n e d from t h e match w e r e k H / P = 1 . 5 2 ~ 1 0 - ~ m ~ / p a . s eand c Q6tH = 8 ~ l O - ~ m / p .

where A s d e n o t e s t h e change i n t h e w a t e r l e v e l . Equation ( 1 ) simply s t a t e s t h a t t h e water e n t e r i n g t h e w e l l (9,) must l e a v e t h e w e l l (9,) o r be c o n t a i n e d i n the w e l l , c a u s i n g a change i n t h e water l e v e l ( A s ) C e r t a i n l y a f t e r some t i m e a s t e a d y s t a t e c o n d i t i o n w i l l be r e a c h e d where t h e f l o w rates a t t h e w e l l h e a d and a t t h e s a n d f a c e are i d e n t i c a l and c o n s e q u e n t l y t h e water l e v e l i s s t a b l e (As = 0). However, f o r t h e K r a f l a w e l l s ( c a s i n g d i a m e t e r 9 5/8 in) , t h e w e l l b o r e s t o r a g e e f f e c t s w i l l l a s t f o r approximately one and one h a l f h o u r s , and t h e r e f o r e t h e v a r i a b l e f l o w r a t e approach must be employed i n t h e test analysis.

The t r a n s m i s s i v i t y (kH) of t h e r e s e r v o i r c o u l d n o t be determined, as it w a s n o t obvious i f t h e v i s c o s i t y (V) of t h e c o l d i n j e c t i o n water o r t h e h o t r e s e r v o i r water s h o u l d be used i n t h e analysis. Furthermore, t h e t o t a l compress i b i l i t y ( B t ) c o u l d n o t be e x p l i c i t l y c a l c u l a t e d , as t h e p o r o s i t y ($) and t h e e f f e c t i v e r e s e r v o i r t h i c k n e s s (H) w e r e not known. F u r t h e r d i s c u s s i o n of t h e r e s e r v o i r p a r a m e t e r s determined from t h e i n j e c t i o n t e s t i s given l a t e r i n t h i s s e c t i o n .

.

Now l e t u s examine t h e a p p a r e n t i s o t h e r m a l b e h a v i o r observed i n t h e i n j e c t i o n t e s t d a t a . S i n c e t h e f l u i d v i s c o s i t y changes by more t h a n an o r d e r of magnitude f o r t h e t e m p e r a t u r e range 2 0 ° t o 32OoC, one would n o t e x p e c t isothermal p r e s s u r e behavior i n t h e d a t a e s p e c i a l l y when t h e d a t a i s t a k e n d u r i n g both i n j e c t i o n and f a l l o f f p e r i o d s . The r e a s o n f o r t h i s i s t h a t f o r a Theis- type r e s e r v o i r , t h e p r e s s u r e changes d u r i n g i n j e c t i o n w i l l correspond t o t h e c o l d w a t e r f l u i d p r o p e r t i e s ,

Because of t h e two-phase n a t u r e of t h e K r a f l a r e s e r v o i r and t h e nonisothermal e f f e c t s i n t r o d u c e d by t h e c o l d water i n j e c t i o n , a t t e m p t s w e r e made t o model t h e i n j e c t i o n t e s t d a t a u s i n g t h e two-phase s i m u l a t o r SHAFT79 ( P r u e s s and S c h r o e d e r , 1980) and t h e noni s o t h e r m a l s i m u l a t o r PT (Bodvarsson, 1981).

20

K R A F L A W E L L KJ - 13 Injection Test July I I , 1980

60

-

I

Parameters

AI00

220

% 30

-..20 Lo -

---

+

e

-

Sandfoce flow roles corrected for wellbore

10

lL

0

0

20

40

60

80

100

120

140

160

180

200 220

240

260

280

Time (minutes) m a

~ 0 ~ ~ 1 7

Figure 5 .

Comparison between observed and c a l c u l a t e d water l e v e l s f o r i n j e c t i o n test of w e l l KG-13.

-73-

whereas during the falloff period, the pressure changes will correspond to the fluid properties of the hot reservoir (Bodvarsson and Tsang, 1980).

mt

m-

In an attempt to explain the isothermal behavior of the data from the injection test, two possibilities must be explored: 1) that the undisturbed reservoir conditions (e.g., T = 32OOC) control the pressure response at the well; and 2) that the temperature of the injected water is the controlling factor. As cold water has been injected into the well at all times during drilling (approximately 45 days) and also during the few days after drilling, there must be a cold water zone around the well. Consequently, the first possibility seems unlikely. If the cold water zone around the well is to explain the isothermal behavior in the data, this zone must extend further from the well than the pressure disturbance during each injection step.

20

’

/

A=2J/m,r..C D*1000m

C.

=4200J/kg,OC

R = 2 6 5 0 ka/m3

lot Time (days) xkmti-uI

Figure 6 Advancement of the cold water front with time along a horizontal fracture.

Multiplying the numerator and the denominator by the effective thickness of the fracture zone H I the parameter groups determined from the well test (see equations 2 and 3) can be used to determine the radius of influence. For an injection step lasting 1 hour, a radius of influence of 16.5 m can be calculated. As this value (16.5 m) is less than the calculated radial extent of the cold water zone (- 50 m), isothermal pressure behavior can be expected. If this analysis is correct, the fluid parameters corresponding to the cold injection water should be used, and consequently this implies a transmissivity of kH = 15 Darcy-meters.

In order to study the advancement of the cold-water front along a fracture intercepting the well, the theory developed by Bodvarsson and Tsang (1981) was used. A single horizontal fracture, representing a permeable layer between lava beds, is assumed to absorb the total injection rate. Heat conduction from the rocks above and below the fracture retards the advancement of the cold water front. The equation governing the advancement of the cold water front along the fracture away from the well is:

The fracture zone (aquifer) feeding the Krafla well KG-13 is believed to be very thin, or on the order of 1 m (Stefansson, personal comunication, 1981). If one assumes a value for the porosity (4) for this zone, say 4 = .lo, a very high total compressibility, 6, = 8x10+ pa-l can be calculated using equation (3). This high total compressibility can be explained by the two-phase conditions in the reservoir, or by high fracture compressibility. Due to the uncertainity in explaining the isothermal behavior of the test, both possibilities will be explored.

where r is the radial distance of the cold water front away from the injection well, q is the injection rate, A is the thermal conductivity, and Pwcw and prcr are the volumetric heat capacities of the injected water and the rock matrix respectively. Figure 6 shows the advancement of the cold water front along the fracture versus time. The parameters used in the calculations are shown in Figure 6 ; they represent the average injection rate prior to the injection test, and average thermal properties for basaltic rocks. Figure 6 shows that, if one considers only the injection after drilling (2-3 days), the cold water front will have advanced approximately 50 m away from the well when the second injection test begins. It is of interest to note that this estimate is independent of the fracture aperature (Bodvarsson and Tsang, 1981).

The compressibility of two-phase fluids is two to four orders of magnitude larger than those of single phase liquid or steam water (Grant and Sorey, 1979). The two-phase compressibility depends on many parameters, such as the temperature, saturation, porosity, and the relative permeability curves. Figure 7 shows the relationship between fluid compressibility and vapor saturation for various values of porosity. In calculating the curves shown in Figure 7 a reservoir temperature of 30OoC and the Corey relative permeability curves are used. Comparison of the total compressibility 8, (previously determined fit = 8 ~ 1 0 -pa-l) ~ to Figure 7 yields a porosity value of 0 = .05 and vapor saturation of S, < .20. These values agree very well with values of porosity

In comparison, the radius of influence for the pressure disturbance due to a typical injection step can be calculated directly from the reservoir diffusivity as follows: (3)

-74-

IO-'

'

Temperature = 300°C Corey curves

KRAFLA WELL KG 12

4h

Time (minutes)

Figure 8

Vapor saturation

x n nw~ 1300 a

I n j e c t i o n t e s t d a t a for K G 1 2 .

XBL 817- 3293

Figure 7 F l u i d c o m p r e s s i b i l i t y a s a function of vapor s a t u r a t i o n f o r v a r i o u s v a l u e s of porosity.

s i m u l a t i o n ( b r o k e n l i n e ) t o account f o r t h e wellbore s t o r a g e e f f e c t s . A s t h e f i g u r e shows, t h e c a l c u l a t e d v a l u e s compare v e r y w e l l w i t h t h e observed d a t a . However, t h e e n t i r e test could not be simulated using a constant For t h e i n i t i a l f a l l o f f and v a l u e f o r kH/u. the f i r s t injection- falloff cycle the data were matched r e a s o n a b l y w e l l u s i n g kH/U = 1 . 2 ~ 1 0 - ~ %;however, approximately 200 minutes a f t e r t h e i n j e c t i o n t e s t began, a d e c r e a s e i n t h e water l e v e l was observed, a l t h o u g h t h e i n j e c t i o n r a t e remained c o n s t a n t ( F i g u r e 9 ) T h i s i m p l i e s a change i n t h e t r a n s m i s s i v i t y of t h e r e s e r v o i r . T h i s i s v e r i f i e d by t h e numerical s i m u l a t i o n , s i n c e i f t h e kH/p f a c t o r i s k e p t c o n s t a n t a t kH/u = 1.2x10-* over t h e e n t i r e simulation, t h e calculated pressure changes w i l l g r e a t l y exceed t h e observed ones. Therefore, i n t h e simulation t h e transmiss i v i t y had t o be i n c r e a s e d t o account f o r t h e a p p a r e n t s t i m u l a t i o n due t o t h e c o l d water

and vapor s a t u r a t i o n i n f e r r e d from o t h e r f i e l d d a t a ( S t e f a n s s o n , 1981). However, it i s r a t h e r doubtful t h a t t h e high compressibility determined from t h e i n j e c t i o n tests i s due t o t h e p r e s e n c e of two-phase f l u i d s , because of t h e c o l d water zone s u r r o u n d i n g t h e w e l l . It is p o s s i b l e t h a t t h e h i g h c o m p r e s s i b i l i t y i s due t o deformable f r a c t u r e s . I n t h a t case, t h e increase i n w e l l losses during injection tests may be due t o opening up of f r a c t u r e s caused by i n c r e a s e d p o r e p r e s s u r e s .

.

The second i n j e c t i o n t e s t t h a t was a n a l y z e d was performed on w e l l KG-12 ( W 1 2 ) . The w e l l is c a s e d ( 9 5/8 i n c a s i n g ) t o a d e p t h of 952 m, and below t h a t t o t h e bottom of t h e w e l l (2222 m ) , a 7 i n s l o t t e d l i n e r i s i n place. T h i s w e l l is a l s o completed i n t h e lower two-phase r e s e r v o i r . The major f r a c t u r e zone i s l o c a t e d a t a d e p t h of 1600 m, b u t some c o n t r i b u t i o n t o t h e p r o d u c t i o n from t h e w e l l may come from f r a c t u r e s l o c a t e d a t a d e p t h of 1 0 0 0 m. The i n j e c t i o n t e s t d a t a , c o n s i s t i n g of water l e v e l d a t a and wellhead f l o w r a t e s are given i n F i g u r e 8. A s t h e f i g u r e shows, f o r s e v e r a l c?ays p r i o r t o t h e t e s t , c o l d w a t e r a t a r a t e of 30 l/s was i n j e c t e d i n t o t h e w e l l . After an i n i t i a l f a l l o f f l a s t i n g f o r approximately one and one h a l f h o u r s , f o u r i n j e c t i o n / f a l l o f f segments w i t h i n c r e a s i n g i n j e c t i o n r a t e s w e r e used. On t h e a v e r a g e , each of t h e i n j e c t i o n s t e p s o n l y l a s t e d 4 0 minutes, so t h a t w e l l b o r e s t o r a g e e f f e c t s a r e q u i t e important. A n a l y s i s of t h e i n j e c t i o n t e s t of w e l l KG-12 w a s c a r r i e d o u t u s i n g t h e s i m u l a t o r PT i n i t s i s o t h e r m a l mode. F i g u r e 9 shows t h e b e s t match o b t a i n e d between t h e observed and t h e c a l c u l a t e d water l e v e l v a l u e s . Figure 9 a l s o shows t h e v a r i a b l e f l o w r a t e used i n t h e

F i g u r e 9 Match of c a l c u l a t e d and observed waterlevel data a t K G 1 2 .

-75-

i n j e c t i o n . I n t h e s i m u l a t i o n shown i n F i g u r e 9 , t h e k H h w a s i n c r e a s e d by a f a c t o r of two, from a n i n i t i a l v a l u e of 1.2x10-8 m3 t o a f i n a l v a l u e of 2 . 4 ~ 1 0 -m3 ~ Injectp i oh; ns

References Bodvarsson, G.S. ( 1 9 8 1 ) , "Mathematical Modeling of t h e Behavior of Geothermal Systems under E x p l o i t a t i o n " , (Ph .D. d i s s e r t a t i o n ) Lawrence Berkeley L a b o r a t o r y , Berkeley, California.

.

t e s t data from s e v e r a l o t h e r k r a f l a w e l l s have shown s i m i l a r i n c r e a s e s i n i n j e c t i v i t y d u r i n g i n j e c t i o n . The e x p e r i e n c e a t K r a f l a h a s i n d i c a t e d t h a t c o l d w a t e r i n j e c t i o n can stimul a t e t i g h t w e l l s i n t o becoming f a i r p r o d u c e r s . S i m i l a r i l y , i n c r e a s e s i n p r o d u c t i v i t y of flowing w e l l s due t o t h e r m a l c o n t r a c t i o n have been r e p o r t e d by S t e f a n s s o n and S t e i n g r i m s s o n (1980)

Bodvarsson, G.S., and Tsang, C.F. ( 1 9 8 0 ) , "Thermal E f f e c t s i n W e l l T e s t s of F r a c t u r e d Reservoirs," Proceedings, Third I n t e r n a t i o n a l Well- Testing Symposium, Lawrence Berkeley Laboratory (March 26- 28).

I n t h e s i m u l a t i o n shown i n F i g u r e 9 a c o n s t a n t s t o r a t i v i t y v a l u e was u s e d , QBtH = E x ~ O - ~ m/p. This value is i d e n t i c a l t o t h e value o b t a i n e d from t h e a n a l y s i s of w e l l KG-13. This indicates e i t h e r a r a t h e r constant d i s t r i b u t i o n of t h e f l u i d r e s e r v e s ( 9 and Sv r a t h e r u n i f o r m ) , o r a f a i r l y uniform fracture compressibility.

Bodvarsson, G. S., and Tsang, C.F. ( 1981) , " I n j e c t i o n and Thermal Breakthrough i n Fract u r e d Geothermal R e s e r v o i r s , " t o be p u b l i s h e d i n t h e J o u r n a l of Geophysical Research, LBL-12698. G r a n t , M.A., and Sorey, M.L. ( 1 9 7 9 ) , "The C o m p r e s s i b i l i t y and H y d r a u l i c D i f f u s i v i t y of Water-Steam Flows," Water Resources Research, v o l . 15, no. 3, p. 684-686.

Conclusions I n j e c t i o n t e s t d a t a from t h e K r a f l a geothermal f i e l d i n I c e l a n d have been a n a l y z e d u s i n g numerical s i m u l a t i o n techn i q u e s . The r e s u l t s i n d i c a t e t h a t a l t h o u g h t h e i n j e c t e d water i s of a much lower tempera t u r e than t h e undisturbed r e s e r v o i r water, t h e r e a r e no a p p a r e n t nonisothermal e f f e c t s observable i n t h e data. One p o s s i b l e explana t i o n is t h a t a c o l d w a t e r zone e x i s t s around t h e w e l l due t o t h e c o l d d r i l l i n g water and t h a t t h e p r e s s u r e d i s t u r b a n c e d u r i n g t h e i n j e c t i o n t e s t s does n o t e x t e n d beyond t h e c o l d water zone. Thus, t h e r e s e r v o i r param e t e r s must be e v a l u a t e d based on t h e f l u i d p r o p e r t i e s corresponding t o t h e i n j e c t e d fluid.

McEdwards, D.G., and Benson, S.M. ( 1 9 8 1 ) , "User's Manual f o r ANALYZE, Variable- Rate, Multiple- Well, L e a s t Squares Matching Routine f o r W e l l - T e s t A n a l y s i s , " Lawrence Berkeley L a b o r a t o r y , Berkeley, C a l i f o r n i a , LBL-10907. P r u e s s , K . , and Schroeder, R.C., (1980), "SHAFT79 U s e r ' s Manual ," Lawrence Berkeley L a b o r a t o r y , Berkeley, C a l i f o r n i a , LBL-10861. S i g u r d s s o n , O . , and S t e f a n s s o n , V. ( 1 9 7 7 ) , " L e k t i borholum i K r o f l u " , ( i n I c e l a n d i c ) N a t i o n a l Energy A u t h o r i t y of I c e l a n d . S i g u r d s s o n , 0. ( 1 9 7 8 ) , " R e n n s l i s e i g i n l e i k a r e f r a j a r d h i t a k e r f i s i n s i K r o f l u , " ( i n Icel a n d i c ) N a t i o n a l Energy A u t h o r i t y of I c e l a n d .

Numerical modeling s t u d i e s of i n j e c t i o n t e s t s from t w o of t h e K r a f l a w e l l s ( K G - 1 2 and KG-13) y i e l d e d t h e t r a n s m i s s i v i t y and s t o r a t i v i t y of t h e formation s u r r o u n d i n g t h e w e l l s . The results indicate t h a t the injection t e s t s t i m u l a t e d w e l l K G - 1 2 , s i n c e an a p p a r e n t i n c r e a s e i n t h e t r a n s m i s s i v i t y was observed d u r i n g t h e t e s t . Also, f o r b o t h of t h e w e l l s , t h e modeling r e s u l t s i n d i c a t e d a high t o t a l compressibility. The h i g h c o m p r e s s i b i l i t y can e i t h e r be due t o t h e two-phase c o n d i t i o n i n t h e r e s e r v o i r o r a high f r a c t u r e compressibility.

S t e f a n s s o n , V. ( 1 9 8 1 ) , "The K r a f l a Geothermal F i e l d , N o r t h e a s t I c e l a n d , " Geothermal Systems, L. Ryback and L . J . P . M u f f l e r ( e d i t o r s ) .

S t e f a n s s o n , V . , and S t e i n g r i m s s o n , B . ( 1 9 8 0 ) . "Production c h a r a c t e r i s t i c s of w e l l s t a p p i n g two-phrase r e s e r v o i r s a t K r a f l a and Namafjall" Proceedings. 6 t h Workshop Geothermal R e s e r v o i r Engineering, Stanford University, Stanford C a l i f o r n i a , SGP-TR-50, 49-59

Acknowledgements T h i s r e s e a r c h was performed a s a p a r t of a c o o p e r a t i v e s c i e n t i f i c i n v e s t i g a t i o n of t h e K r a f l a geothermal f i e l d between NEA, SEPW, E R I , and SI of I c e l a n d and LBL o f U.S.A. T h i s work was s u p p o r t e d by t h e A s s i s t a n t S c e r e t a r y of Conservation and Renewable h e r g y , O f f i c e of Renewable Technology, D i v i s i o n of Geothermal and Hyropower Technologies of t h e U . S . Department of Energy under C o n t r a c t N o . W-7405-ENG-48.

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Proceedings Seventh Workshop Geothermal Reservoir Ewineering Stanford, December 1981. SGP-TR-55.

FIRST RESULTS OF A REINJECTION EXPERIMENT AT LARDERELLO

Anselmo Giovannoni,Giovanni Allegrini and Guido Cappetti ENEL-Unit2 Nazionale Geotermica, Pisa, Italy Romano Celati CNR-Istituto Internazionale per le Ricerche Geotermiche,Pisa,Italy Abstract Reinjection, which began at Larderello in 1974 as a means of disposing of excess steam condensate, is now envisaged as a method for improving heat recovery. The behavior of the geothermal field when subjected to production and injection is difficult to predict because of the very heterogeneous fractured reservoir. More information is needed on circulation patterns and heat sweeping processes to estimate the long-term behavior of the reservoir and to avoid detrimental effects. A series of reinjection experiments is now under way in different parts of the Larderello reservoir, aimed at improving knowledge o f these points before starting a widescale injection program. This bout that over

a1.,1977a; Baldi et a1.,1980). The success of the new wells is tied to the possibility, still to be verified, of recovering fluids from zones outside the present margins of the field and from deep horizons of the reservoir (more than 2 k m depth). - O

1

2km

paper presents the results of aone year of injection in an area has been exploited intensely for 2 0 years.

During this test the following were noted: - almost complete vaporization of the injected water; - significant production increases and no temperature decrease in the wells around the injector. Introduction Production from the Larderello field, under exploitation for more than 5 0 years, has been kept more or less constant during the last 30 years by drilling new wells. This policy has proved to be less than satisfactory during the last few years because of the large decrease in pressure throughout the field (Fig.l), and in the more productive zones in particular (Ferrara et a1.,1970; Celati et

Figure 1 Pressure distribution at the top of the reservoir in the Larderello field, showing injection well w 0 and the study area. Pressure in bar. Another possible approach is to inject large quantities of water back into the reservoir. Theoretically a "secondary recovery" of heat from this greatly depleted reservoir is possible as the temperature in most of the explored volume is still within the 24Oo-26O0C range;

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temperature values of more than 300'C have also been recorded more or less everywhere at depths below 2 km. Mathematical models and a limited field experience (Celati et a1.,1977b; Celati and Ruffilli,l980; O'Sullivan and Pruess, 1980; Schroeder et al. ,1980) have shown that it is possible to increase both the recovery factors in the long term and the production rates in the short term, by exploiting reservoirs with pressures below saturation values. Favourable conditions for obtaining significant production increases can be found in the horizons most exploited nowadays, over the wide zones of Larderello characterized by high permeability and low pressures.

w4

0

0

w8. w 15

D

w7 0

.wO

w3 0

wl

0

0

-

0

500 m

Figure 2 Location of the wells 1)injection well; 2)productive wells; 3 ) shut-in wells.

In the present energy situation this seems to be a highly attractive possibility. At the moment, however, we have not a sufficient knowledge of the spatial distribution of the fractures, nor, consequently, of the path taken by the injected fluid in the reservoir and the sweep efficiency attainable. "Short-circuits" have frequently occurred between wells at the drilling stage, after a circulation loss, and productive wells. For these reasons, before defining a large-scale injection program for the Larderello field, the decision was taken to run a series of tests in different places and situations. The objectives of these tests are to study field behavior,select the most suitable sites for injection wells and develop some tracing methods capable of throwing light o n the evolution of the phenomena. First injection test The zone chosen for the first reinjection test is that shown in Fig.1. The main reasons for choosing this zone were: - high permeability tied to a diffuse fracturing. The initial flow-rate in some of these wells exceeded 300 tlh; - high density of productive wells and, hence, possibility of studying the propagation of the effects of injection (Fig.2);

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-

-

marked decrease in production andresrevoir pressure with time (Fig.31, with temperatures remaining around 24OQ-26O'C; marked stability of the chemical characteristics of the fluids during the last few years, and more or less uniform spatial distribution of the isotopic composition around the injection well.

All the wells in the area vary in depth from 400 to 600 m , their steam entries lying within the carbonate-evaporitic formation. The first test was conducted from January to August 1979, keeping the flowrate of the injected water on quite low values (usually 30 and 50 m3/h, and about 105 m3/h for a short period only). After a 3 month break injection began again with higher flow-rates. All the wells from wl to w14 in Fig.2 were affected to varying degrees, in the form of production increases and changes in fluid composition. The most significant changes were those affecting the isotopic composition of the fluid (Nuti et a1.,1981). Figure 4 shows the flow-rate of the injected water, the total production increase of wells wl-w14, the wellhead

production of the various wells (Nuti analyses of the et al.,l981).Systematic isotopic composition of the fluid have been made o n four wells only (wl,w2, w7 and wll). We can thus estimate how much of the steam produced by the injected water joins the fluid produced by these wells. Figure 5 shows that they produce about 60% of the injected water and that this contribution alone is higher than the increase in flowrate observed throughout the area.

20

. :

?

j

y

900

18

aoo

16

700

14

600

12

500

10

; -

400

8

:

300

6 %

0

J

!

-

? ?

2

200

4

100

55

2

3

2

60

65

70

75

years

80

Figure 3 Decline i n total flow-rate in wells wl to w14 and in shut-in pressure in well w 1 5 . pressures and temperatures of the seven most productive wells in the area and the average gas content of wells w l w14. The steam flow-rate was strongly affected by the variations in wellhead pressure, which increased notably during this period as a result of certain operations in the power-plants. The increase in flow-rate was lower than the amount of water injected. Wellhead temperature in the productive wells varied very little, even i n the wells nearest the injector. The average gas content of the fluid decreased to 7 0 % o f its pre-injection value, which, along with increased pressure, led to an increase in conversion efficiency. The variations in the gas/ steam ratio appear to be tied to the flow-rate of injected fluid. The latter has a negligible gas content s o that the steam i t produces merely dilutes the gas in the original steam. The studies of the isotopic composition of the fluid have shown that it is possible to calculate the contribution of the injected water to the

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Towards the end of the injection period (204th day i n Fig.5) an isotopic survey was made of all the wells affected by reinjection. According to the results of Nuti et al., more than 9 0 % o f the flow-rate of injected water was reproduced by the wells. The variations in the gas/steam ratio can also be used to evaluate, albeit approximately, the contribution of injected water to production in the area, assuming that the fluid produced is a mixture of original steam with a constant gas content and injected water containing n o gas. This calculation, however, is incorrect as the gas content in the original steam flowing to any given well is not constant because the flow pattern in the reservoir is altered by reinjection. The error made in computing gas dilution can be reduced by using the average gaslsteam ratio in the fluid produced from all the wells in the area, but it cannot be eliminated altogether. Nevertheless, the gas/ steam ratio is known for all the wells affected by reinjection and for the entire duration of the test; we can thus estimate approximatively the fraction of the water injected i n thetotal fluid produced in the area. Figure 5 shows that the contribution o f injected water to production, calculated in this way, is more or less the same as the injection rate. On the whole we may conclude that, in this first test phase of small injection rates, almost all the water injected is vaporized and joins the fluid produced. The total increase i n flowrate, however, is much smaller than this contribution, which means that the flow of original steam towards the wells decreased during injection. In this case, the phenomenon was mainly

50

100

150

200

2 50

7-

x * 0

6-

0) al

;

5-

a

4-

Dl

Figure 4

I n j e c t i o n r a t e i n w e l l w 0 and t o t a l p r o d u c t i o n i n c r e a s e i n w e l l s wl to w14. W e l l h e a d t e m p e r a t u r e and p r e s s u r e i n t h e s e v e n m o s t productive w e l l s of the area. A v e r a g e g a s c o n t e n t i n w e l l s w l to w14.

-80-

0

100

1) 2)

+ 3)

L

f 0

c o) c

r

50

e

I

3

L.

-0

\

LL

\ \

\

\

0

50

Figure 5

150 100 T I ME ( d a y s )

I 250

200

Injected water recovered through wells wl,w2,w7 and wl , from isotopes,l); injected water recovered through wells wl to w14, from gas content,2) and from isotopes,3).

caused by the back-pressure increases on the wells, deriving from operations in the power-plants. These back-pressure increases have a considerable effect on the rise of steam from great depths and a much lesser effect on the steam coming from the shallower, very permeable formations. These observations are in agreement with the results of the numerical simulation (Schroeder et a1.,1980), indicating that effects of this type can also have a certain importance when producing at constant wellhead pressure.

L

-c

-3 0

&

20

rro

50

100

150

200

250

Figure 6 shows the trend o f fluid flowrate and the contribution of injected water to production for wells w 7 and w9. In w7, which is very near the injection well, this contribution is much higher than the increase in flow-rate, whereas in w9, relatively further ! away o from the injection point, the increase in flow-rate is higher than in w7, but the contribution of injected water is very low. The flow of original steam thus decreases in w 7 , and increases in w9 as a consequence of an increase in reservoir pressure.

Figure 6 Fluid production and injected water recovered in wells w7 and w9. 'From isotopes. *From gas content.

Reinjection was always conducted with

no back-pressure at the wellhead, and

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3

LL

Injected l 0

50

100

150

200

250

days

the injectivity of the well showed no variations throughout the duration of the test; the pressure at the top of the permeable sector of the borehole never varied more than 0 . 5 ata from static pressure. Despite the fact that a total of 2.3 x 105 m3 of water was injected into well w0 during this first phase, at an average rate of 5 0 m3/h, the well had already reached its usual shut-in pressure at wellhead 1 0 minutes after injection ended, and no liquid phase was found in the borehole. The well was kept shut for twenty days, during which the pressure remained constant and the temperature in the bore was at saturation values. On opening the well the steam rapidly became superheated and the wellhead temperature quickly rose to 185OC after only 8 days (Figure 7), and 22OoC at wellbottom after 4 0 days production.

Conclusions No breakthrough phenomena were observed during this first phase o f the experiment with low injection rates, even with such a reduced well spacing. On the contrary, the conditions appear to b e favourable for a good penetration of the water into the fractured medium, and for a good rockiluid thermal exchange. A second injection phase is now under way to verify the earlier results when injecting at higher flow-rates. Other experiments are beginning in nearby areas with productive wells deeper than the injection wells. These tests hopefully will also shed light on the penetrating capacity of the injected fluid at depth. References Baldi,P.,Bertini,G., Calore,C.,Cappetti,G.,Cataldi,C., Celati,R. and Squarci, P.(1980)," Selection of Dry Wells in Tuscany for Stimulation Tests", Proc. Second DOE-ENEL Workshop for Cooperative Research in Geothermal Energy, Berkeley,p.98-115. Celati,R.,Squarci,P.,Stefani,G.C. and Taffi,L.(1977a),"Study of Water Levels in Larderello Region Geothermal Wells for Reconstruction of Reservoir Pressure Trend", Proc. of Simposio International sobre Energia Geotermica En America Latina, Ciudad de Guatemala, 1976, IILA, Rome, p.501-526. Celati,R.,McEdwards,D.,Ruffilli,C., Schroeder,R.,Weres,O. and Witherspoon, P.(1977b),"Study of Effect of Reinjection with a Mathematical Model",Proc. ENEL-ERDA W o r k s h o p , L a r d e r e l l o , l 9 7 7 , p . 256-298.

1

Celati,R. and Ruffilli,C.(1980),"Simulazione numerica della iniezione in sistemi a vapore-dominante". ENEL-CNR Report,Pisa, 19 pp.

140

1 Ferrara,G.C., 0 10 20 30

Panichi, c . , Stefani, G (1g70), "Remarks on the geothermal Phenomenon in an intensively exploited field. Results of an experimental well", Geothermics, Special IsSue 2, V.2, p t m l , p .5 7 8- 5 8 6 .

time days1

Figure 7 Wellhead temperature in w0, producing at the end o f the injection period.

uti, ~ . , ~ a l o r e , Cand . Noto,P.(1981) "use of Environmental Isotopes as Natural Tracers in a Reinjection Experiment

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a t L a r d e r e l l o " , P r o c . 7 t h W o r k s h o p Geothermal Reservoir Engineering, Stanford, 15- 17 D e c . , 1 9 8 1 . O'Sullivan,M.J. and Pruess7K.(1980)"Num e r i c a l S t u d i e s o f t h e E n e r g y Sweep i n Five- Spot Geothermal Production-Inject i o n S y s t e m s " , P r o c . 6 t h W o r k s h o p Geothermal Reservoir Engineering, Stanf o r d , 16- 19 D e c . , 1 9 8 0 7 p . 2 0 4 - 2 1 2 .

Schroeder,R.C.,O'Sullivan,M.J.,Pruess, K.,Celati,R. and R u f f i l l i , C . ( 1 9 8 0 ) , "Re i n j e c t i o n S t u d i e s o f Vap o r - D o m i n a t e d S y s t e m s " , P r o c . 2 n d D O E - E N E L Works h o p f o r C o o p e r a t i v e R e s e a r c h i n Geot h e r m a l Energy, B e r k e l e y , 198O,p.381433.

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Proceedings Seventh Workshop Geothemal Reservoir Engineering Stanford, December 1981. SGP-TR-55.

Sergio Nuti, Claudio m o r e and Pietro Noto

Istituto Internazionale per le Ricerche ceOtermiche,(Cm), Via del Buongusto 1, 56100 Pisa, Italy Abstract Reinjection of the discharge f m the pawer-plants of a geothermal field can cause serious, irreversible damage to the field itself,so that such operations must be mnitored continuously. Tracer techniques are particularly useful for t h i s purpose.

Larderellorwitha heterogeneous reservoir and fracture-derived -ability, one cannot predict with any certainty the effects induced by reinjection. For example, one could expect a decrease in steam enthalpy and/or in the aamxult of steam produced. In order to avoid eventual irreversible negative effects to the field sane experiments must be made prior to launching a full-scale reinjection programre.

In rn reinjection studies tritiated water was used as a tracer, but t h i s methcd has cert a i n disadvantages as well as advantages, one disadvantage being the destruction of the natural tritium balance in the reservoir.

These experimnts will provide useful information on the canplex phenmology of vapourdcminated systems only if the tools used are adequate.

The natural abundances of tritium at Larderello were, and are still, used to study field recharge f m mteoric waters, so that another tracing methcd had to be applied to avoid disrupting these studies. The discharge fluid f m the p3crFer-stations is traced naturally by its 180 and D ampsitions with respect to the steam ccmposition of the field. As these isotopes do not create the s a difficulties as tritium we are checking their possible utilization as tracers.

In this respect the tracer tests assum a role of considerable hprtance, as they can prcrvide informtion on the processes occurring in the reservoir and on eventualmdifications to these processes; they can also be used to calculate the m u n t of injected water reappearing as steam in the fluids produced and individuate any preferential pathways of the fluids The problem is to find a routine work mthcd that creates as little disturbance as possible to the system, but which also produces the mimumof data.

This papr presents the results obtained by applying this methd in the first phase of a reinjection experhnt now under way in a central area of the field which has been exploited for long periods. Sane limits of t h i s method are also discussed.

In the reinjection tests currently being conducted in a central area of the Larderello field, which has been exploited for the past 20-0dd years, a study is being m d e of the potential of the stable isotopes 180 and D as tracers.

Intrcduction Because of the limited natural recharge of the Larderello geothermal field (Petracco and Squarci,1975), the decision w a s taken to study the possibility of recharging it artificially by reinjecting the waste f m the geothemlectric pier-plants back underground in s a suitable points of the field itself. This procedure w l d also solve the problem of getting rid of these effluents whose ccmposition is such that they cannot be discharged into the runoff waters or the shallm aquifers.

The reasons for undertaking a reinjection prog r m at Larderello, the problems connected

with this programne and the results f m n the engineering viewpoint are described in a p p r by Giavannoni et al. (1981). Enviromtal isotopes as natural tracers In scme geothermal fields tritiated water has been used as a tracer in reinjection studies, as in The Geysers (Gulati et al. ,1978) and at

In a vapur-dcsninated field such as that of

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.

Ahua&apan (Einarsson et al. ,1975) As outlined by Gulati et al., t h i s method has its negative as well as positive aspects. Among the former are the fact that the method requires expensive and time-consuming enrichrent analyses and that its application destroys the natural tritium balance in the reservoir.

interference entails, including those to the environment. The only disturbance i s thus that braght on by reinjection itself. The sampling and analysis techniques can be carried out as mutine field operations, W c h are easy to apply and cheaper than tritium masurenwts

.

!Che natural tritium abundance at Larderello

bbeover,this mthcd., as o p p s e d to the pulse techniques, pennits us to mnitor the system systemtically throughout the reinjection test, so that we can imdiately obtain informtion on any variations underqone by processes occurring in the reservoir. Theoretically sans negative aspects could exist in this mthcd.There could be an isotopic exchange between the water and the rocks, but this phenmon, if it did take place, would affect the oxygen only. Again, an isotopic re-equilibration could take place between the water and gas species, but t h i s is a theoretical possibility only, as the latter represents only 2.8 mles percent of the geothermal fluid at Larderello and could, therefore, have little effect on the isotopic ccmposition of the water. Furthemre, 90% of the gas is CO2, so that the hydrqen muld, again, be unaffected.

has been, and is still, used in studies of field recharge by meteoric waters (Panichi et al.,1974,1978). Another tracer mthod had thus to be adopted to avoid ccanprcmising these studies. Enviromtal tracers have, on the other hand, been used for s m time now in geological studies in mrit of their being natural tracers. The isotopic abundances of oxygen-18 and deuterium in the waters have helped to solve m y hydroseological problems. Problems m r e closely tied to an exploited geothermal field have also been investigated by means of the stable isotopes. During recent years especially, scam interesting conclusions have been reached on subjects such as deep tmpratures, physical state of the water and origin of sam? ccanponen"^ of the geothermal fluid ( N o t o et al., 1979; Panichi et a1.,1979; Nuti et al.,1980).

The last, and m s t serious, negative aspect is the possibility of an isotopic fractionation of the water during phase chanqe.

In the reinjection experimnt now under way at Larderello, in an area with high temperatures and pressures of about 5 ata, the steam f m n the mnitored productive wells had a canpsition ranging betwen -1.5 and -3 in 6 l b and -37 to -42 in 6D. All the d values given in t h i s paper refer to differences permil fran VIENNA-SKW, the international isotope standard for waters defined by the International Atanic Energy Agency of Vienna (Gonfiantini, 1978).

Results of the tracing experiment The injection well chosen for the experirrwt, W, lies in a central area of the field, where exploitation has been under way for m r e than 20 years but temperatures have ranained high (mre than 248C in the reservoir). The mitored wells are distributed all round well W, at distances of 150 to m r e than 700 metres (Fiq.1) The isotopic capsition of the steam produced by the wells in this zone is nearly uniform, so that it wuld be only sliqhtly affected by any chanqe in the oriqinal fluid flcw pattern ensuing frcm reinjection.

.

After leaving the turbines the steam is condensed, and the condensed water passes to the cooling tawers where a considerable fraction is lost to the amsphere in the form of vapour. Because of t h i s process the residual water is greatly enriched in the heavy isotopes oxygen-18 and deuterium. Throughout the experimnt the isotopic ccmposition of the injected water was in fact m r e or less constant and 6 D ,both beinq with respect to 6% near to a value of +5, and far f m the value of the "undisturbed" wells.% the discharge water is traced naturally by its stable isotopic canposition and no artificial or radioactive tracers need be added to t k system, avoiding all the negative effects t h i s type of

In the first phase of this experbent a total of 30-35 m3/h of water was injected for about 3 mnths, 105 m3/h for about 20 days and 50 m 3 h for another 3 mnths

.

After iiijection kegan the isotopic composition of the fluid in the mnitored wells shifted tawards the positive values of the reinjected water, and kegan decreasing when reinjection was reduced to XI m3h.

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wells. 0

0

II!.

O

.

O0 w1 ".'.wo

3

0 w2 0

0

0

0

.

0

0 0 0

0

Figure 3 shaws the trend of the isotopic cc*nposition of the fluids produced in wells W2 and ~7 during t h i s phase of reinjection, in a 6 ~ 6l80 diagram. The high linear correlation CGefficients and the position of the points along straight lines joining the "undisturbed" canposition of the wells to that of the reinjected w a t e r suggest that a s-le mixing process has W e n place, with no disturbing m a n e n a related to isotopic exchange or fractionatim. The fact that no isotopic fractionation has occurred suggests that the vaprization process does not produce two phases but a steam phase only, i.e., every portion of the injected water participating in the boiling process is ccmpletely vaporized.

0 500m

I

Figure 1 Location of the wells in the area west of Larderello affected by the reinjection test. 1)reinjection well; 2)production wells; 3) shut-in wells.

In these conditions the m u n t of injected water returning in the fluid produced by the wells can be calculated by the follming balance equation:

Once the test ended the canposition returned to its earlier "undisturbed" values,while well KI started producing steam whose isotopic canposition was slightly m r e positive than that of the nearby wells (Fig.2). The deuterium shmed the sam behaviour as the oxygen-18. This alone SWS that at least part of the injected water vaporizes and that t h i s steam joins the fluid produced by the surrounding

(6, - 6,/6, -6,

1, where G is the flaw-rate, I, FiH and R are subscripts rep resenting the fluid caning f m injected water, fluid produced by the wells and original fluid respectively.

G~ =

Figure 4 s h m the the spatial distribution of the fraction of injected water versus the to-

8

8'

'

- 44 8 wo 0 WI

fU 100.

; F 50-

0 w2 0 w7 0 WO -I

x- c 5%

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(1niect.d water)

(Delirored fluid)

tal fluid prcdud by the wells abut 20 days before the test ended.

The fact that the wells m s t affected by reinjection lie along a NW-SE aligrrment, the lack of correlation between the distance frcm the reinjection well to the others and the contribution of reinjected water in their steam show that the injected water has found preferential pathways in the underground. This consequently confirms that the reservoir cannot be c a p x e d to a hamgeneous and iso-

tropic porous rredium.

In conclusion, artificial tracers are unnecessary in vapur-dcmhated fields with certain favourable conditions, as the stable isotopic ccsnposition of the reinjected water already acts as a natural tracer. By mnitoring the isotopic capsition of the condensate of the productive wells in the area affected by reinjection we were able to ascertain that the injected water vaporizes and contributes to production, to calculate the m u n t of injected water re-entering the surrounding wells and to individuate the preferential pathways taken by t h i s water in the underground.

Injected water

I

-2

W2

r-.99Q

0 W7

r - ,998

I

I

I

0

2

4

6"O

VSY

SMOW

Figure 3 Variation in isotopic ccmposition, in a 6 D - 6l80 diagram, of the condensate of the fluid produd by wells W2 and W7 during the reinjection test.

In our opinion these results prove that tritium is not always the best tracer for reinjection studies in vapur-dcminated geothermal systems. References

0

0

Einarsson,S.S.,Vides,A. and Cuellar,G. (19751, "Disposal of Geothermal Waste Water by Reinjection" .Proc.2nd U.N.Symp. ,San Francisco,Vol.2, 1349-1363.

110

Giovannoni,A.,Allegrini,G. ,Cappetti,G. and Celati,R. (1981),"First Results of a Reinjection Ekprimnt at Larderello". Proc. 7th Workshop Geothermal Reservoir Engineering, Stanford, 16-18 December,1981. Gonfiantini,R. (1978),"Standards for Stable Isotopes Measurements in Natural Cmpunds". Nature, Vo1.271, 534-536. Gulati,M.S., Lip,S.C. and Str&l,C.J.(1978), 'Tritium ?kacer survey at The Geysers". Geothermal Resources Counci1,Trans. 2,237-239. Noto,P.,Nuti,S.,Panichi,C. and Gonfiantini,R. (1979) "Enviromntal Isotopes in Geotheml Water Investigation". Prcc.2nd Workshop on Isotopes in Nature, Leipzig.

Figure 4 Contribution of injected water to production. 1),2) and 3) as in Fig.1; 4) prcentage ratio of injected water recovered as steam to the total fluid prcdud by the wells.

Nuti,S., Noto,P. and Ferrara,G.C. (1980)"The System H20 c02-cH4 -H at Travale, Italy: 2

-88-

Tentative Interpretation".Geothermics,Val.93/4,287-296.

PdCkirC. r Nuti,S. and NotorP. (1979)r ' T J S e Of Isotopic -ters in the Larderello Geo1Field". In: Imtop Hydrolw 1978, v01.2, IAFA, Vienna, 613-630.

Panichi,C.,Celati,R.,Noto,P.,Squarci,P.,Taffi, L. and Tongiorgi,E. (1974)"oxYgen and Hydrogen Isotope Studies of the Larderello,Italy, Geothermal Systgn". In:IsOtope T e c h n i w s in Groundwater Hyitmlogy ,Vol.2, lEAE, Vienna , 328.

Petracco,C. and Squarci, P. (1975),"yrdrological Wane of Larderello Geothermal Fegion". Proc. 2nd U.N. Symp. M the Developnent and U s e of Geothermal ~esaurces,San Francisco, VOl.1, 521-530.

Panichi,C. ard Gonfiantini,R. (1978)"Environmtal Isotopes Fn Geothermal Studies". Geothedcs, Vo1.6-3/4,143-161.

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T

W

W WOLUTION OF THE COMPOSITION OF THE FLUID FRO4 SERRAZZANO ZONE (LARDERELLO)

Franco D'Amore,Claudio Calore and Romano Celati Istituto Internazionale per le Ricerche Geotermiche (CNR) Via del Buongusto 1, 56100 Pisa, Italy Abstract A temporal evolution in fluid composition has been noted during exploitation of the Serrazzano geothermal area, on the northwestern margin of the Larderello field. Strong variations have been recorded in the contents of NH3 ,H3B03 , HC1 and the uncondensable gases. The largest variations were recorded in the 1955-70 period, when new boreholes were being drilled.

(Calore et al. ,1980) has now been studied from the pint of view of evolution of fluid composition during about 40 years of production. This paper describes the first attempt at defining a conceptual model of Serrazzano reservoir capable of explaining the observed fluid composition history. We assume that, during exploitation, different steam sources contribute, to varying degrees, to fluid production.

One possible explanation of these changes may lie in the fact that the fluid discharged from the wells comes from diverse sources and that exploitation has gradually modified the contribution from each of these sources.

Evolution of steam composition at Serrazzano We consider a limited zone, including the old densely drilled area and the productive wells north of it (Fig.1). The geological features are well known (Calore et a l . ,1980).

Introduction During the last few years there has been an increase in interest in time and space variations of fluid composition in vapour-dominated fields. The studies of fluid composition at Larderello and The Geysers have already shown interesting possibilities of the application of geochemical methods in field development and reservoir engineering (Celati et a1.,1973; Panichi et a1.,1974; D'Amore et a1.,1977; Truesdell et a1.,1977; Truesdell and Nehring, 1978; Mazor, 1978; D'Amore and Truesde11,1979; Calore et al.,1980).

i

09

loo

O8

The long production histories of Larderello reveal different trends of fluid composition in different areas of the field, both as regards space distribution and time evolution. Space distribution appears to be controlled by two major phenomena: 1) mixing of original reservoir fluid with recharge waters and 2)gradual condensation of steam flowing from vaporization zones towards the field boundaries.

0

7

60

04

5O 0

-

Time variations observed in some old wells of the central area of Larderello have been interpreted as the consequences of changes in the contribution of different steam sources to fluid production (D'hore and Truesdell,l979).

0

100 200

300m

3 02 01

Figure 1 Producing wells in Serrazzano area Serrazzano area, where space distribution appears to be controlled by steam condensation

Wells nos.1 to 6 were drilled in the period

-91-

I Figure 2

"Y.

Gas/steam ratio in the steam of the producing wells of Serrazzano area (litres S.T.P./kg)

-.

0

Io. ID..

m.

j@ Figure 3

.

...

.

..

H3BOg in the steam of the producing wells of Serrazzano area (ppm)

I

I

Figure 4

NH

3

in the steam of the producing wells of Serrazzano area (ppm)

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1928-1941, with a very reduced spacing, close to surface manifestations. From 1941 to 1954 no new wells were drilled in the area; wells 7,8 and 9 started production between 1954 and 1957 and well 10 in 1966. The gas/steam ratio, boric acid and amnonia concentrations are generally available from 1940 on, while other chemical and isotopic analyses are relatively recent in this area. Figures 2,3 and 4 show the changes with time of the gas/steam ratio and boron and ammonia in wells 2 to 9. Figure 5 shows the available C1-data for the wells in which its concentration was detectable.

: 1CIPPm 10

:I,, 2

50

4

I I , I I I ,

60

years

70

80

I

Figure 5 C1- (ppm) in the steam of the wells at Serrazzano.Wells not reported in the figure have undetectable C1- concentrations. By smoothing the data some definite trends appear and in different wells show some correlation. Minor features are apparent in the gas/ steam ratio only, as no boric acid and ammonia data are available for certain periods. Discussion The study of space variations in fluid composition throughout Serrazzano area led to a conceptual model for this part of the field (Calore et a1.,1980) that has now been tested to explain the time changes. A sketch of this model is shown in Fig.6. A steam source is assumed to exist in a high

temperature (about 27OoC) zone east of the small area of our present study. This steam initially flowed towards the north, north-west boundaries, condensing on its way. The condensation should have been very effective near the 'cold walls' at the top of the reservoir and on the northern boundary, where there are low permeability formations, with the added probability of being cooled by a limited meteoric water infiltration through the ophiolitic

Figure 6 Conceptual model of Serrazzano reservoir. rocks outcropping north of the study area. Along the vertical we can distinguish three different portions of the reservoir: 1) upper condensation zone, A, characterizad by high fracture-derived permeability (Celati et a1.,1975), high initial liquid saturation and a fluid very rich in boron and poor in ammonia and COz; 2) intermediate two-phase zone, F, with a lower permeability and low water saturation, capable of producing a fluid rich in CO2 and NH 3 and poor in H3B03; 3) lower liquid-saturated zone, C ,whose existence is documented by the wells in the northern area and is inferred elsewhere; the liquidtwo-phase boundary descends towards the south, so that the continuous liquid phase is close to the bottom of the wells in the northern part, gradually moving away as we move southwards. We assume this liquid is made up of a high salinity water, surmounted by a layer of condensate, thinning from north to south; the effect of this condensate probably becomes negligible on the southern edge of the densely drilled zone. Both in A and F, NH3 and CO2 gradually increase and H3B03 decreases as we move towards the north. The deep liquid is assumed to be generally very poor in COz, boron and annnonia,but boron and ammonia increase towards the south where the brine contribution becomes significant. Prior to 1954 only wells 1 t o 6 were producing. During the second world war, from 1941 to 1946, when production probably decreased, some condensation occurred in the reservoir and the gas/steam ratio continually increased in wells 3 to 6. In 1946 production returned to normal and in a few years the fluid composition re-

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turned more or less to pre-war ya1ues:the gas acmlation was depleted and the condensate deposited vaporized. Boron and ammonia data are scarce for this period; however, a decrease in boron and a contemporaneous increase in ammonia are clear in a few wells.

more boiling in the old zone too, and interrupted the previous flow of fluid from the north, so that a fast decrease also took place in the gas/steam ratio. The effects of the new wells were felt first in the upper zone A, due to its high permeability. Unfortunately boron and amnonia data are missing for this period.

Production in this period came mainly from the upper section A of the reservoir. Before 1954 the gadsteam ratio was increasing, probably due to an increase in the contribution to fluid production from section F and from the north. This trend was halted by the start of production in new wells (no.?' in 1954, no.9 in 1955, n0.8 in 1957). Production from these wells was more than total prhction from the wells producing previously in the area. There was a strong interference between the new and old wells as shown in Fig.7: reservoir pressure is not available for the period 1954-1957 but the sharp decrease in total flow-rate of wells 2, 4 , 5 and 6 clearly shows this interference.

The flow pattern continued to vary in the reservoir: the contribution from the two-phase intermediate zone F increased in all the wells, thus increasing the gas/steam ratio. The behaviour of the northern (nos.6,7,8 and 9) and southern (nos.1,2,3,4 and 5) wells differed in this period. For the northern wells the two-phase zone F has a limited thickness, the deep,continuous liquid phase is close to the bottom of the wells and becomes rapidly the main source of the fluid produced. The steam generated at the top of the deep liquid phase is very poor in gas, boron and ammonia, so that the concentrations of all these components decrease in time.

The flow pattern in the reservoir was completely altered; the new wells exhibited a large and rapid decrease in gas/steam ratio and ammonia as they drained fluid from the south and the pressure decrease induced liquid boiling. The interference from the new wells brought on

I

A

For the central wells zone F eventually becomes the main source of fluid, with a minor contribution also from C. Thus, while boron decreases continuously in the period of decreasing A contribution, ammonia increases with the in-

gas steam

l

pressure

l

Figure 7 Total steam flow-rate and average gas/steam ratio of wells nos.2, 4, 5 and 6 . Reservoir pressure and the date of starting production in new wells are also reported.

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crease in contribution from F and C.

chemical history of Serrazzano. Tne space variations of the F sources are consistent with a process of condensation of steam flowing from ESE to "I"I (Fig.8)

In the southernmost wells the contribution from the brine is responsible for the relatively high NH3 content and for the appearance or increase in C1-.

.

These processes are accelerated by the entry into production of well 10 and its consequent interference to already producing wells. In Fig.7 this interference is apparent both in the trend of reservoir pressure (measured in a shut-in well) and in the production of the old wells: both start decreasing after a period of rough stabilization. The contribution of deep liquid continues to increase, with a decrease in the gas/steam ratio in the majority of the wells and essentially a levellingoff of boron and ammonia.

I C co

I

I

Figure 8 Space distribution of the assumed concentration changes of H3BO3, Mi3 and C02 in steam source F. Figures 9 and 10 show the variations in time of the contributions from the different sources, along with the experilental data (dots) and model results (circles) for two wells characteristic of northern and southern zones.

The three-sources model applied by D'Amore and Truesdell(1979) to some wells of the central area of Larderello has been applied to the wells at Serrazzano. This model assumes that three sources, A, F and C, each delivering a fluid of constant composition, contribute to the production of each well. The three sources differ from one well to another.

These simple models roughly confirm the hypotheses outlined above. Obviously, these models are not completely adequate to describe the well behaviour in this area.In fact, they totally ignore horizontal flow of fluid in the reservoir. Strictly speaking, only a distributed parameter model would be capable o f simulating this system adequately.

The chemical characteristics attributed to the fluid coming from the different sources (Table 1) and the variations in their relative contribution to well production were chosen so as to be consistent with the conceptual model just described and to give a good match of the geo-

Table 1 Concentrations in the steam delivered by the sources A, F and C. Hell

Source

Gaslsteam

H3B03

(litres STP/kg) No.8

N0.7

N0.4

No.2

A F C

75.O

(PPI

NH3

(PPI

HC1

(PF)

400

0.0

375 20 55

A F C

17.5 52.0 0.0

425 40 65

300

0 0

50

0

A F

500 120 85

65 230

C

12.o 30.0 0.0

0 0 5

A

12.o

500

F C

22.5 0.0

170 110

40 200 150

22.5

-95-

125 40

110

100

0 0 0

0 0

15

6 4 2 0

e 300

.. .-. . .... 0

0

..

.

.o.

200

loo 200

sa

Q...

mc

*.......9

0 '

.'. e

le

.0..* . . d' . .. .

If Id

Gaslsteam ( I I k g ) % Contribution

Upper leyer

7

T ~ p O h e layer ~

8c

7

Upper layer

condensate

0

4c

0-

45

50

55

60

BS

m

55

80

65

70

years Figure 10 rktching of experimental data of well no.7 with a three-source model and contribution of the three sources.

Brlne

1935 4(r

I950

75

years Figure 9 Matching of experimental data of well no.2 with a three-source model, and contribution of the three sources.

and Isotopic Composition of the Geothermal Fluids in Larderello Field".Geothermics,Vo1.5, 153-163. D'Amore,F. and Truesdel1,A.H. (1979)'Nodels for Steam Chemistry at Larderello and The Geysers". Proc.5th V!orkshop Geothermal Reservoir Engineering,ll-14 December,1979,283-297.

References Calore,C.,Celati,R.,D'Amore,F.,Squarci,P. and Truesdel1,A.H. (1980),"A Geologic,Hydrologic and Geochemical bbdel of the Serrazzano Zone of the Larderello Geothermal Field". Proc.6th Workshop Geothermal Reservoir Engineering (H. J .Ramey,Jr and P .Kruger ,Eds.) ,Stanford,l6-18 December,1980, 21-27.

iYazor,E. (1978) ,"Noble Gases in a Section Across the Vapor-dominated Geothermal Field of Larderello,Italy".Pure and Applied Geophysics,Vol. 117,262-275.

.

Panichi,C.,Celati,R.,Noto,P.,Squarci,P.,Taffi, L. and Tongiorgi,E. (1974) ,"Oxygen and Hydrogen Isotope Studies of the Larderello (Italy) Geothermal System". In:Isotope Techniques in Groundwater Hydrology,l974.IAEA,Vienna,Vol.2 , 3-28.

Celati,R. ,Noto,P.,Panichi,C.,Squarci,P. and Taffi,L. (1973) , "Interactions between the Steam Reservoir and Surrounding Aquifers in the Larderello Geothermal Field".Geothermics, V01.2-3/4,174-185. Celati,R.,Neri,G.,Perusini,P. and Squarci,P. (1975) , "An Attempt at Correlating kh Distribution with the Geological Structure of Larderello Geothermal Field". Geothermal Reservoir Engineering,Stanford,Geothermal Program, 15-17 December,1975, 37-41.

Truesdell,A.H. ,Nehring,N.L. and Frye,G.A. (1977) "Steam Production at The Geysers,California, Comes from Liquid Water Near the Well-bottom". (Abs)Geol.Soc.Am.Abstracts with Programs,Vo1.9, 1206. Truesdel1,A.H. and Nehring,N.L. (1978) ,"Gases and Water Isotopes in a Geochemical Section across the Larderello,Italy,Geothermal Field". PAGEOPH, V01.117, 276-289.

D'Amore,F.,Celati,R.,Ferrara,G.C. and Panichi, C. (1977) ,"Secondary Changes in the Chemical

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SUMMARY OF HOT DRY ROCK GEOTHERMAL RESERVOIR TESTING 1978 TO 1980

Z. V.

Dash and H. D. Murphy ( E d i t o r s )

Los Alamos N a t i o n a l L a b o r a t o r y Los Alamos, NM 87544

A b s t r a c t Experimental r e s u l t s and re- eval ua-the Phase I Hot Dry Rock Geothermal Energy r e s e r v o i r s a t t h e Fenton H i l l f i e l d s i t e a r e summarized. This report traces r e s e r v o i r growth as demonstrated d u r i n g Run Segments 2 t h r o u g h 5 (January 1978 t o December 1980). R e s e r v o i r growth was caused n o t o n l y by p r e s s u r i z a t i o n and h y d r a u l i c f r a c t u r i n g , b u t a l s o by heat e x t r a c t i o n and thermal contraction effects. Reservoir heat- transfer area grew from 8000 t o 50 000 m2 and r e s e r v o i r Desf r a c t u r e volume grew from 11 t o 2663m p i t e t h i s r e s e r v o i r growth, t h e water l o s s r a t e increased o n l y 30%, under s i m i l a r press u r e environments. For comparable temperature and pressure c o n d i t i o n s , t h e f l o w impedance ( a measure o f t h e r e s i s t a n c e t o c i r c u l a t i o n o f water t h r o u g h t h e r e s e r v o i r ) remained essent i a l l y unchanged, and i f reproduced i n t h e Phase I 1 r e s e r v o i r under development, c o u l d Geochemical and r e s u l t i n " s e l f pumping." seismic hazards have been n o n e x i s t e n t i n t h e The produced water i s Phase I r e s e r v o i r s . r e l a t i v e l y l o w i n t o t a l d i s s o l v e d s o l i d s and shows l i t t l e tendency f o r c o r r o s i o n o r s c a l ing. The l a r g e s t microearthquake a s s o c i a t e d w i t h heat e x t r a c t i o n measures l e s s t h a n -1 on t h e e x t r a p o l a t e d R i c h t e r scale.

Run Segment 3 (Expt. 186), t h e Hiqh BackPressure F1 ow Experiment (Brown, i n preparat i o n ) was r u n d u r i n g September and October 1978 f o r 28 days. The purpose o f t h i s e x p e r i ment was t o e v a l u a t e r e s e r v o i r f l o w c h a r a c t e r i s t i c s a t h i g h mean- pressure l e v e l s . The h i q h back pressure was induced by t h r o t t l i n g t h e p r o d u c t i o n w e l l . F o l l o w i n g Run Segment 3, t h e EE- 1 c a s i n g was recemented near i t s c a s i n g bottom t o p r e v e n t leakaqe o f f l u i d i n t o t h e annulus. An e n l a r g e d r e s e r v o i r was t h e n formed b y e x t e n d i n q a h y d r a u l i c f r a c t u r e from an i n i t i a t i o n depth o f 2.93 km (9620 f t ) i n EE-1, about 200 m deeper t h a n t h e f i r s t f r a c t u r e i n EE- 1. The f r a c t u r i n g was conducted i n March 1979, w i t h two f r a c t u r i n g experiments. These experiments a r e r e f e r r e d t o as "massive" hyd r a u l i c f r a c t u r i n g (MHF) Expts. 203 (March 14) and 195 (March 21). P r e l i m i n a r y e v a l u a t i o n o f t h e new r e s e r v o i r was accomplished d u r i n g a h e a t - e x t r a c t i o n and reservoir23- day assessment experiment t h a t began October 23, 1 9 7 9 (Murphy, 1 9 8 0 ) . T h i s segment o f o p e r a t i o n w i t h t h e EE-l/GT-2B w e l l p a i r was Run Segment 4, o r Expt. 215.

.

The l o n g - t e r m r e s e r v o i r c h a r a c t e r i s i c s were i n v e s t i g a t e d i n Run Segment 5, o r Expt. 217, which began March 3, 1980 ( Z y v o l o s k i , 1981). Because o f t h e l a r g e s i z e and r e s u l t i n q slow thermal drawdown, a l e n q t h y f l o w t i m e o f 286 days was necessary t o e v a l u a t e t h e r e s e r v o f r. Run Segment 5 ended w i t h t h e 2-day S t r e s s U n l o c k i n g Experiment (SUE) (Murphy, 1981).

Introduction The HDR r e s e r v o i r s a t Fenton a r e l o c a t e d i n t h e Jemez Mountains o f n o r t h e r n New Mexico. The r e s e r v o i r s were formed between two w e l l s , GT-2B and EE- 1, d r i l l e d i n t o h o t , low p e r m e a b i l i t y rock and h y d r a u l i c a l l y fractured. Reservoir performance was f i r s t e v a l u a t e d by a 75-day p e r i o d o f c l o s e d - l o o p o p e r a t i o n from January 28 t o A p r i l 13, 1978 ( T e s t e r and A l b r i g h t , 1979). The assessment o f t h i s f i r s t r e s e r v o i r i n EE-1 and GT-2B i s r e f e r r e d t o as "Run Segment 2" o r t h e "75- day t e s t . " (Run Segment 1 c o n s i s t ed o f a 4-day p r e c u r s o r experiment conducted i n September 1977.) Hot water from GT-2B was d i r e c t e d t o a w a t e r - t o - a i r heat exchanger where t h e water was cooled t o 25°C b e f o r e reinjection. Makeup water, r e q u i r e d t o r e p l a c e downhole l o s s e s t o t h e r o c k s u r r o u n d i n g t h e f r a c t u r e , was added t o t h e cooled w a t e r and pumped down EE- 1, and then t h r o u g h t h e f r a c t u r e system. Heat was t r a n s f e r r e d t o t h e c i r c u l a t i n g water by thermal conduction t h r o u g h t h e n e a r l y impervious rock a d j a c e n t t o t h e f r a c t u r e surfaces. Hill

I n t h e t h r e e y e a r s d u r i n g which t h e s e r e s e r v o i r t e s t s were conducted, o u r understanding o f r e s e r v o i r b e h a v i o r has s t e a d i l y improved. S i m p l i f i e d models t h a t were developed f o r Run Segment 2 were s i g n i f i c a n t l y m o d i f i e d by t h e t i m e o f Run Segment 5. Consequently, t h e p r e v i o u s t e s t s were reanalysed i n a c o n s i s t e n t manner u s i n g t h e l a t e s t models. Further, t h e growth of t h e r e s e r v o i r w i t h t i m e was t r a c e d and p e r i o d s o f growth a t t r i b u t e d t o thermal c o n t r a c t i o n and h e a t e x t r a c t i o n e f f e c t s were i d e n t i f i e d as apposed t o qrowth caused by p r e s s u r i z a t i o n and h y d r a u l i c f r a c t u r i n g . H e a t P r o d u c t i o n and H e a t - t r a n s f e r M o d e l i n g H e a t - t r a n s f e r model i n g o f t h e r e s e r v o i r s has been p e r f o r m e d w i t h t w o n u m e r i c a l m o d e l s . Both models use twodimensional s i m u l a t o r s i n

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n e t area e f f e c t i v e i n h e a t exchange was o n l y 8000 m'.

which heat i s t r a n s p o r t e d b y c o n d u c t i o n w i t h i n t h e r o c k t o t h e f r a c t u r e s . The most r e c e n t l y developed model ( t h e m u l t i p l e - f r a c t u r e model) assumes t h a t t h e r e s e r v o i r c o n s i s t s o f t h r e e p a r a l l e l f r a c t u r e s i d e a l i z e d as r e c t a n g l e s i n which t h e f l o w i s d i s t r i b u t e d u n i f o r m l y a l o n g t h e bottom o f each f r a c t u r e and withdrawn u n i f o r m l y across t h e top. The f l o w i s t h u s o n e - d i m e n s i o n a l , and t h e s t r e a m l i n e s a r e straight vertical lines. Consequently f l u i d dynamic c o n s i d e r a t i o n s do n o t d i r e c t l y e n t e r i n t o t h e h e a t - e x t r a c t i o n process, t h e sweep e f f i c i e n c y i s i m p l i c i t l y assumed t o be 100%. However, a r i g o r o u s t w o - d i m e n s i o n a l h e a t conduction s o l u t i o n i s incorporated f o r t h e r o c k between t h e f r a c t u r e s , and t h i s p e r m i t s Val i d c o n s i d e r a t i o n o f thermal - i n t e r a c t i o n e f f e c t s between t h e f r a c t u r e s . I n contrast, t h e o l d e r model ( t h e i n d e p e n d e n t - f r a c t u r e s model), assumes t h a t t h e f r a c t u r e s (two i n number) a r e c i r c u l a r and a l l o w s p r o p e r l o c a l p o s i t i o n i n g o f t h e i n l e t and o u t l e t s , i.e., the point- like intersection o f the injection w e l l w i t h t h e f r a c t u r e can be modeled, as can t h e i n t e r s e c t i o n o f t h e main h y d r a u l i c f r a c t u r e s and t h e s l a n t i n g j o i n t s t h a t p r o v i d e t h e However, as was cauc o n n e c t i o n s t o GT-26. t i o n e d e a r l i e r , w h i l e t h e f l u i d dynamic e f f e c t s o f t h e j o i n t s / o u t l e t s can be f a i t h f u l l y modeled, t h e h e a t - t r a n s f e r e f f e c t o f t h e j o i n t s cannot; t h e area o f t h e j o i n t s must be I n view o f lumped w i t h t h e main f r a c t u r e s . t h i s more f a i t h f u l r e p r e s e n t a t i o n o f i n l e t and o u t l e t s , and t h e f a c t t h a t a complete twodimensional s o l u t i o n t o t h e Navier- Stokes f l u i d dynamic e q u a t i o n s i s i n c o r p o r a t e d , t h e i n d e p e n d e n t - f r a c t u r e s model r e s u l t s i n a more r e a l i s t i c assessment o f t h e e f f e c t o f f l u i d dynamics and sweep e f f i c i e n c i e s upon heat extraction. The p e n a l t y , however, i s t h a t i n t h e p r e s e n t twodimensional v e r s i o n o f t h e code, thermal i n t e r a c t i o n as t h e t e m p e r a t u r e waves i n t h e r o c k between f r a c t u r e s o v e r l a p cannot be r e a l i s t i c a l l y represented, as i t i s w i t h t h e m u l t i p l e - f r a c t u r e model.

D u r i n g Run Segment 3 ( t h e h i g h back- pressure experiment) thermal drawdown suqgested t h a t , a c c o r d i n g t o t h e i n d e p e n d e n t - f r a c t u r e s model, t h e e f f e c t i v e h e a t area was n e a r l y t h e same. However, f l o w r a t e ( s p i n n e r ) surveys i n GT-2 i n d i c a t e d t h a t because of t h e h i g h e r p r e s s u r e l e v e l most of t h e f l o w was e n t e r i n g GT-2B a t p o s i t i o n s t h a t averaged 25 m deeper t h a n d u r i n g Run Segment 2. In effect the reservoir It was f l o w paths were shortened about 25%. concluded t h a t w h i l e p r e s s u r i z a t i o n d i d indeed result i n p a r t i a l short c i r c u i t i n g o f the streamlines, i t also resulted i n a notable decrease i n impedance, which a f f o r d e d b e t t e r f l u i d sweep and b a t h i n g o f t h e r e m a i n i n g area. The r e s e r v o i r was e n l a r g e d d u r i n g t h e f r a c t u r i n g o p e r a t i o n s o f 1979, t h e MHF Expts. 195 F o r t h e i n d e p e n d e n t - f r a c t u r e s model and 203. t h e e n l a r g e d r e s e r v o i r i s p o r t r a y e d as two f r a c t u r e s , t h e o l d one o p e r a t i v e i n Run Segments 2 and 3, w i t h a new and l a r g e r one. The en1 arqed r e s e r v o i r was e v a l u a t e d d u r i n g Run Segment 4 and Run Segment 5. To summarize t h e Run Segment 4 s t u d i e s , i t was found t h a t t h e o l d f r a c t u r e had an e f f e c t i v e h e a t - t r a n s f e r area o f 15 000 m2 and t h e new f r a c t u r e had an The e f f e c t i v e area o f a t l e a s t 30 0002 m area determined i n Run Segment 4 f o r t h e o l d f r a c t u r e was a t l e a s t t w i c e t h a t determined i n Run Segment 2. T h i s t r e n d o f i n c r e a s i n g area i s now a t t r i b u t e d t o thermal s t r e s s c r a c k i n g e f f e c t s (Murphy, 1979).

.

B e t t e r e s t i m a t e s o f t h e t o t a l e f f e c t i v e heatt r a n s f e r area of b o t h f r a c t u r e s were o b t a i n e d i n Run Segment 5, d u r i n g which t h e thermal drawdown was o n l y 8°C. The mean o u t l e t temperature a c t u a l l y increased s l i g h t l y during t h e e a r l y p o r t i o n o f Run Seqment 5. This temporary i n c r e a s e i s due t o t r a n s p o r t o f deeper, h o t t e r w a t e r t o t h e p r o d u c t i o n w e l l , as w e l l as t o some i n t e r a c t i o n o f t h e f r a c t u r e s . For s i m p l i c i t y t h e e f f e c t was n e g l e c t ed i n t h e i n d e p e n d e n t - f r a c t u r e s model as i t i s f a i r l y small, l e s s t h a n 2OC. The d a t a a r e f i t v e r y w e l l b y a model w i t h a combined area o f 50 000 m2, some 5000 m' g r e a t e r t h a n t h e area t e n t a t i v e l y e s t i m a t e d d u r i n g Run Segment 4.

Independent- Fractures Model ing The f i r s t app l i c a t i o n of t h i s model was t o t h e e a r l v r e s e a r c h r e s e r v o i r , when o n l y a s m a l l s i n g l e hydraulic f r a c t u r e existed. This reservoir was t e s t e d e x t e n s i v e l y d u r i n g Run Segment 2. Based upon s p i n n e r and t e m p e r a t u r e surveys i n t h e p r o d u c t i o n w e l l , t h e depths o f t h e i n t e r sections o f the production well with the s l a n t i n g j o i n t s were e s t i m a t e d as w e l l as t h e f l o w r a t e s communicated b y each j o i n t . I n t h e c a l c u l a t i o n s , t h e a c t u a l temporal v a r i a t i o n s o f p r o d u c t i o n and i n j e c t i o n f l o w r a t e s were u t i l i z e d . With t h i s information, estimates o f t h e thermal drawdown were c a l c u l a t e d w i t h t h e model f o r v a r i o u s t r i a l v a l u e s o f f r a c t u r e r a d i i and v e r t i c a l p o s i t i o n o f t h e f r a c t u r e A f r a c t u r e r a d i u s o f 60 m w i t h an inlet. i n l e t l o c a t e d 25 m above t h e f r a c t u r e bottom r e s u l t e d i n a good f i t t o t h e measurements. A r a d i u s o f 60 m i m p l i e s a t o t a l f r a c t u r e area (on one s i d e ) of 11 000 m2; however, because of hydrodynamic f l o w sweep i n e f f i c i e n c i e s t h e

A summary of t h e heat- exchange areas d e t e r mined w i t h t h e i n d e p e n d e n t - f r a c t u r e s model i s presented i n Fig. 1. As can be s,een, a steady i n c r e a s e , from 8000 t o 50 000 m , i s i n d i c a ted. As i n d i c a t e d by t h e q u e s t i o n marks i n F i g . 1, t h e area i n c r e a s e due t o t h e MHF exp e r i m e n t s (195 and 203), i s u n c e r t a i n . The h e a t - t r a n s f e r area was n o t measured u n t i l t h e Consequently, l a t e r stages of Run Segment 4. t h e area i n c r e a s e measured i s due t o t h e comb i n e d e f f e c t s o f a l l t h e f r a c t u r i n g and Run Segment 4 o p e r a t i o n s , and cannot be i n d i v i d u a l l y ascribed t o t h e separate operations. Multiple- Fracture Modeling For t h e m u l t i p l e f r a c t u r e model t h e f o l l o w i n g procedure was used t o f i t t h e data.

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The i n i t i a l area o f 7500 m 2 was e s t a b l i s h e d b y many p r e s s u r i z a t i o n s and some c o o l i n g . This As area grew t o 1 5 000 m2 i n Run Segment 2. i n d i c a t e d e a r l i e r t h e h i g h back p r e s s u r e of Run Segment 3 caused a r e d i s t r i b u t i o n o f f l o w r e s u l t i n g i n f l u i d dynamic s h o r t - c i r c u i t i n g . However, u n l i k e t h e i n d e p e n d e n t - f r a c t u r e s model, t h e new model i n d i c a t e s t h a t t h e i n i t i a l heat- exchange area was a c t u a l l y l e s s t h a n t h a t o f Run Segment 2, s t a r t i n g a t 6000 m2; b u t i t t h e n grew t o 12 000 m 2 d u r i n g t h e 28day t e s t . The system was p r e s s u r i z e d t o h i g h p r e s s u r e s s e v e r a l t i m e s d u r i n g MHF Expts. 203 and 195 and Run Segment 4 b u t no area o r v o l ume measurements were made u n t i l Run Segfient 4. A f t e r Run Segment 4, t h e EE- 1 temperatur: l o g s i n d i c a t e d t h a t between 6000 and 9000 m had been added t o t h e l o w e r p a r t o f t h e r e s e r v o i r b y t h e recementing and p r e s s u r i z a t i o n p r i o r t o and d u r i n g Run Segment 4. This i n creased t h e measured heat-2exchange area t o The area measbetween 2 1 000 and 24 000 m urements d u r i n g Run Segment 5 a r e somewhat uncertain. The b e s t e s t i m a t e s a r e t h a t t h e heat- exchange area was q r e a t e r t h a n 45 000 m 2 a t t h e end o f t h e experiment. The l a c k o f recovery o f t h e o u t l e t temperature i n d i c a t e s t h a t t h e a d d i t i o n a l area i s i n t h e d e p l e t e d upper h a l f o f t h e r e s e r v o i r o r was p a r t l y added t o t h e l o w e r h a l f as Run Segment 5 p r o ceeded.

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F i g u r e 1 Heat t r a n s f e r area growth d e t e r mined b y t h e independent f r a c t u r e s model i n t h e Phase I r e s e r v o i r s d u r i n g Run Segments 2 through 5

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The measured GT-2B f l o w r a t e and e s t i mated r e s e r v o i r i n l e t t e m p e r a t u r e were programmed as f u n c t i o n s o f time. 0 The i n i t i a l f r a c t u r e area was a d j u s t e d t o o b t a i n t h e b e s t f i t a t e a r l y times. 0 The f r a c t u r e area was a l l o w e d t o i n c r e a s e so as t o p r o v i d e a good f i t t o t h e r e m a i n i n g data. F o r c o m p u t a t i o n a l simp l i c i t y , t h e area i n c r e a s e was assumed t o occur i n d i s c r e t e s t e p s r a t h e r t h a n i n a smooth, p i e c e - w i s e l i n e a r , f a s h i o n . As i n d i c a t e d e a r l i e r , t h e independentf r a c t u r e s model was n o t a b l e t o d e t e c t any i n c r e a s e i n t h e e f f e c t i v e h e a t - t r a n s f e r area d u r i n g a c t u a l drawdown, b u t t h e m u l t i p l e f r a c t u r e model i n d i c a t e s t h a t t h e h e a t - t r a n s f e r area i n c r e a s e d by a f a c t o r o f two. 0

T r a c e r S t u d i e s and F r a c t u r e Volume Growth The main o b j e c t i v e s o f r e s e r v o i r t r a c e r s t u d i e s a r e t o assess t h e volume changes a s s o c i a t e d w i t h t h e c r e a t i o n o f t h e Phase I system and t o d e t e r m i n e dynamic b e h a v i o r o f t h e system v o l ume as t h e system undergoes l o n g - t e r m h e a t ext r a c t i o n . The f r a c t u r e modal volume i s s i m p l y t h e volume o f f l u i d produced a t GT-2B between t h e t i m e t h e t r a c e r p u l s e was i n j e c t e d and t h e t i m e t h e peak t r a c e r c o n c e n t r a t i o n appeared i n t h e produced f l u i d . The w e l l b o r e volumes a r e s u b t r a c t e d from t h e t o t a l volume produced t o g i v e t h e t r u e f r a c t u r e modal volumes. The modal volume i s c o n s i d e r e d t h e most r e l i a b l e i n d i c a t o r o f r e s e r v o i r volume change. Large changes i n t h e modal volume a r e observed a f t e r t h e h y d r a u l i c f r a c t u r i n q o f t h e system between Run Segments 3 and 4 and d u r i n g t h e SUE, which f o l 1owed Run Segment 5.

S i m i l a r modeling was c a r r i e d o u t f o r Run Segments 3, 4 and 5. F i g u r e 2 summarizes t h e growth o f t h e heat- exchange area, a c c o r d i n g t o t h e m u l t i p l e - f r a c t u r e s model, t h r o u g h o u t Phase I. The general s i m i l a r i t y w i t h t h e summary o f t h e i n d e p e n d e n t - f r a c t u r e s model, F i g . 1, i s noted, b u t t h e r e a r e d i f f e r e n c e s i n d e t a i l . mnm MULTIPLE FRACTURE MODEL

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A complete r e v i e w o f t h e t r a c e r - t e s t d a t a f r o m Segments 2 t h r o u g h 5 has r e v e a l e d p e r t i n e n t i n f o r m a t i o n r e g a r d i n g t h e growth o f t h e r e s e r v o i r due t o h e a t e x t r a c t i o n and p r e s s u r i z a t i o n e f f e c t s . The r e s e r v o i r growth due t o heat ext r a c t i o n i s , t o be p r e c i s e , r e a l l y a t h e r m a l c o n t r a c t i o n e f f e c t -- as t h e r o c k s u r r o u n d i n g t h e f r a c t u r e s s h r i n k s , t h e f r a c t u r e s , and cons e q u e n t l y , t h e measured volumes, expand. In s p i t e o f n o n l i n e a r coupled e f f e c t s of thermal c o n t r a c t i o n , p o r e and f r a c t u r e i n f l a t i o n due t o s u s t a i n e d p r e s s u r i z a t i o n , and l o c a l i r r e v e r s i b i l i t i e s r e s u l t i n g i n f r a c t u r e propagat i o n , a s i m p l e c o r r e l a t i o n between A V and A E e x i s t s . Furthermore, t h i s s i m p l e r e l a t i o n s h i p p e r s i s t s even i n t h e presence o f t h e c o n f i n i n g stresses surrounding t h e a c t i v e reservoir,

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F i g u r e 2 Heat t r a n s f e r area growth d e t e r mined by model f i t s t o drawdown d a t a and w e l l b o r e t e m p e r a t u r e l o g s i n t h e Phase I r e s e r v o i r s d u r i n g Run Segments 2 t h r o u g h 5

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which i n d u c e a c o n s t r a i n e d behavior. For p r a c t i c a l purposes, t h e r e g i o n between t h e low- pressure d a t a and t h e f r e e thermal volume 1 i n e s d e f i n e s an envelope o f r e s e r v o i r operat i n g conditions. As s t r e s s e s a r e r e l i e v e d , f o r example d u r i n g SUE, o r t h e h i g h backp r e s s u r e t e s t o f t h e o r i g i n a l r e s e r v o i r (Run Segment 3 ) , o r t h e high- pressure, h y d r a u l i c f r a c t u r i n g s t a g e a t t h e b e g i n n i n g o f Run Segment 4, one moves away from t h e n o r m a l l y c o n s t r a i n e d c o n d i t i o n toward t h e f r e e thermal expansion l i n e .

o r i n E n g l i s h customary u n i t s , pounds p e r square i n c h p e r g a l l o n p e r m i n u t e (psi/gpm). Because p r e s s u r e s a r e u s u a l l y measured a t t h e surface, a "buoyancy" c o r r e c t i o n s h o u l d be made f o r t h e d i f f e r e n c e i n h y d r o s t a t i c pressures i n t h e h o t p r o d u c t i o n w e l l and t h e c o l d i n j e c t i o n well. The d e p t h a t which t h i s c o r r e c t i o n i s c a l c u l a t e d corresponds t o t h e b o t tom o f t h e i n j e c t i o n w e l l , t h a t i s , buoyancy i n s i d e t h e f r a c t u r e i s included i n t h e hot leg. Impedances o f about 1 GPa s/m3 a r e cons i d e r e d d e s i r a b l e . F o r example, i n t h e deeper and h o t t e r Phase I 1 r e s e r v o i r b e i n g completed now, such a l o w v a l u e o f impedance c o u l d a c t u a l l y r e s u l t i n "self-pumping" o f t h e r e s e r v o i r because o f buoyancy e f f e c t s .

Perhaps t h e most p r o m i s i n g aspect o f t h e t r a c e r tests i s t h e i r potential f o r estimating the e f f e c t i v e heat- transfer surface area o f a reservoir. T h i s becomes c l e a r when t h e modal volume ( p l o t t e d vs t i m e i n Fig. 3 ) i s compared t o t h e c o r r e s p o n d i n g h e a t - t r a n s f e r area ( p l o t t e d vs t i m e i n Figs. 1 and 2 ) ; t h e s i m i l a r i t i e s o f t h e growth o f area and volume a r e quite striking. T h i s can be q u a n t i f i e d b y c o n s i d e r i n g t h e r e l a t i o n s h i p s between area, volume, and a p e r t u r e ( o r e f f e c t i v e f r a c t u r e opening). The volume, V, i s s i m p l y t h e produ c t o f t h e area, A, and t h e mean a p e r t u r e , w: V = A w D u r i n g h e a t e x t r a c t i o n and/or p r e s s u r i z a t i o n , t h e area and a p e r t u r e can b o t h v a r y ; t h e r e f o r e t h e volume i s a f u n c t i o n o f t w o v a r i a b l e s r a t h e r t h a n one. For constant a p e r t u r e , t h e t r a c e r volumes s h o u l d s c a l e Further d i r e c t l y w i t h h e a t - t r a n s f e r area. development o f t h i s e m p i r i c a l c o r r e l a t i o n c o u l d p r o v i d e a d i r e c t and independent method of d e t e r m i n i n g r e s e r v o i r h e a t - t r a n s f e r area w i t h o u t r e q u i r i n g thermal drawdown, which i s t i m e consuming and expensive t o o b t a i n , p a r t i c u l a r l y so f o r t h e l a r g e r Phase I 1 r e s e r v o i r under development

F i g u r e 4 summarizes t h e impedance h i s t o r y o v e r Segments 2 t h r o u g h 5 and t h e SUE experiment. Impedance i s dependent on f r a c t u r e a p e r t u r e , w. T h e o r e t i c a l l y , i t decreases as l / w 3 i n b o t h l a m i n a r and t u r b u l e n t f l o w . A p e r t u r e may be (1) b y p r e s s u r i i n c r e a s e d i n s e v e r a l ways: zation o f t h e fracture, (2) coolinq o f the s u r r o u n d i n g rock, ( 3 ) d i s s o l u t i o n o f m i n e r a l s l i n i n g t h e c r a c k b y chemical t r e a t m e n t o f t h e f l u i d , and ( 4 ) b y g e o m e t r i c changes r e s u l t i n g f r o m r e l a t i v e displacement o f one f r a c t u r e .face w i t h r e s p e c t t o t h e o t h e r . Run Segments.2 and 3 were e s p e c i a l l y u s e f u l i n d e m o n s t r a t i n g t h e c o r r e l a t i o n between impedance and p r e s s u r e and temperature. The i m p e d a n c e c h a n g e s observed a f t e r SUE were p r o b a b l y due t o a d d i t i o n a l " s e l f - p r o p p i n g " caused by s l i p p a g e along t h e f r a c t u r e faces near t h e e x i t o r by o t h e r pressure- induced g e o m e t r i c chanqes.

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The c o n c e n t r a t i o n o f impedance near t h e e x i t , shown i n a l l t h e l o w back- pressure f l o w experiments, may be d e s i r a b l e when t h e system i m pedance i s reduced by m u l t i p l e f r a c t u r e s . In t h i s mode o f r e s e r v o i r development, t h e poss i b i l i t y o f u n s t a b l e "runaway" (one f r a c t u r e c o o l i n g and t a k i n g much o f t h e f l o w ) e x i s t s , and t h e e x i t i m p e d a n c e c o n c e n t r a t i o n w i l l p r e v e n t t h i s u n t i l r e s e r v o i r c o o l i n g has been extensive. E v e n t u a l l y , t h e problem o f f l o w c o n t r o l i n t h e i n d i v i d u a l f r a c t u r e s may a r i s e ,

.

Impedance C h a r a c t e r i s t i c s The impedance o f a c i r c u l a t i n g geothermal r e s e r v o i r i s u s u a l l y d e f i n e d as t h e p r e s s u r e d r o p between t h e i n l e t and o u t l e t o f t h e f r a c t u r e caused b y f l o w i n t h e fracture, divided by t h e e x i t volumetric f l o w r a t e . I t s u n i t s a r e pressure-s/volume, and i n t h i s r e p o r t we t y p i c a l l y use Giga Pascals p e r c u b i c meter p e r second (GPa s/m3)

4.0

-I

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2.3.0 . 2 P

a

1

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1078 CAlENDIR YEAR

1SW

18A

1878

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Figure 4 Flow impedance b e h a v i o r i n t h e Phase I r e s e r v o i r s d u r i n g Run Segments 2 through 5

F i g u r e 3 Growth of t r a c e r modal volume i n t h e Phase I r e s e r v o i r s d u r i n g Run Segments 2 throuqh 5

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and methods o f f l o w c o n t r o l near t h e f r a c t u r e e n t r a n c e may be r e q u i r e d . D u r i n g normal , l o w back- pressure c o n d i t i o n s f r a c t u r e impedance appears t o be c o n c e n t r a t e d near t h e e x i t w e l l , a t l e a s t a f t e r a short p e r i o d of o p e r a t i o n , and t o t a l impedance does n o t depend s t r o n g l y on w e l l b o r e s e p a r a t i o n . Impedances a r e s u f f i c i e n t l y l o w t o a l l o w opera t i o n o f e f f i c i e n t HDR g e o t h e r m a l e n e r g y e x t r a c t i o n systems. The impedance i n a l a r g e system does n o t change r a p i d l y , and t h e prognosis f o r operation o f t h e multiple- fracture, Phase I 1 system seems f a v o r a b l e . Water Losses The w a t e r l o s s o f an HDR system i s v e r y i m p o r t a n t because t h i s w a t e r must be p r o v i d e d from some o u t s i d e source. This i n f o r m a t i o n can be v i t a l f o r environmental as w e l l as economic reasons. The w a t e r - l o s s r a t e , t h a t i s , t h e r a t e a t which w a t e r permeates t h e rock formation surrounding t h e f r a c t u r e system, i s t h e d i f f e r e n c e between t h e i n j e c t i o n r a t e and t h e produced, o r recovered, This loss r a t e i s a strong r a t e a t GT-26. f u n c t i o n o f system p r e s s u r e s and f l o w r a t e and would a l s o be expected t o be a f u n c t i o n o f r e s e r v o i r size. The w a t e r - l o s s f l o w - r a t e d a t a o f each e x p e r i ment c o n t a i n many t r a n s i e n t s due t o o p e r a t i o n s h u t d o w n s , pump l i m i t a t i o n s , and v a r i o u s leaks. Consequently, t h e a c c u m u l a t i v e volume o f water l o s s i s b e s t s u i t e d f o r comparisons s i n c e many o f t h e t r a n s i e n t s a r e smoothed o u t , and t h i s c o m p a r i s o n i s p r e s e n t e d i n Run F i g u r e 5, f o r Run Segments 2, 3, and 5. Segment 4, o n l y 23 days long, was excluded from t h i s comparison because o f t h e d i s p a r a t e c o n d i t i o n s under which i t was conducted. Comparisons between Run Segments 2 and 5, b o t h conducted under normal, l o w back- pressure cond i t i o n s , can be made as f o l l o w s . D i r e c t comp a r i s o n s i n d i c a t e t h a t t h e w a t e r l o s s f o r Run Segment 5 i s a p p r o x i m a t e l y 40% h i g h e r t h a n t h a t o f Run Segment 2 a t comparable t i m e s a f t e r the beginning o f heat extraction. However, because t h e o p e r a t i n g p r e s s u r e was 10% h i g h e r d u r i n g Run Segment 5, t h e w a t e r @RUN SEGMENT 2 @RUN SEGMENT 2, SCALED ON PRESSURE @RUN SEGMENT 3

l o s s f o r Run Segment 2 s h o u l d be s c a l e d up by 10% as i n c u r v e 2 o f t h e f i g u r e , i n o r d e r t o be d i r e c t l y comparable t o Run Segment 5. Then i t i s seen t h a t t h e Run Segment 5 w a t e r l o s s i s o n l y 30% h i g h e r t h a n Run Segment 2, d e s p i t e a s e v e r a l - f o l d i n c r e a s e i n h e a t - t r a n s f e r area and volume. An obvious c o n c l u s i o n i s t h a t t h e heat- exchange system u t i l i z e s o n l y a small p o r t i o n of a much l a r g e r f r a c t u r e system t h a t c o n t r o l s water loss. This large, potential f r a c t u r e system was n o t a l t e r e d t o any l a r g e e x t e n t b y t h e MHF experiments o f Segment 4. Furthermore, i n comparison t o t h e heatt r a n s f e r areas, t h e s e o t h e r areas d i d n o t grow s i q n i f i c a n t l y from Run Segments 2 t h r o u q h 5. F l u i d Geochemistry Analysis o f t h e f l u i d c h e m i s t r y d a t a from t h e Phase I r e s e r v o i r s shows s e v e r a l i n t e r e s t i n g f e a t u r e s t h a t a r e pertinent t o the size o f the reservoirs. S t r o n g evidence f r o m each o f t h e Phase I heate x t r a c t i o n experiments i n d i c a t e s t h e e x i s t e n c e o f e s s e n t i a l l y t w o p a r a l l e l f l o w p a t h s : (1) a f r a c t u r e - d o m i n a t e d f l o w p a t h (perhaps c o n s i s t i n g o f m u l t i p l e fractures) t h a t includes t h e h e a t - t r a n s f e r s u r f a c e s , and ( 2 ) a h i g h impedance f l o w p a t h c o n s i s t i n g o f t h e connect e d m i c r o f r a c t u r e s and pores i n t h e r o c k s u r rounding t h e heat- extraction p o r t i o n o f t h e reservoir. Displacement o f t h e i n d i g e n o u s pore f l u i d c o n t a i n e d i n t h i s high- impedance f l o w p a t h i s t h e s i n g l e most i m p o r t a n t geochemical e f f e c t observed i n t h e h e a t - e x t r a c t i o n experiments t o date. This i s discussed f u r t h e r by G r i g s b y e t a l . (1981). I n summary, s e v e r a l c o n c l u s i o n s s h o u l d b e drawn f r o m g e o c h e m i s t r y r e s u l t s t o d a t e . F i r s t o f a l l , the overall circulating f l u i d q u a l i t y . i n a HDR system i s l a r g e l y f i x e d b y t h e p o r e - f l u i d c o n c e n t r a t i o n and displacement rate. Under t h e v e r y w o r s t c o n d i t i o n s ( t h a t i s , 100% o f t h e produced f l u i d i s pore f l u i d ) t h e maximum c o n c e n t r a t i o n o f d i s s o l v e d s o l i d s would be around 5000 mg/R f o r t h i s r e s e r v o i r w i t h i n t h e Environmental P r o t e c t i o n Agency (EPA) w a t e r q u a l i t y s t a n d a r d f o r c o n t i n u o u s However, i r r i g a t i o n o f salt- tolerant plants. t h e steady- state concentration o f t o t a l diss o l v e d s o l i d s i s t y p i c a l l y 2500 mg/R -- s i m i l a r t o w a t e r used f o r human consumption i n many p a r t s o f t h e c o u n t r y . The pH o f t h e w a t e r i s 6.5 f 0.5, n e a r l y n e u t r a l , and problems w i t h c o r r o s i o n o r d e p o s i t i o n upon s u r f a c e equipment such as p i p i n g , h e a t exchangers, and pumps have been minimal.

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A second conclusion from t h e fluidgeochemistry s t u d i e s concerns t h e v e r y l a r g e volume o f pore f l u i d t h a t has been d i s p l a c e d from t h e r o c k s u r r o u n d i n g t h e f r a c t u r e system i n t o t h e f r a c t u r e system. Because t h i s f r a c t u r e system i s everywhere p r e s s u r i z e d above h y d r o s t a t i c pressure, c i r c u l a t i n g f l u i d s h o u l d be c o n t i n u o u s l y l o s t t o t h e s u r r o u n d i n g mat r i x , which i s s u b h y d r o s t a t i c . P o r e f l u i d f r o m t h i s s u b h y d r o s t a t i c p r e s s u r e f i e l d would have t o f l o w against a pressure gradient i n order

TIME Idnp)

Figure 5 Cumulative water l o s s e s vs t i m e f o r Run Segments 2, 3, and 5

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c o n t r a s t w i t h t h e Run Segment 2 r e s e r v o i r t h a t e x h i b i t e d a sharp d e c l i n e i n t h e impedance, presumably due t o t h e l a r g e t h e r m a l drawdown t h a t t h e system experienced. Geochemical m o n i t o r i n g o f t h e system p r o v i d e d v a l u a b l e i n s i g h t c o n c e r n i n g p o r e - f l u i d displacement and flow connections i n t h e reservoir. The concentrations o f dissolved chemicals i n t h e produced w a t e r were r e l a t i v e l y l o w and t h e pH was near n e u t r a l , so t h e produced water was o f good q u a l i t y and problems w i t h c o r r o s i o n o r s c a l i n g of s u r f a c e equipment have been m i n i mal. Seismic a c t i v i t y i n t h e Phase I r e s e r v o i r s has been i n s i g n i f i c a n t . Events a s s o c i a t e d w i t h heat e x t r a c t i o n have measured l e s s t h a n minus one on t h e e x t r a p o l a t e d R i c h t e r s c a l e,

t o e n t e r t h e f l o w i n g system. However, seconda r y f l o w p a t h s w i t h impedance i n t e r m e d i a t e t o t h a t o f t h e main f r a c t u r e system and t h a t o f t h e u n f r a c t u r e d r e s e r v o i r rock p r o v i d e a means f o r t h e p r e s s u r e l e v e l i n t h e main f r a c t u r e ( s ) t o d i s p l a c e t h e p o r e f l u i d i n t o t h e f l o w system. F i n a l l y , t h e f l o w from these secondary p a t h s appears t o be p a r t i a l l y s e n s i t i v e t o t h e p r e s s u r e d i f f e r e n c e b e t w e e n t h e i n l e t and o u t l e t and p r o b a b l y , t o t h e o v e r a l l l e v e l o f pressurization o f the reservoir. Seismicity Seismic m o n i t o r i n g was conducted f o r a l l t h e run segments w i t h a s u r f a c e s e i s mic a r r a y and d u r i n g p o r t i o n s o f Run Segments 4 and 5, and SUE, w i t h downhole geophone packages p o s i t i o n e d i n t h e r e s e r v o i r v i c i n i t y . The o b j e c t i v e o f t h i s m o n i t o r i n g was t o e v a l u a t e p o t e n t i a l seismic r i s k s associated w i t h HDR geothermal energy e x t r a c t i o n . The l a r g e s t event d e t e c t e d i n Run Segment 4 w i t h t h e downh o l e package had a magnitude o f -1.5. The ene r g y r e l e a s e o f a -1.5 magnitude m i c r o s e i s m i c event i s r o u g h l y e q u i v a l e n t t o t h a t o f a 10 kg mass dropped 3 m. Furthermore, t h i s event o c c u r r e d d u r i n g t h e h i g h back- pressure stage. D u r i n g t h e low back- pressure stage, more t y p i cal o f ordinary heat- extraction conditions, t h e l a r g e s t event was -3. D u r i n g t h e 286-day Run Seqment 5, 13 microearthquakes r a n g i n g between -1.5 and 0.5 were recorded by t h e s u r f a c e seismic a r r a y . These events were l o c a t e d about 200 m n o r t h o f EE-2 a t a depth o f about 1 km. The events a r e n o t r e l a t e d t o Run Segment 5 a c t i v i t i e s , b u t r a t h e r t o t h e d r i l l i n g o f EE-2 and EE-3.

Acknowledgments T h i s work was supported by t h e D i v i s i o n o f G e o t h e r m a l E n e r g y , U.S. Department o f Energy. The c o n t r i b u t i o n s o f R. L. Aamodt, R. G. A g u i l a r , D. W. Brown, D. A. Counce, H. N. F i s h e r , C. 0. G r i g s b y , H. Keppler, A. W. L a u g h l i n , R. M. P o t t e r , J. W. T e s t e r , P. E. T r u j i l l o , Jr., and G. A. Zyvoloski t o t h i s summary r e p o r t a r e g r a t e f u l l y acknowledged. .References Brown, D. W. (ed.), " R e s u l t s o f Expt. 186, The H i g h B a c k - P r e s s u r e F l o w E x p e r i m e n t ,'I Los A1 amos N a t i o n a l L a b o r a t o r y r e p o r t LA-8941-HDR ( t o be p u b l i s h e d ) . G r i g s b y , C. O., J. W. T e s t e r , P. E. T r u j i l l o , D. A. Counce, J. Abbott, C. E. H o l l e y , and L. A. B l a t z , "Rock-Water I n t e r a c t i o n s i n Hot Dry Rock Geothermal Systems: F i e l d I n v e s t i g a t i o n s o f I n S i t u Geochemical Behavior," s u b m i t t e d t o J . 3 m a n o l . and Geotherm. Res., 1981.

Conclusions The r e s e r v o i r s o f t h e Phase I HDR geothermal energy system have e x h i b i t e d growth t h r o u g h a l l segments o f o p e r a t i o n . This growth r e s u l t e d from p r e s s u r i z a t i o n , c o o l i n g ( t h e r m a l c o n t r a c t i o n ) , and f r a c t u r e - f a c e d i s placement o r movement. D u r i n g t h e e a r l y t i m e experiments (Run Segments 2 and 3) thermal drawdown was s i g n i f i c a n t due t o t h e small s i z e o f t h e r e s e r v o i r i n v o l v e d (90°C f o r Segment 2 I n the l a t e r experiand 37OC f o r Segment 3). ments, drawdown was much l e s s s i g n i f i c a n t due t o the larger reservoir. No drawdown was observed d u r i n g Segment 4, and d u r i n g Segment 5 o p e r a t i o n s , t h e r e s e r v o i r s u s t a i n e d o n l y an 8OC thermal drawdown a f t e r 286 days. Modeling o f t h e Phase I r e s e r v o i r s l e d t o an e s t i m a t e d h e a t - t r a n s f e r area o f 8000 m 2 f o r Run Segment 2, w h i l e by t h e end o f Run Segment 5 t h e h e a t t r a n s f e r area was e s t i m a t e d t o be 45 000 t o Measured 50 000 m2, about s i x t i m e s l a r g e r . t r a c e r volumes suggested a f r a c t u r e area o f Modal 80 000 m 2 by t h e end o f Segment 5. volume o f t h e r e s e r v o i r has grown from 11 t o 266 m 3 t h r o u g h t h e course o f Phase I e x p e r i ments.

Murphy, H. D. "Thermal S t r e s s C r a c k i n g and t h e Enhancement o f Heat E x t r a c t i o n from F r a c t u r e d Geothermal Reservoi r s ," Geothermal Energy 22-29 (1979).

1,

Murphy, H. D. (ed.), " P r e l i m i n a r y E v a l u a t i o n of t h e Second Hot Dry Rock Geothermal Energy Reservoir: R e s u l t s o f Phase 1, Run Segment 4," Los Alamos S c i e n t i f i c L a b o r a t o r y r e p o r t LA-8354-MS (May 1980). Murphy, H. D. (ed.), " R e l a x a t i o n o f Geothermal Stresses Induced by Heat Production," Los A1 amos N a t i o n a l L a b o r a t o r y r e p o r t LA-8954-MS (August 1981). T e s t e r , J. W. and Dry Rock Energy Days of O p e r a t i o n Fenton H i l l ,I' Los r e p o r t LA-7771-MS

Water l o s s e s were v e r y encouraging because, f o r comparable o p e r a t i n g p r e s s u r e c o n d i t i o n s , o n l y a 30% i n c r e a s e o f water l o s s was observed f o r a s i x f o l d i n c r e a s e i n h e a t - t r a n s f e r area. The impedance remained c o n s t a n t t h r o u g h o u t Run This i s i n Segment 5 a t about 1.6 GPa s/m3.

J. N. A l b r i g h t (eds.), "Hot E x t r a c t i o n F i e l d Test: 75 o f a Prototype Reservoir a t Alamos S c i e n t i f i c L a b o r a t o r y ( A p r i l 1979).

Z y v o l o s k i , 6. (ed.), " E v a l u a t i o n o f t h e Second H o t Dry Rock G e o t h e r m a l E n e r g y R e s e r v o i r : R e s u l t o f Phase I , Run Segment 5," Los Alamos N a t i o n a l L a b o r a t o r y r e p o r t LA-8940-HDR (August 1981).

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Proceedings Seventh Yorkshop Geothermal Reservoir Engineering Stanford. December 1981. SGP-TR-55.

D I S P E R S I O N I N TRACER FLOW I N FRACTURED GEOTHERMAL SYSTEMS

Roland N. Horne Fernando Rodrig7tez Dept. of Petroleum Enginering Stanford U n i v e r s i t y S t a n f o r d , CA 94305 Abstract Recent f i e l d experiments i n Japan have emphasized t h e importance of performing tracer tests i n any geothermal u t i l i z a t i o n i n which r e i n j e c t i o n i s i n use or i s planned. This is because rapid short- circuiting between r e i n j e c t i o n and production w e l l s may occur due t o t h e f r a c t u r e d n a t u r e of t h e system. I n c a s e s where f r a c t u r i n g i s such that preferred pathways exist in the r e s e r v o i r , t h e r e s u l t may be a r a p i d thermal drawdown of t h e f i e l d production. Tracer t e s t i n g p r o v i d e s a method of e v a l u a t i n g t h e magnitude of such problems. Previous methods used t o a n a l y z e t h e Onuma, Hatchobaru, and Otake tracer tests have used e a r l y and long t i m e d a t a , t h i s paper d i s c u s s e s t h e use of the field concentration/time profile in f r a c t u r e d systems, and t h e l i k e l y forms of d i s p e r s i o n l i k e l y t o dominate i n the process. I n t r o d u c t i o n R e i n j e c t i o n of waste h o t water is practiced in many liquiddominated geothermal f i e l d s (namely Ahuachapan, Otake, Onuma, Kakkonda, Onikobe, Hatchobaru, MakBan, E a s t Mesa, Brawley, and R a f t R i v e r ) . The fundamental purpose of r e i n j e c t i o n i s t o d i s p o s e of t h e unused h o t w a t e r , although it h a s o f t e n been suggested t h a t t h e r e s e r v o i r p r o d u c t i v i t y may be i n c r e a s e d c o n c u r r e n t l y . I n f a c t t h e r e has been only s c a n t evidence t o show support of r e s e r v o i r performance by r e i n j e c t i o n (see Horne 19811, and i n f a c t i n some c a s e s it has been seen t o be d e t r i m e n t a l t o production due t o e a r l y invasion of t h e cooler injected water through high permeability paths in the reservoir. Furthermore, o b s e r v a t i o n s on t h e e f f e c t s of r e i n j e c t i o n have emphasized t h e need t o pay close a t t e n t i o n t o t h e f r a c t u r e d n a t u r e of geothermal r e s e r v o i r s . The benevolence o r malevolence of r e i n j e c t i o n i n geothermal r e s e r v o i r s has been seen t o be closely related to the degree of fracturing. The degree of f r a c t u r i n g h a s been most s u c c e s s f u l l y determined by u s i n g tracer tests. For example, tracer tests sumnarized i n Horne (1981) i n d i c a t e d a high degree of f r a c t u r i n g i n Wairakei, Kakkonda, and Hatchobaru, a m d e r a t e or mixed degree i n Onuma and Ahuachapan, and a l o w degree i n Otake. I n view of t h e subsequent experience i n r e i n j e c t i o n it w a s concluded t h a t

understanding t h e f r a c t u r e system through t h e use of tracers should be t h e f i r s t s t e p i n designing a reinjection program. Unfortunately however, the methods of analysis appropriate t o tracer flow i n fractured systems are not yet fully developed, mst surveys t o d a t e having used only t h e e a r l y time (or i n one i n s t a n c e t h e l a t e t i m e ) data. A method of a n a l y z i n g t h e f u l l tracer r e t u r n p r o f i l e is demonstrated i n t h i s work, and a d i s c u s s i o n o f f e r e d on t h e form of t h e a p p r o p r i a t e t r a n s f e r f u n c t i o n . E x i s t i n g Tracer Analysis Methods The c l a s s S c petroleum r e s e r v o i r methods f o r a n a l y s i s of tracer tests have commonly been based on uniform "sweep" flow through a porous medium i n a given c o n f i g u r a t i o n ( u s u a l l y a 5- spot) see f o r example Brigham and Smith ( 1 9 6 5 ) , Baldwin (1966) and Wagner (1977). In theee a n a l y s e s t h e system i s modelled as a " s t a c k " of non-connecting l a y e r s of porous media which are uniform b u t which n e v e r t h e l e s s have differing properties. The tracer " breaks through" d i f f e r e n t l a y e r s a t d i f f e r e n t times, g i v i n g rise t o t h e c h a r a c t e r i s t i c m u l t i p l e hump return illustrated in Figure 1. Geothermal systems however show very d i f f e r e n t r e t u r n s because of t h e l i m i t a t i o n of flow t o f r a c t u r e s and commonly show a s i n g l e hump r e t u r n as i n F i g u r e 2. The absence of mre than one s t r o n g tracer r e t u r n i t s e l f emphasizes t h e h i g h l y f r a c t u r e d n a t u r e of geothermal r e s e r v o i r s . It i s c l e a r l y i n a p p r o p r i a t e t o u s e t h e uniform sweep model of t h e petroleum i n d u s t r y i n such i n s t a n c e s .

-

9

"1

Figure return

-103-

1

1

1

,

1

1

,

,

,

,

WELL 'D' TRACER COYClNTKATIOU

1:

Multiple

- from Brigham

&

breakthrough Smith (1965)

tracer

.*

*. 1 MAR

?.

e x i s t s a need t o formulate a means of analyzing t h e of t h e s i n g l e humped tracer response with s p e c i f i c r e f e r e n c e t o flow i n f r a c t u r e s . An attempt t o i s o l a t e t h e f e a t u r e s of t r a c e r t r a n s p o r t in f r a c t u r e s is r e p o r t e d here.

._*. . .. -.---. ._*._ 4PR

XAY

. .

Jlm Jim

Figure 2 : Onuma geothermal t r a c e r t e s t from I t o , Kubota E, Kurosawa (1978)

-

Without r e s o r t i n g t o a flow m d e l t h e r e are t w o important items of i n f o r m a t i n which can be derived from a tracer t e s t . The f i r s t is simply t h e speed of f i r s t r e t u r n between an i n j e c t o r and p e r m e a b i l i t y connection between t h e two. A " c o n n e c t i v i t y " map of t h e r e s e r v o i r can be drawn from such r e s u l t s . The second i t e m i s t h e long term d i l u t i o n of tracer i n a t e s t i n which t h e produced tracer i s r e c y c l e d ( a s i s o f t e n t h e case i n a n o p e r a t i o n a l system). This long t e r m d i l u t i o n h a s been used by I t o , Kubota, and Kurosawa (1978) t o estimate t h e volume of f l u i d c i r c u l a t i n g through t h e system a t Onuma field.

Tracer Transport i n F r a c t u r e s Methods of s i g n a l a n a l y s i s are r e a d i l y a p p l i c a b l e t o t h e i n t e r p r e t a t i o n of tracer r e t u r n c o n c e n t r a t i o n h i s t o r i e s , reducing t h e observed p r o f i l e t o t h e sum of i t s component signals. For example, t h e tracer c o n c e n t r a t i o n in a producing w e l l t h a t r e c e i v e s flow from an injection well through two intervening f r a c t u r e s w i l l demonstrate t h e superposed t r a n s f e r f u n c t i o n corresponding t o t r a c e r flow through t h o s e two f r a c t u r e s . The d i f f i c u l t y i n decoupling t h e response i n t o i t s component parts depends on d e f i n i n g t h e f e a t u r e s of t h o s e component p a r t s . For example , tester, Bivins, and P o t t e r ( 1979) d e s c r i b e a method t o r e p r e s e n t t h e t r a c e r c o n c e n t r a t i o n C a t a production p o i n t i n terms of M independent components, t h u s : H

c

-2

tj C, (x,,

ej,

Pej )

(1)

J-1

where 5 j is t h e f r a c t i o n of flow i n "path"; and non-dimensional d i s t a n c e and t i m e a r e d e f i n e d by:

These t w o c a l c u l a t i o n s involve t h e u s e of only t h e e a r l y and l a t e measurements i n t h e test. I n t h e petroleum l i t e r a t u r e , Brigham and Smith ( 1 9 6 5 ) demonstrated a method of "matching" the intermediate time tracer return c o n c e n t r a t i o n , e s s e n t i a l l y by a t r i a l and e r r o r apaproach. Their a n a l y s i s w a s based on a model of t h e r e s e r v o i r as a series of non-connecting l a y e r s with different p e r m e a b i l i t i e s which gave rise t o s e p a r a t e "peaks' i n t r a c e r r e t u r n . Yuen, Brigham, and Cinco ( 1 9 7 9 ) l a t e r extended t h e method of analysis to calculate the p e r m e a b i l i t i e s of v a r i o u s l a y e r s by matching t h e c o n c e n t r a t i o n s a t t h e v a r i o u s peaks with t h e analytical solution. Both of t h e s e s t u d i e s considered a 5-spot c o n f i g u r a t i o n , however t h e methodology would be a p p l i c a b l e to other configurations. Despite the c o n s i d e r a b l e e x t r a information t h a t t h i s " i n t e r m e d i a t e t i m e " a n a l y s i s can provide, it s t i l l e s s e n t i a l l y u s e s only d a t a a t t h e peaks of the response curve. Also, the a n a l y s i s is based on a l a y e r model that may h o l d f o r hydrocarbon r e s e r v o i r s , but which would be i n a p p r o p r i a t e f o r m o s t geothermal systems.

(3)

where x Lj, q j and Vj are t h e p o s i t i o n w i t h i n , j i e n g t h o f , flow r a t e through and volume of t h e j - t h "path" through t h e P e j r e p r e s e n t s t h e P e c l e t number of system. flow through t h e j- th p a t h , d e f i n e d as:

(4)

where n j i s t h e d i f f u s i v i t y (or d i s p e r s i o n c o e f f i c i e n t ) of tracer d u r i n g t r a n s p o r t . Tester, Bivins, and P o t t e r ( 1 9 7 9 ) proposed t h e a n a l y s i s of N measured v a l u e s of e x i t tracer c o n c e n t r a t i o n C i by minimizing t h e o b j e c t i v e f u n c t i o n F, where:

Features of Geothermal Tracer Tests Geothermal r e s e r v o i r s a r e u s u a l l y very h i g h l y f r a c t u r e d . As a r e s u l t , and as an i n d i c a t i o n of t h i s f a c t , t h e t r a c e r response almost always shows j u s t a s i n g l e peak. Thus, although t h e e a r l y and l a t e t i m e a n a l y s e s are s t i l l p o s s i b l e , t h e a n a l y s i s of t h e s i n g l e peak c o n c e n t r a t i o n would provide l i t t l e e x t r a information, and does i n any case require t h e formulation of a flow model. Thus, t h e r e

N

(5) i=1

and C is given by equation ( 1 ) . v a r i a b l e s w i l l be Pej, qj, and Vj.

-104-

Decision

T h i s method can s t r a i g h t f o r w a r d l y provide e s t i m a t e s of t h e P e c l e t numbers a s s o c i a t e d with t h e v a r i o u s flow p a t h s , and t h e i r r e l a t i v e ( b u t not a b s o l u t e ) rates of flaw and r e l a t i v e ( b u t not a b s o l u t e ) values. It does however depend s t r o n g l y upon t h e t r a n s f e r f u n c t i o n C ( Xj, j, P e . ) assumed i n equation ( 1 ) f o r t h e t r a n s p o r t 0% t h e t r a c e r . Brigham and Smith ( 1 9 6 5 ) based t h e i r determination of t h e t r a n s f e r f u n c t i o n on flow through a porous medium between w e l l s i n a f i v e s p o t formation. Tester, Bivins, and P o t t e r ( 1 9 7 9 ) determined t r a n s f e r f u n c t i o n f o r one- and two-dimensional f l o w through porous media.

(9) The mean c o n c e n t r a t i o n a t p o i n t given by: c

-%

t'-2

C

t'

-

C [H(t

-

t*)

Nx)

4

H(t

- At -

t*)

(13)

Figure 4 shows concentration normalized t i m e injection t i m e F i g u r e s 4 and 2

F r a c t u r e flow c o n f i g u r a t i o n

The p r o f i l e a c r o s s t h e span of t h e f r a c t u r e f o r laminar flow is given by: - b2

b2

-

1

[,

x>o

(14) rco

and t is t h e l e n g t h of t i m e t h e t r a c e r is injected.

-b

(y2

(12)

where H(x) is t h e Heavisi.de s t e p f u n c t i o n :

X

-6

x

3 u

G-

b

-

(10)

I n a p r a c t i c a l case, of course, the t r a c e r would not be i n j e c t e d continuously nor would it be i n j e c t e d a t a c o n c e n t r a t i o n of 100%. Equation (11) may however be used to superpose t h e behavior of t h e l e a d i n g edge and t r a i l i n g edge of a tracer s l u g of i n i t i a l c o n c e n t r a t i o n Co, a f t e r which:

3.

u(y)

i s then

where t * is t h e f i r s t a r r i v a l time of tracer and is given by:

Convective Dispersion i n F r a c t u r e Flow These t w o s t u d i e s do n o t , however, correctly r e p r e s e n t t h e flow through a f r a c t u r e i n t h a t a tracer f r o n t i s modelled as p r o p a g a t i n g p e r p e n d i c u l a r l y t o t h e d i r e c t i o n of flow. In a f r a c t u r e , however, i n e i t h e r laminar or t u r b u l e n t f l o w , t h e f l u i d w i l l be t r a n s p o r t e d f a s t e r i n t h e c e n t e r of t h e f r a c t u r e t h a n on t h e w a l l s ( i n f a c t , due t o boundary l a y e r e f f e c t s , it w i l l not be t r a n s p o r t e d along t h e w a l l s a t a l l ) . T h i s is i l l u s t r a t e d i n Figure

Figure 3:

x

t h e normalized tracer r e t u r n as a function of t/t* f o r v a r i o u s v a l u e s of t/t*. The s i m i l a r i t y between should be noted. C/Co

)

where U i s t h e average v e l o c i t y and b i s t h e half- width of t h e crack. Equation ( 6 1 is the well known parabolic velocity d i s t r i b u t i o n and g i v e s r i s e t o a maximum v e l o c i t y a t t h e c e n t e r of t h e crack ( a t y=O) of 3/2 U. Now, i f a continuous s l u g of tracer were t o be i n j e c t e d a t t i m e t = O , the distance x moved by t h e t r a c e r f r o n t , assuming no d i s p e r s i o n , w i l l be given by: X(Y)

-

u(y)t

(7)

and t h u s the t r a c e r w i l l have " a r r i v e d " a t over a range of y given by t h e equation:

x

Figure 4 :

t/t*

0.1

t o 1.0,

increments of

0.1

The end result of this non-uniform "convective" displacement of the tracer slug's l e a d i n g and t r a i l i n g edges i s a d i s p e r s i o n of t h e tracer s l u g over t h e e n t i r e d i s t a n c e between t h e i n j e c t i o n p o i n t and the

t h e s o l u t i o n of which is:

-105-

For i n i t i a l s l u g widths ranging from 0.01 t o 0.5 of t h e t o t a l c a v i t y width, t h e d i f f e r e n c e between the centerline and wall c o n c e n t r a t i o n s of tracer reduces e f f e c t i v e l y t o z e r o w i t h i n a dimensionless t i m e tDof 0.5 i n every case. The molecular d i f f u s i v i t y D may be of o r d e r cm2/sec and t h e f r a c t u r e half- width b may be of o r d e r 0.5 nun, s u g g e s t i n g t h a t t h e t r a n s v e r s e d i f f u s i o n w i l l equalise any c o n c e n t r a t i o n d i f f e r e n c e s w i t h i n 125 seconds ( d u r i n g which t i m e the tracer s l u g could be considered t o move no f u r t h e r than 40 an). C l e a r l y this t r a n s v e r s e diffusion will rapidly overcome the convective d i s p e r s i o n i n a f i e l d case.

observation point. T h i s smearing of t h e s l u g may be termed t h e “convective d i s p e r s i o n ” of t h e tracer, although it must be remembered t h a t t h e e f f e c t is l i t e r a l l y t o d i s p e r s e t h e tracer and not to diffuse it. The “ d i s p e r s i v i t y ” of t h i s p r o c e s s may not be determined i n t h e common sense of t h e term, however a q u a l i t a t i v e impression of i t s o r d e r of magnitude may be estimated. Comparing Equation (13) with t h e s o l u t i o n f o r a p u r e l y dispersive transport:

it is seen t h a t , s i n c e t h e e r r o r f u n c t i o n is roughly l i n e a r over i t s e a r l y range, t h e r e is a rough correspondence between t h e r e c i p r o c a l square r o o t of t i m e terms i n t h e two equations. Thus it may be observed t h a t 4 /L2 behaves l i k e l / t * which is i t s e l f 1.5U/L. The e f f e c t i v e d i s p e r s i v i t y of t h e convection p r o c e s s i s t h e r e f o r e l i k e :

neff

=

T h i s combination o f t r a n s v e r s e d i f f u s i o n and convective d i s p e r s i o n i s known a s “Taylor Dispersion” and w a s d e s c r i b e d f o r p i p e flow by Taylor (1953). The n e t r e s u l t of Taylor d i s p e r s i o n is t h a t t h e t r a c e r f r o n t propogates with t h e speed of the flow i n s p i t e of t h e f a c t t h a t t h e f a s t e s t moving f l u i d i n t h e c e n t e r of t h e channel moves a t t w i c e t h e speed i n t h e case of p i p e flow, or 3/2 t i m e s t h e speed i n t h e case of f r a c t u r e flow. The n e t l o n g i t u d i n a l d i s p e r s i o n was determined by Taylor f o r p i p e flow, and w a s d e r i v e d d u r i n g t h i s i n v e s t i q a t i o n f o r f r a c t u r e flow t o be:

;

UL

and t h e e f f e c t i v e P e c l e t number i s always of o r d e r 8/3. Taylor Dispersion i n F r a c t u r e Flow Even thouqh molecular d i f f u s i o n i n t h e a x i a l d i r e c t i o n i s s e v e r a l o r d e r s of magnitude smaller t h a n a l l o t h e r e f f e c t s ( t y p i c a l l y t h e P e c l e t number f o r molecular d i f f u s i o n may be compared t o t h e value 8/3 determined f o r convective d i s p e r s i o n ) , t h e convective smearing of t h e tracer g i v e s r i s e t o l a r g e concentration gradients across t h e narrow width of t h e f r a c t u r e . With t h i s large concentration gradient molecular d i f f u s i o n tends t o rapidly equalise t h e t r a c e r concentration across t h e f r a c t u r e , thus counteracting t h e e f f e c t of convective d i s p e r s i o n . Figure 5 shows t h e molecular d i f f u s i o n of a tracer s l u g a c r o s s t h e width of a c a v i t y .

It should be noted that the Taylor d i s p e r s i v i t y is i n v e r s e l y p r o p o r t i o n a l t o t h e molecular d i f f u s i v i t y , hence n e t d i s p e r s i o n is g r e a t e r when molecular diffusion is smaller. This analysis cannot be e x t r a p o l a t e d t o z e r o molecular d i f f u s i v i t y because of assumptions made in the d e r i v a t i o n , however the maximum d i s p e r s i v i t y i n t h a t c a s e would be t h a t determined i n Equation ( 16 ).

With a mean flow speed of 3 cm/sec and a f r a c t u r e width of 1 mm, a t y p i c a l value of t h e d i s p e r s i v i t y would be 40 cm2/sec. For a 100 m long f r a c t u r e t h i s would give rise t o a P e c l e t number of o r d e r 1000. Turbulence Turbulence would t e n d t o i n c r e a s e t h e rate of t r a c e r d i f f u s i o n a c r o s s t h e f r a c t u r e and would t h u s d e c r e a s e t h e t o t a l effective dispersivity and increase the P e c l e t number.

0 001

01

0 01 ?D-

I

D t b2

F i g u r e 5: Concentration d i f f e r e n c e between w a l l and c e n t e r l i n e a s a f u n c t i o n of t i m e and initial fracture

tracer

penetration

across

the

-106-

Discussion Various d i s p e r s i o n mechanisms have been discussed. Comparing their i n d i v i d u a l relevance t o t h e f i e l d problem of flow through f r a c t u r e s it is seen t h a t : ( 1) Longitudinal molecular d i f f u s i o n i s insignificant, ( 2 ) Taylor d i s p e r s i o n w i l l dominate over convective d i s p e r s i o n , (3) Turbulence w i l l r e i n f o r c e t h e e f f e c t s of Taylor d i s p e r s i o n , ( 4 ) The PeClet number f o r one-dimensional flaw w i l l t h e r e f o r e t y p i c a l l y be g r e a t e r than 1000.

I n a c t u a l f a c t , f i e l d s t u d i e s on t h e f r a c t u r e system a t Los A l a m o s (Tester, Bivins and Potter, 1979) indicated values of the c a l c u l a t e d P e c l e t numbers of o r d e r 2. The f u r t h e r reduction i n t o t a l t r a n s f e r Peclet number i s due t o t h e f a c t t h a t t h e tracer s p r e a d s i n two o r t h r e e dimensions away from t h e i n j e c t i o n p o i n t and converges a g a i n towards t h e production p o i n t , t h u s being f u r t h e r dispersed. The s i n g l e f i e l d r e s u l t from Los Alamos s u g g e s t s t h a t t h i s multi- dimensional d i s p e r s i o n e f f e c t is i n f a c t a t l e a s t 3 o r d e r s of magnitude greater t h a n t h e simple one-dimensional d i s p e r s i o n mechanisms. This conclusion is being t e s t e d i n t h i s c o n t i n u i n g study by c a l c u l a t i o n of t y p i c a l flow c o n f i g u r a t i o n s . However t h e principal and somewhat unsatisfying conclusion of t h i s work so f a r i s t h a t t h e tracer r e t u r n is dominantly determined by t h e l a r g e s c a l e flow c o n f i g u r a t i o n .

F a l l Meeting, 1979.

Sept.

-

Yuen,

C., Brigham, W. E. and Cinco-L., "Analysis of Five-Spot T r a c e r T e s t s t o Determine Reservoir Layering," U. S. Department of Energy Report SAN-1265-8.

D.

H.:

References Baldwin, D. E., Jr. : " P r e d i c t i o n of Tracer Performance i n a Five-Spot P a t t e r n , " JPet. Tech. (Apr. 1966), 513. W. E., and Smith, D. H.: Brigham, " P r e d i c t i o n of Tracer Behavior i n FiveSpot Flow," paper SPE 1130 p r e s e n t e d a t t h e SPE-AIME 40th Annual F a l l Meeting, Denver, Colorado, O c t . 1965.

Horne, R. N.: "Geothermal R e i n j e c t i o n Experience i n Japan," paper SPE 9925 presented at the 1981 California Regional Meeting of SPSE, B a k e r s f i e l d , C a l i f o r n i a , March 1981, ( t o appear i n J. Pet. Tech.). J., Kubota, "Tracer T e s t s

Nevada,

Wagner, 0. R.: "The U s e of T r a c e r s i n Diagnosing Interwell Reservoir Heterogeneities F i e l d R e s u l t s , " JPet. Tech. (Nov. 19771, 11410.

Acknowledgement T h i s work forms p a r t of t h e cooperative qeothermai program between Stanford University and Instituto de Investigaciones Electricidad, Mexico. Funding for Stanford University's p a r t i c i p a t i o n i n t h i s work w a s provided by t h e U . S . Department of Energy under c o n t r a c t number DE-AT-03-80SF11459.

Ito,

Las Vegas,

and Kurosawa, M.: t h e Geothermal Hot Water a t t h e Oruna Geothermal Power Station," Japan Geothermal Energy A s s o c i a t i o n J o u r n a l , Vol. 15, no. 2 , ( s e r i e s 571, ( J u n e 1978) p. 87, ( i n Japanese 1. Y.,

of

T a y l o r , G. I. : "Dispersion of Soluble Matter i n Solvent Flowing Slowly Through Proceedings of t h e Royal a Tube," S o c i e t y (19531, p. 186. Tester, J. N., Bivins, R. L., and P o t t e r , R. M.: " I n t e r w e l l T r a c e r Analysis of a Anonit i c Hydraulically Fractured Geothermal R e s e r v o i r , " paper SPE 8270 p r e s e n t e d a t t h e SPE-AIME 54th Annual

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LOW-TEMPERATURE GEOTHERMAL RESOURCE ASSESSMENT OF THE UNITED STATES

Michael L. Sorey and M a r s h a l l J . Reed

U. S. G e o l o g i c a l Survey Menlo Park, CA 94025 Introduction Geothermal r e s o u r c e assessment i s t h e e s t i m a t i o n o f t h e amount o f thermal energy t h a t m i g h t be e x t r a c t e d f r o m t h e e a r t h and used a t c o s t s c o m p e t i t i v e with o t h e r forms o f energy a t a f o r e s e e a b l e t i m e under reasonable assumptions o f t e c h n o l o g i c a l improvement. A r e g i o n a l o r n a t i o n a l r e s o u r c e assessment p r o v i d e s a framework f o r l o n g - t e r m energy p o l i c y and s t r a t e g y d e c i s i o n s by government and i n d u s t r y . A r e source assessment i s n o t i n t e n d e d t o e s t a b l i s h s p e c i f i c reserve figures f o r short- term investment and m d r k e t i n g d e c i s i o n s , b u t i n s t e a d t o g i v e an o v e r a l l p e r s p e c t i v e a t a p a r t i c u l a r time, u s i n g u n i f o r m methodology and data. The f i r s t s y s t e m a t i c e f f o r t t o e s t i m a t e t h e geothermal r e s o u r c e s o f t h e U n i t e d S t a t e s was p u b l i s h e d i n 1975 as U.S. G e o l o g i c a l Survey C i r c u l a r 726 (White and W i l l i a m s , eds., 1975). T h i s assessment and a f o l l o w u p assessment publ i s h e d i n 1979 as C i r c u l a r 790 ( M u f f l e r , ed., 1979) focused on t h e q u a n t i t i e s o f geothermal energy a v a i l a b l e i n r e g i o n a l c o n d u c t i v e e n v i ronments, igneous- re1a t e d geothermal systems, hydrothermal c o n v e c t i o n systems, and geopressured- geothermal systems. E s t i m a t e s were g i v e n of t h e thermal energy r e c o v e r a b l e from hydrothermal c o n v e c t i o n systems a t temperatures above 90°C and geopressured systems. I n a d d i t i o n , t h e 1979 assessment i n c l u d e d a compilat i o n o f d a t a on t h e occurrence o f low- temperat u r e geothermal w a t e r l e s s t h a n 90°C ( S a m e l , 19791, b u t no a t t e m p t was made t o e s t i m a t e t h e q u a n t i t i e s o f thermal energy a s s o c i a t e d w i t h such occurrences. Low- temperature geothermal r e s o u r c e s o c c u r i n two t y p e s o f g e o h y d r o l o g i c environments. These i n c l u d e hydrothermal c o n v e c t i o n systems, comnonly in v o l v i ng upward f l o w o f thermal w a t e r a l o n g f a u l t s i n areas o f above-normal h e a t f l o w , and conduction- dominated areas such as sedimentary b a s i n s where a q u i f e r s o f l a r g e a r e a l e x t e n t o c c u r beneath a t h i c k i n s u l a t i n g b l a n k e t o f r o c k s h a v i n g l o w thermal c o n d u c t i v i ty. As discussed by Same1 (19791, low- tempera t u r e geothermal r e s o u r c e s o c c u r t h r o u g h o u t t h e U n i t e d S t a t e s , w i t h some a t r e l a t i v e l y s h a l l o w depths, and appear t o have t h e p o t e n t i a l f o r s i g n i f i c a n t u t i l i z a t i o n i n space h e a t i n g and a g r i c u l t u r e on a 1oca1 b a s i s . To p r o v i d e e s t i m a t e s o f t h e q u a n t i t i e s o f t h e r -

mal energy s t o r e d i n and r e c o v e r a b l e from lowtemperature r e s e r v o i r s i n t h e U n i t e d S t a t e s , t h e G e o l o g i c a l Survey has made a new assessment based on an updated i n v e n t o r y o f low-temp e r a t u r e geothermal occurrences and on t h e development o f new methodology f o r e s t i m a t i n g r e c o v e r a b l e energy. We have been a i d e d i n t h i s t a s k by t h e d a t a g a t h e r e d under programs o f many s t a t e agencies and s e v e r a l p r i v a t e cont r a c t o r s which a r e supported by t h e S t a t e Coupled Geothermal Program o f t h e Department o f E n e r g y ' s D i v i s i o n o f Geothermal Energy. The assessment i s n e a r l y complete; r e s u l t s w i l l be p u b l i s h e d a s a USGS c i r c u l a r i n 1982. We p r e s e n t h e r e an o u t l i n e o f t h e methods used and general d e s c r i p t i o n s o f t h e r e s u l t s o b t a i n e d . Methods f o r Assessing Low-temperature Eeothermal Re sources Assessment o f geothermal energy r e s o u r c e s i n v o l v e s a two- step p r o c e s s o f f i r s t d e t e r m i n i n g t h e l o c a t i o n , e x t e n t , and g e o h y d r o l o g i c c h a r a c t e r i s t i c s o f each r e s o u r c e area and t h e n e s t i m a t i n g t h e amount o f thermal energy s t o r e d i n each r e s e r v o i r ( t h e r e s o u r c e base) and t h e amount o f energy which can be produced a t t h e l a n d surface ( t h e resource). I d e n t i f i e d resource a r e a s i d e a l l y s h o u l d meet t h e c r i t e r S a t h a t a r e s e r v o i r e x i s t s w i t h s u f f i c i e n t permeab i l i t y t o s u p p l y l o n g - t e r m p r o d u c t i o n and t h a t r e s e r v o i r temperatures exceed some minimum temp e r a t u r e - d e p t h r e l a t i o n . As d e p i c t e d i n f i g u r e 1, we have used a l o w e r - t e m p e r a t u r e l i m i t w h i c h i s 10°C above t h e mean annual a i r temperature a t t h e s u r f a c e and i n c r e a s e s a t a r a t e o f 25'C/km w i t h depth. T h i s a v o i d s c o n s i d e r a t i o n o f t h e enormous q u a n t i t y o f c o o l , s h a l l o w groundwater whi 1e enabl ing us t o in c l ude a r e a s w i t h anomalous c o n c e n t r a t i o n s o f h e a t a s s o c i a t e d w i t h hydrothermal c o n v e c t i o n systems and deep sedimentary b a s i n s o r c o a s t a l embayments where temperatures i n c r e a s e r e 1 a t i v e l y r a p i d l y with depth. The r e s o u r c e base f o r each low- temperature a r e a i d e n t i f i e d i n t h i s assessment i s c a l c u l a t e d as: q

R

= pc.a-d.(t-t

ref

(1)

where: q = r e s o u r c e base ( s t o r e d thermal energy) p! = v o l u m e t r i c s p e c i f i c h e a t o f r o c k p l u s w a t e r (2.6 J/cm3"C) a = r e s e r v o i r area -109-

d = reservoir thickness t = reservoir temperature

tref = reference temperature (15°C) Statistical methods outlined in Circular 790 were used t o quantify the uncertainty in these calculations and t o provide probability distributions for total resource base, resource, and beneficial heat estimates. For each reservoir, estimates were made of the minimum, maximum, and most likely characteristic values f o r various parameters; these values were then used t o form triangular probability densities from which the mean and standard deviation f o r each parameter and for the various energy quanti t i e s were calculated. The approach used in previous assessments t o estimate recoverable energy, or well head thermal energy, was t o assume a recovery factor of 25 percent of the stored thermal energy. This value i s based on a process involving injection of cold water into the reservoir t o replace h o t water withdrawn during production and t o sweep o u t the energy stored i n the rock. In uniformly-permeable reservoirs, up t o 50 percent of the stored energy can be recovered before coldwater breakthrough occurs (Nathenson, 1975); 25 percent recovery was assumed to account for comnonly-encountered non-uni form permeabi 1 i ty conditions. In the present assessment, wellhead thermal energy calculations were made b o t h for a recovery factor of 25 percent for small systems and for a development plan involving production wells discharging for 30 years in the absence of cold-water injection for large systems. In t h i s production model, we1 1head thermal energy i s given by: 'WH

= ( P C ) .N*Q*(30y e a r s ) * ( t - t 1 f ref

(2)

where : qWH= energy recovered a t the wellhead over 30 years ( p C I f = volumetri specific heat of fluid (4.1 J/cm 5 " C ) N = number of production wells Q = average discharge rate of each well This approach i s bel ieved to yield more real ist i c estimates of recoverable energy for most low-temperature areas, where lower energy content of the resource f l u i d and relatively large reservoir areas make the economics of injection much less attractive t h a n for higher-temperature geothermal areas. Limited hydrologic data precluded determination of optimum values for N and Q in equation ( 2 ) for most of our identified low-temperature reservoirs. Instead, we selected a production pl an i nvol v i ng evenly-spaced we1 1s di schargi ng a t 0.0315 m3/s (500 gpm) for 30 years w i t h a maximum drawdown of 152 m (500 f t ) . Then for a range of realistic values of reservoir transmissivity and storage coefficient and of con-

fining-bed properties, we developed correspondi n g curves relating reservoir area t o well area, defined as the square of the well spacing. An example, calculated using the leakyaquifer solutions of Hantush (1960) w i t h confining-bed properties considered typical for sedimentary basins, i s presented i n figure 2. The curves i n figure 2 show several significant features of a production model which does n o t involve injection. Most notable i s the fact t h a t a s reservoi r area increases the appropri ate we1 1 spacing increases because of drawdown interference between wells. Only for very large-area reservoirs, such as the Madison limestone and Dakota sandstone in the northern Great Plains, does the well spacing become constant with reservoir area thereby permitting the number of wells t o increase in proportion t o the size o f the reservoir. For smaller reservoir areas, the number of wells a reservoir can support does n o t increase in proportion t o the size of the reservoir. Consequently, under t h i s production plan, the energy recoverable from a 100 km2 reservoir i s only about 1.5 times t h a t from a 10 km2 reservoir w i t h the same hydraulic characteristics. Equivalent recovery factors w i t h this production stratagy vary from about 0.1 percent f o r the largestarea reservoi rs in sedimentary basins t o 25 percent for the smal lest-area reservoirs associated w i t h thermal springs. The 25 percent figure i s approached for small reservoir areas where breakthrough of cold, induced recharge from surrounding regions rather than drawdown interference 1 imits energy recovery. Curves such as those in figure 2 can be used t o estimate minimum, maximum, and most likely values for the well area, a,, which define the corresponding triangular probability density and the mean value %. The assumption was made in developing these curves that each reservoir area was uniformly transmissive. The uncertainty associated with t h i s assumption i s treated i n a manner similar t o that used with the recovery factor approach by assigning a correction factor k t o the estimated number of production we1 1 s w i t h minimum, maximum, and most likely values of 0, 1, and 0.5, respectively. The effects o f t h i s correction factor are t o decrease the estimate of N and t o i n crease the confidence limits on estimates of q i and beneficial h e a t , The mean value for N i s then given by (ka/aw) and the mean we1 1head thermal energy becomes:

where: M = (0.0315 m3/s).(30 years) = 3 x 107 m3 =

mean wellhead temperature reservoir temperature).

(E

mean

For small-area reservoirs, our resource e s t i mates based on equation ( 3 ) are close t o those based on a recovery factor of 25 percent, b u t -110-

for 1arge-area reservoirs the we1 1-spacing calculation yields a much smaller value. Differences between these estimates reflect the importance of time-scale over which production continues and the choice of injection versus no-injection developments. Maximum recovery fractions are obtained w i t h an efficient injection plan designed t o give cold-water breakthrough a t the end of the designed l i f e of the development. I f cold water i s not injected, h i g h recovery fractions can s t i l l be attained for small-area reservoirs over a 30-year period, b u t for large-area reservoirs production would have t o continue for much longer times to obtain h i g h recovery fractions. Despite low recovery fractions, total energy recovery from sedimentary basins could be large because the estimated reservoir areas are so 1arge. The amount of each resource that can be directly applied to non-electric uses i s termed beneficial heat, qB. Whereas wellhead thermal energy, qWHs represents energy above a reference state, beneficial heat represents energy that can be applied to specific processes such as heating a i r . Estimates of beneficial heat from geothermal fluid can be compared with thermal energy obtainable from other fuels. Selection of appropriate uses for low-temperature geothermal water depends partly on the reservoir temperature, and different uses i n volve different rejection temperatures for the geothermal waste water. We have used the following equation t o calculate the mean temperature drop, A t , a s a function of mean wellhead temperature: 0 . 6 * ( t -25°C) WH

A t =

(4)

From this, mean values for beneficial heat are calculated as:

-=

qB

( pc 1

f

- (ka/a-)

W

-M-0.6-(t -25°C) WH

(5)

Note that for values of tG < 25°C i n equations 4 and 5 the useful A t and the beneficial heat are zero. This limit i s obtained both from defining low-temperature resources as being a t least 10°C above the mean annual air temperature, which averages 15°C across the United States, and from consideration of heatpump applications for which ground-water temperatures above 25°C significantly improve coefficients of performace.

Th e totals are each on the order of 200 x 10 J (about 200 Quads).

f8

The major portion of the identified low-temperature resource occurs within the sedimentary basins east of the Rocky Mountains. Among these are portions of the Powder River and Williston Basins in Wyoming, Montana, North Dakota, and South Dakota where regional aquifers w i t h resources a t temperatures of 40" t o 90°C exist in Paleozoic rocks of the Madison Group (limestone) and Cretaceous rocks of the Dakota Group (sandstone). A1 though the reservoir areas involved i n these basins are large, the methods used in t h i s assessment t o estimate recoverable thermal energy appear t o yield rea l i s t i c results. For example, our preliminary estimate of recoverable energy for the Madison limestone i n the Powder River and Williston Basins i n Montana i s 20 x 10185. This e s t i mate involves 1800 production wells spread over an area of about 180,000 km2. An alternative cal cul ation , i nvol vi ng small er-scal e developments a t each of about 60 population centers w i t h i n this region, yields the same resource total f o r individual well fields w i t h 30 wells per town. In terms of total numbers of identified lowtemperature areas, most areas occur in the western United States and are associated w i t h hydrothermal convection systems of various kinds. We have identified approximately 1,900 individual areas i n the United States; roughly 1,850 of these occur west of the Rocky Mountains. In most of these areas, evidence for the existence of a geothermal reservoir consists mainly of the presence of a single thermal spring or spring group, comnonly along one or more active normal faults. In such cases a most likely reservoir volume of 1 km3 and a thermal energy recovery factor of 25 percent were assigned. The resource estimate for such a system,-with a reservoir temperature of 70"C, i s 4 x 10165. There are, however, numerous areas involving hydrothermal convection systems where sufficient information exists t o delineate a larger reservoir size. Most notably, i n portions of Idaho's Snake River Plain, thermal water moves upward along marginal faults and leaks laterally through permeable strata a t relatively shallow depths towards the center of the Plain. Our preliminary estimate of the t o t a l recoverable energy for 24 low-temperature areas w i t h i n he Snake River Plain i s approximately 2 x 101 5.

Q

Estimates of Resource Base, Resource, and Beneficial Heat Preliminary results of our assessment o f lowtemperature resources i n the United States i n dicate that the total recoverable thermal energy a t temperatures less than 90°C (based on our well-spacing calculations) i s of similar magnitude to corresponding estimates from previous assessments for identified intermediate-temperature (90" t o 150°C) and high-temperature (above 150°C) hydrothermal convection systems.

Low-temperature areas identified within the Eastern United States include the warm spring areas i n the Appalachian Mountains of Virginia, West Virginia, and North Carolina i n the south and of New York and Massachusetts in the north. Other warm spring areas i n Arkansas and Georgia were identi fed as containing resources, and portions of the Atlantic Coastal Plain in Delaware, Maryland, Virginia, and North Carolina were also included as low-temperature -111-

areas. The western side of the Allegheny Basin i n Pennsylvania contains a thick section of Devonian shale having low thermal conductivity, b u t i t i s n o t known i f aquifers exist below the shale to form a geothermal reservoir. Outside these areas, however, the generally low crustal heat flow and low thermal gradients appear t o r e s t r i c t the occurrence of significant low-temperature resources.

References Rantush, t4. S . , 1960, Modification of the theory of leaky aquifers: Journal of Geophysical Research, v. 65, n. 11, p. 3713-3725. Muffler, L. J . P., ed., 1979, Assessment o f geothermal resources of the United States--1978: U.S. Geological Survey Circular 790, 163 p.

Limited geohydrologic data requires us t o t r e a t many areas which appear favorable for the exi stence of 1ow-temperature reservoirs as containing undiscovered, rather than identified resources. Undiscovered resources were assumed to be associated w i t h 1) aquifers in sedimentary basins and coastal embayments where existing data could not support a quantitative assessment, 2 ) halos around identified intermediate and high-temperature systems, and 3) thermal energy i n systems whose locations are as yet unknown. Estimates of the total undiscovered resource base, resource, and beneficial heat for various geologic provinces will be included in the final report along with estimates for identified areas.

0

y

20

Nathenson, Manuel, 1975, Physical factors determining the fraction of stored energy recoverable from hydrothermal convection systems and conducti on-domi nated areas : U.S. Geological Survey Open-file Report 75-525, 35 p. Samnel, E. A., 1979, Occurrence of lowtemperature geothermal waters in the United States, i n Muffler, L. J . P., ed., Assessment of geothermal resources of the United States--1978: U.S. Geological Survey Circular 790, p. 86-131. White, D. E., and Williams, D. L., eds., 1975, Assessment of geothermal resources of the United States--1975: U.S. Geological Survey Circular 726, 155 p.

TEMPERATURE ,OC 40 60

80

LOW-TEMPERATURE GEOTHERMAL RESOURCES € 1

24

I

c

n n W

2

I I

Thermal water less favorable for development ( n o t identified in this assessment)

Figure 1. Diagram of temperature-depth c r i t e r i a used t o identify lowtemperature geothermal resources. MA = mean annual a i r temperature. Maximum temperature limit = 90°C. Minimum temperature limit given by line with slope of 25"C/km and intercept of MA+lO"C. Heavy solid line shows minimum temperature limit for example case MA = 10°C.

-112-

10"

F i g u r e 2.

10'

loJ 10' 1oc RESERVOIR AREA, km2

1o3

Curves o f re1 1 r e a ( w e l l s p a c i n g squared) v e r s u s r e s e r v o i r a r e a developed f o r a p r o d u c t i o n p l a n i n v o l v i n g e v e n l y spaced w e l l s d i s c h a r g i n g f o r 30 y e a r s a t 0.0315 m3/s (500 gpm) w i t h a maximum drawdown o f 152 m (500 f t ) T = reservoir t r a n s m i s s i v i t y (m2/s), s = r e s e r v o i r s t o r a g e c o e f f i c i e n t , K' = h y d r a u l i c c o n d u c t i v i t y o f c o n f i n i n g beds ( m / s ) , and S ' = s p e c i f i c s t o r a g e o f c o n f i n i n g beds ( m - 1 ) .

.

-113-


GEOTHERMAL WELL LOGGING AND ITS INTEXPRETdTION

S e i i c h i Hirakawa and S h i n j i Yamaguchi

The U n i v e r s i t y of Tokyo, Facul t y of Engineering

AF3STRACT For Japanese geothermal d e v e l o p e r s of p r i v a t e companies, some conventional l o g s , such as temperature l o g , SP l o g and r e s i s t i v i t y l o g , are a v a i l a b l e i n geothermal f i e l d s . On t h e o t h e r hand, improvement i s b e i n g made i n w e l l l o g g i n g techniques on geothermal w e l l s i n t h e " National Sunshine P r o j e c t . " The purpose of this paper i s (1) t o e x p l a i n t h e h i g h temperature well l o g g i n g developed on t h e Sunshine P r o j e c t , and ( 2 ) t o p r e s e n t a l g o r i t h m which can e s t i m a t e p o r o s i t y d i s t r i b u t i o n s and d e t e c t f r a c t u r e d zones from conventional geothermal w e l l logging.

i d e a l i z e d model of a f r a c t u r e d geothermal r e s e r v o i r considered i n this s t u d y , i t is assumed that t h e s p a c i n g between p a r a l l e l e pipeds r e p r e s e n t s t h e fractures. T h i s model c o n s i d e r s b o t h f r a c t u r e d p o r o s i t y and matrix p o r o s i t y . T h i s a l g o r i t h m c o n s i s t s of t h e b a s i c e q u a t i o n s i n formation e v a l u a t i o n , empir i c a l e q u a t i o n s o f t e n used i n o i l r e s e r v o i r e v a l u a t i o n and e q u a t i o n s d e r i v e d from doublep o r o s i t y model theory. To show t h e v a l i d i t y of this algorithm, one example was chosen from w e l l loggings. The t o t a l p o r o s i t y d i s t r i b u t i o n s and t h e d i s t r i b u t i o n s of t h e f r a c t i o n of t o t a l pore volume made up of f r a c t u r e s t o t h e t o t a l pore volume of t h e system were obtained. The f r a c t u r e d %ones e s t i m a t e d from t h e s e r e s u l t s agreed approximately w i t h t h e f r a c t u r e d zones e s t i m a t e d from d r i l l i n g c h a r t s . T h i s a l g o r i t h m may be one of e f f e c t i v e methods f o r t h e e s t i m a t i o n of p o r o s i t y d i s t r i b u t i o n s i n a f r a c t u r e d geothermal r e s e r v o i r .

HIGH TFiKPERATURE LOGGING TOOL High temperature l o g g i n g t o o l s must be developed f o r geothermal r e s e r v o i r p o t e n t i a l e v a l u a t i o n . The p l a n f o r developing l o g g i n g t o o l s f o r geothermal wells s t a r t e d i n 1974 as a p a r t of t h e " National Sunshine P r o j e c t " . E i g h t b a s i c l o g g i n g t o o l s had been developed by 1979. They are 1) Multi- Spacing E l e c t r i c a l Log 2) P-S Acoustic Log/Caliper Log 3) Micro- Spherically Focused Log/Caliper Log 4) Temperature Log 5) P r e s s u r e Log 6) Flowmeter 7) Bottom Hole Sampler 8) Borehole Televiewer. Table 1 shows t h e l o g g i n g t o o l s i n c l u d e d i n t h e p r o j e c t and t h e i r endurance a b i l i t y . These l o g g i n g t o o l s were chosen a f t e r wide r e s e a r ches on t h e n e c e s s i t y of new t o o l s . The developing p l a n i n t h e Sunshine P r o j e c t i s d i v i d e d i n t o two s t e p s a c c o r d i n g t o t h e maxim u m temperature which l o g g i n g t o o l s can endure. The maximum temperature limits of t h e l o g g i n g t o o l s were 275OC i n t h e f i r s t s t e p (1976-1979). The l o g g i n g t o o l s developed were t e s t e d i n geothermal r e s e r v o i r s and good r e s u l t s were obtained. Now many d a t a a r e being g a t h e r e d . I n t h e second s t e p (19€30*), t h e g o a l s of the maximum temperature were f i x e d a t 350.C.

Theorv The i d e a l i z e d model considered i n this s t u d y i s p r e s e n t e d i n F i g u r e 1. I n this model i t i s assumed that t h e s p a c i n g between parallelepipeds represents the fractures. I t i s s i m i l a r t o t h e one p r e s e n t e d by Warren and Root t o analyze p r e s s u r e behavior i n f r a c t u r e d r e s e r v o i r s and t o t h e one p r e s e n t e d by Towle t o study the r e l a t i o n s h i p between formation r e s i s t i v i t y f a c t o r and p o r o s i t y . I t i s also s i m i l a r t o t h e one p r e s e n t e d by Aguilera t o analyze n a t u r a l l y f r a c t u r e d o i l r e s e r v o i r s from well l o g s . The d i f f e r e n c e between Aguilera's model and this model i s t h a t t h e former c o n s i d e r s only f r a c t u r e p o r o s i t y and t h e l a t t e r considers both f r a c t u r e porosity and m a t r i x p o r o s i t y . For t h e b a s i s of this s t u d y , the f o l l o w i n g b a s i c r e l a t i o n s h i p s i n formation e v a l u a t i o n are assumed t o be a p p l i c a b l e t o both t h e system and t h e matrix.

INTERPRETATION TECHNIQUES Some conventional l o g s , such as temperature l o g , SP l o g and r e s i s t i v i t y l o g , are a v a i l a b l e i n Japanese geothermal f i e l d s . Algorithm i s shown w i t h which p o r o s i t y d i s t r i b u t i o n s i n a f r a c t u r e d geothermal r e s e r v o i r can be e s t i m a t e d from conventional geothermal w e l l l o g g i n g s . I n t h e

Rft

If.=-=Ff

Ff

-115-

=+

R,

Rft Rfo

c a l c u l a t e d by t h e following equation.

(3) where

I

= r e s i s t i v i t y index f o r t h e system

f

where

Rfo = r e s i s t i v i t y f o r the system a t 100percent formation water s a t u r a t i o n Ff = formation f a c t o r f o r the system

6

= t o t a l porosity, f r a c t i o n

Rmf a t any tempeltature can be c a l c u l a t e d u s i n g Arp' s equation.

m = double- porosity system exponent F = matrix formation f a c t o r = matrix p o r o s i t y , f r a c t i o n

ab

mb

Rmf

= r e s i s t i v i t y of f l u s h e d zone a t 24%, ohm-m R,,, = r e s i s t i v i t y of d r i l l i n g muds a t 24*C, ohm-m K,, = c o e f f i c i e n t

Rft = t r u e r e s i s t i v i t y f o r t h e system

= matrix p o r o s i t y exponent

Rw = connate water r e s i s t i v i t y r e s i s t i v i t y of t h e f l u s h e d zone a t

The following equations can be derived on t h e b a s i s of t h e double- porosity model.

6 f,

= 1-

x3

+ x36

r e s i s t i v i t y of t h e f l u r h e d zone a t T 1 , ohm-m

(4)

T i = temperature, 'C

1-~3

=

T2, ohm-m

(5)

T2 = temperature, 'C

1-x3+x36

1-X + F( l-X+X@b) (X2-X+1 ..

Temperature i n above equations can be obtained .by a temperature l o g . R,, can be c a l c u l a t e d as:

)

(11)

A

where

X = l e n g t h of t h e block, f r a c t i o n f r = t h e f r a c t i o n of t o t a l pore volume made up of f r a c t u r e s t o t h e pore volume of t h e system On t h e b a s i s of t h e f r a c t u r e d r e s e r v o i r model and equations 1-6, t h e algorithm shown i n Figure 2 has been developed. I n s t e p 4, Rw i s c a l c u l a t e d by t h e following method. The s t a t i c spontaneous p o t e n t i a l value, ESsp, i s r e l a t e d t o Rw by:

%f

-

EssP = -Kc log R W

Kc = 61

+

0.133(1.eT

+ 32)

(7) (8)

where

ESsp = s t a t i c spontaneous p o t e n t i a l , mv

Kc = electrochemical SP c o e f f i c i e n t Rmf = r e s i s t i v i t y of t h e f l u s h e d zone a t r e s e r v o i r temperature, ohm-m R,, = connate water r e s i s t i v i t y a t r e s e r v o i r temperature, ohm-m T = r e s e r v o i r temperature, O C

Rmf can be found from t h e l o g heading or

-1 16-

I n s t e p 6 , l a b o r a t o r y experimental d a t a a r e r e q u i r e d t o determine matrix p r o p e r t i e s . I n t h e c a s e where l a b o r a t o r y experimental d a t a cannot be obtained, conventional formula (e.g. Humble's formula) may be used. T o t a l p o r o s i t y d i s t r i b u t i o n s and f r a c t i o n of t o t a l pore volume made up of f r a c t u r e s t o t h e t o t a l pore volume of t h e system can be estimated with this algorithm. Example And Discussion To show t h e v a l i d i t y of this algorithm, one example i s presented here with t h e well d a t a . A geothermal well was chosen among t h e w e l l s , because i t provided t h e b e s t suite of l o g s . T h i s example u s e s a temperature l o g , spontaneous p o t e n t i a l l o g and an e l e c t r i c a l log. The e l e c t r i c a l l o g c o n t a i n s a long normal ( e l e c t r o d e spacing = 100cm) and a s h o r t normal ( e l e c t r o d e spacing = 25cm). The l o g s were run i n 1975. Four hundred meters were examined, beginning a t a depth of 12OOm. From a temper a t u r e l o g , t h e temperature around t h e depth of i n t e r e s t is 130%. T h i s temperature i s lower than 175"C, which i s t h e maximum temper a t u r e limit of conventional logging t o o l s used i n o i l f i e l d s . So, conventional logging was p o s s i b l e i n this w e l l i n 1975. I n s t e p 4, R,, i s c a l c u l a t e d t o be 1.5 ohm-m. From g e o l o g i c a l s e c t i o n of this well, t h e l i t h o l o g y

rature on t h e Density and E l e c t r i c a l R e s i s t i v i t y of Sodium Chloride S o l u t i o n s , " Petroleum Transactions, AIHE, ~01.198,327330 P i r s o n , S. J . (1957), LO^ I n t e r p r e t a t i o n i n Rocks With Multiple P o r o s i t y TypesWater o r O i l Wet," World O i l , ~ 0 1 . 1 4 4 , June, 196-198 Schlumberger, (1977), "Log I n t e r p r e t a t i o n

i s determined t o be r h y o l i t e t u f f around t h e depth of i n t e r e s t . In s t e p 6 , l a b o r a t o r y experiments with c o r e samples a r e r e q u i r e d t o determine matrix p r o p e r t i e s . I n t h e case where l a b o r a t o r y experimental d a t a connot be obtai n e d , conventional formula may be used. In this c a s e , Humble's formula i s assumed. Matrix formation f a c t o r F i s determined t o be 41 from e l e c t r i c a l l o g and matrix p o r o s i t y $t, i s c a l c u l a t e d t o be 0.14 using t h e Humble's formula. These values a r e s u b s t i t u t e d f o r F and i n equation 6 and t h e r e l a t i o n between Ff and X i s obtained. Figure 3 shows t h e p l o t of Ff v e r s u s X derived from equation 6. If Ff i s obtained from e l e c t r i c a l l o g i n each zone, X can be obtained from this r e l a t i o n and p o r o s i t y can be c a l c u l a t e d using equation 4 i n each zone. The t o t a l p o r o s i t y d i s t r i b u t i o n s and t h e d i s t r i b u t i o n s of t h e f r a c t i o n of t o t a l pore volume made up of f r a c t u r e s t o t h e t o t a l pore volume o f t h e system, which were obtained w i t h this algorithm a r e presented i n Figure 4 and i n Figure 5 r e s p e c t i v e l y . These figures show that depths of 1300111, 1360m and 1440111are highly f r a c t u r e d zones. The f a c t that much f l u i d l o s s i s observed around t h e depth of 1400m on t h e d r i l l i n g c h a r t supports t h e abovementioned r e s u l t s . T h i s algorithm may be one of e f f e c t i v e methods f o r t h e e s t i m a t i o n of p o r o s i t y d i s t r i b u t i o n s i n a f r a c t u r e d geothermal r e s e r v o i r .

4.

5.

Charts" Towle, C. (1962), "An Analysis of t h e Formation R e s i s t i v i t y Factor P o r o s i t y R e l a t i o n s h i p of Some Assumed Pore Ceometries," paper presented a t Third Annual Meeti n g of SPWLA, Houston

6.

6

Start Here

Data Required Resistivity Log Temperature Log s. P. Log

+

CONCLUDING REMARKS Various logging t o o l s for Japanese geothermal f i e l d s a r e being developed as a p a r t of t h e " N a t i o n a l Sunshine P r o j e c t " . On t h e o t h e r hand, some conventional l o g s , such as temperature l o g , SP l o g and r e s i s t i v i t y log, a r e a v a i l a b l e f o r Japanese geothermal developers of p r i v a t e companies. I n t e r p r e t a t i o n techniques of logging r e s u l t s obtained with t h e s e t o o l s have not y e t been p u t i n t o p r a c t i c e i n Japan. I n this paper, one of t h e algorithms f o r a geothermal f i e l d i s presented. With this algorithm p o r o s i t y d i s t r i b u t i o n s i n a f r a c t u r e d geothermal r e s e r v o i r can be e s t i mated. The v a l i d i t y of this algorithm has been shown with f i e l d d a t a .

Step 3

1

Read logs and record data

Temperature Log P. Log

s. Step 5

Geological Section

Step

6

Determine matrix properties F, Bbv mbr ab

ACKNOWLEDGMENT T h i s work has been c a r r i e d out by a grant- in- aid f o r developmental s c i e n t i f i c r e s e a r c h of t h e Ministry of Education, t o which t h e a u t h o r s a r e very g r a t e f u l . REFERENCES 1. Aguilera, R. (1974), "Analysis of N a t u r a l l y Fractured Reservoirs From Sonic and Resist i v i t y Logs," J. P e t . Tech., vo1.26, Nov., 1233-1238 2. Aguilera, R. (1976), "Analysis of Natural l y Fractured R e s e r v o i r s From Conventional Well Logs," J. Pet. Tech., vo1.28, J u l y , 764-772 3. Arps, J. J. (1953), "The E f f e c t of Tempe-

-117-

I

Calculate f Iron equation ( 4 ) . f 5 )

Fig. 2

1 Algorithm

0.0

I d e a l i z e d Model

Fig. 1

fr 120( 120c

0

,

0,2

I

0,4

0,6

124( 124C 12K 1280

132c 1320 1360 1360 h

E

v

x

6

v

1400

E 1400

H

PI

!w a

8 1440

7

1440

1480 1480

1520 1520 I

1560 I

1560

1600 1600

Fig.

4

6 vs

Depth

Fig. 5

-118-

f, vs Depth

0,8

,

0

a

N

0 0

a

0 0

0

0 0

a

0

I O

N

I N

I

0 0

I O I O

I O I O

0

0

r-

a

r-

Ir-

la

la

0 N

lr-

I

*

I O I O

0

ILn

Ln

I O

I N

I O I O I O

0

T-

7

0 I

0 n

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0

0 r

L

-

n

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0

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n

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e

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L

0

n

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M

n

o

d

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s: M

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M

I O I L n

I M

0

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0

1

I

0

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I I

1

C U I

0

1 1

Lnl

E

3 3 3 N a, L

n

L

n

L

r - r - r N

N

n

N

Ln r-

N

Ln

r-

N

LnLn

r-rN N

Lnl

r-I N I

0 0

1 1

N I

I I I

-i

Lnl

c l

c u I

(d c)

r-I

a

E

3 3 3 0

h

P ri

0

d

k

K!

0 a

m r

k

F

P

D

a,.-

e

P

r: 00 4

a,

3

0

4

ri

t3

(d

0

0

4 a,

k 3

P M 0

d

(d

k a,

a

E a,

a

a, a,

5 01

m

a,

k p1

-119-

k a,

e

k

rl

0

22

a,

k

a,

E, > 0

e

T ? r-

R

0

E

.. .

L.7

F9H

d

0 0


MEASURING SALT WATER P E M E A B I L I T I E S BRIAN

D, GOBRAN

ARC0 OIL AND GAS COMPANY A b s t r a c t T h i s s t u d y i n v e s t i g a t e s flow of s a l i n e s o l u t i o n s through sandpacks a t e l e v a t e d temperatures and p r e s s u r e s i n t h e l a b o r a t o r y . The p u r p o s e s of t h i s work are twofold. F i r s t t h e experimental a p p a r a t u s necessary t o measure p e r m e a b i l i t y w i t h s a l t water flow a t e l e v a t e d temperatures must be designed and b u i l t . Then, t h e e f f e c t of temperature on a b s o l u t e p e r m e a b i l i t y t o s a l t w a t e r w i l l be compared t o t h e e f f e c t w i t h d i s t i l l e d water as t h e flowing f l u i d under similar c o n d i t i o n s .

Once t h e s e r e s u l t s were o b t a i n e d , explanat i o n s were sought from t h e l i t e r a t u r e . Chemical e n g i n e e r i n g l i t e r a t u r e was a good s o u r c e f o r e x p l a i n i n g t h e observed permeab i l i t y r e d u c t i o n s w i t h temperature i n c r e a s e . During t h i s period ( t h e e a r l y 1 9 7 0 ' s ) , chemical e n g i n e e r s a l l over t h e world were i n v e s t i g a t i n g anomalous water (Derjaguin and Churaev). Anomalous water w a s water condensed i n very f i n e q u a r t z c a p i l l a r i e s having p r o p e r t i e s s i g n i f i c a n t l y d i f f e r e n t (such as f i f t e e n times t h e v i s c o s i t y ) from normal o r bulk water. So, t h e idea of some unexplained boundary l a y e r phenomenon between q u a r t z and water was used t o e x p l a i n t h e observed p e r m e a b i l i t y r e d u c t i o n s w i t h water flowing through sand.

I n t r o d u c t i o n The a b s o l u t e p e r m e a b i l i t y of a formation occurs a s a v a r i a b l e i n a l l f l u i d flow e q u a t i o n s . Since p e r m e a b i l i t y i s measured i n t h e l a b o r a t o r y , experimenters have been t r y i n g f o r t h e l a s t 40 y e a r s t o simulate reservoir conditions (or a t l e a s t t h o s e c o n d i t i o n s thought i m p o r t a n t ) i n t h e l a b o r a t o r y . The f i r s t s t e p s i n t h i s d i r e c t i o n were t o s u b j e c t c o r e s t o confining pressures s i m i l a r t o those i n the r e s e r v o i r . This ranged from h y d r o s t a t i c ( F a t t and Davis) t o r a d i a l (Wyble) t o t r i a x i a l a t a Poisson r a t i o of 0.33 (Gray, et a l . ) . The r e s u l t s of t h e s e s t u d i e s -showed t h a t a b s o l u t e p e r m e a b i l i t y was a f u n c t i o n of c o n f i n i n g p r e s s u r e while r e l a t i v e p e r m e a b i l i t y ( F a t t ) was not.

Although l a t e r i n v e s t i g a t i o n s found no e f f e c t of temperature on a b s o l u t e permeab i l i t y with d i s t i l l e d water flow (Gobran, Sageev), t h e r e were s t i l l nagging q u e s t i o n s about t h e q u a r t z /water i n t e r f a c e . Perhaps t h e a d d i t i o n of s a l t t o t h e water would cause some i n t e r f a c i a l f o r c e s t h a t would y i e l d a reduction i n permeability with temperature i n c r e a s e . To answer t h e s e q u e s t i o n s and t o f u r t h e r s i m u l a t e a c t u a l reservoir conditions i n the laboratory, s a l t w a t e r p e r m e a b i l i t i e s were ueasured i n t h i s study.

The next approximation t o r e s e r v o i r cond i t i o n s was t o i n v e s t i g a t e t h e e f f e c t of temperature on t h e measured v a l u e s of a b s o l u t e and r e l a t i v e p e r m e a b i l i t i e s . The i n i t i a l s t u d i e s (Poston, e t a l . , Weinbrandt) found a r e d u c t i o n i n a b s o l u t e p e r m e a b i l i t y and a s h i f t i n g of t h e r e l a t i v e p e r m e a b i l i t y c u r v e s when t h e temperature w a s i n c r e a s e d . F u r t h e r s t u d i e s (Casse, Aruna) were i n i t i a t e d with d i s t i l l e d water flow through sandstones and sandpacks t o b e t t e r d e f i n e t h e e f f e c t of temperature on absolute permeability. These r e s e a r c h e r s found a r e d u c t i o n i n p e r m e a b i l i t y w i t h temperature i n c r e a s e when water (and o n l y w a t e r , n o t mineral o i l , g a s or 2- octanol) flowed through sandstone ( c o n s o l i d a t e d sandstone o r u n c o n s o l i d a t e d sand but not l i m e s t o n e ) . This r e d u c t i o n was found t o be r e v e r s i b l e when t h i s was i n v e s t i g a t e d .

A similar work p r e s e n t e d a t t h i s meeting l a s t y e a r ( P o t t e r , e t a l . ) and t h e disc u s s i o n t h a t followed w i l l be used a s a b a s i s of comparison f o r t h e s e r e s u l t s . Experimental Apparatus and Procedure The e x p e r i m e n t a l a p p a r a t u s and procedure have been f u l l y d e s c r i b e d elsewhere (Gobran), however, a b r i e f d e s c r i p t i o n i s a p p r o p r i a t e h e r e f o r t h e d i s c u s s i o n t h a t follows. A schematic diagram of t h e a p p a r a t u s is shown i n F i g u r e 1. The f l u i d flow system b e g i n s w i t h a Ruska c o n s t a n t r a t e pump ( w i t h two 403 s t a i n l e s s steel c y l i n d e r s r a t e d t o 4000 psi). The t u b i n g , c o r e h o l d e r , d i f f e r e n t i a 1 p r e s s u r e t r a n s d u c e r s and o t h e r components of t h e f l u i d flow system a r e m d e of 316 s t a i n l e s s steel.

-121-

experiment. There was, however, a l a r g e loss i n permeability during the cooling c y c l e . The experiment was stopped b e f o r e t h e u s u a l f o u r f l o w r a t e measurements could be made a t 100°F.

The procedure f o r t a k i n g p e r m e a b i l i t y measurements is t h e same as w i t h d i s t i l l e d water (Gobran). A t each temperature f o u r d i f f e r e n t flow rates a r e used: t h r e e h i g h (about one l i t e r per hour) and one low The v a l u e (about 200 m i l l i l i t e r s per hour). of p e r m e a b i l i t y a t each of t h e h i g h flow r a t e s is t h e average of t h e v a l u e s a f t e r f i v e and then t e n i n c r e m e n t a l pore volumes. These v a l u e s are then averaged t o g e t t h e p e r m e a b i l i t y a t a given temperature. The low flow rate is maintained f o r only one pore volume and t h i s value is used a s a check t o be s u r e t h e r e is no f l o w r a t e effect.

F i g u r e 3 shows p e r m e a b i l i t y graphed v e r s u s throughput f o r t h i s experiment. The r e s u l t s of t h i s experiment with s a l t water can be compared w i t h t h e r e s u l t s f o r d i s t i l l e d water flow shown i n F i g u r e s 4 and 5. F i g u r e 4 shows p e r m e a b i l i t y as a f u n c t i o n of throughput a t 100°F. This shows t h e r e d u c t i o n i n p e r m e a b i l i t y t h a t can be expected merely due t o flow. C l e a r l y , t h e r e d u c t i o n i n p e r m e a b i l i t y (20% i n 95 p o r e volumes) i n t h e f i r s t s a l t water experiment was e x c e s s i v e as was t h e r e d u c t i o n during t h e c o o l i n g c y c l e of t h e second experiment.

This system w a s designed f o r d i s t i l l e d w a t e r flow. It w a s used t o measure t h e a b s o l u t e p e r m e a b i l i t y of sandpacks and c o n s o l i d a t e d sandstone c o r e s a s a f u n c t i o n of temperature from 100 t o 300°F, c o n f i n i n g p r e s s u r e from 2000 t o 10,000 p s i and pore p r e s s u r e from 200 t o 4000 p s i . During t h e s t u d i e s w i t h d i s t i l l e d w a t e r , no e f f e c t of contamination was found on t h e i n l e t f a c e of t h e porous media. Also, except f o r a s e t t l i n g o r rearrangement of g r a i n s , t h e r e w a s no red u c t i o n i n p e r m e a b i l i t y caused e x c l u s i v e l y by flow.

F i g u r e 5 shows p e r m e a b i l i t y a s a f u n c t i o n of temperature f o r d i s t i l l e d water flow. This compares very w e l l with t h e h e a t i n g c y c l e i n t h e second s a l t water experiment w i t h both showing no temperature dependence. Examinat i o n of t h e c o r e and t h e two f i l t e r s a t t h e conclusion of t h e second s a l t water e x p e r i ,merit showed t h a t t h e f i l t e r s were plugged with a brown p a r t i c u l a t e and t h a t t h i s same m a t e r i a l coated t h e i n l e t f a c e of t h e core.

R e s u l t s As s t a t e d p r e v i o u s l y , one of t h e g o a l s of t h i s s t u d y w a s t o i n v e s t i g a t e t h e e f f e c t of temperature on a b s o l u t e p e r m e a b i l i t y of u n c o n s o l i d a t e d sandpacks w i t h a 1%N a C l s o l u t i o n as t h e flowing f l u i d . The sandpacks were e i t h e r 100-120 o r 120-200 mesh O t t a w a sand. The c o n f i n i n g p r e s s u r e and t h e pore p r e s s u r e were 2000 and 200 p s i , r e s p e c t i v e l y , f o r a l l experiments.

The r e s u l t s of t h i s experiment i n d i c a t e d t h a t p e r m e a b i l i t y was not dependent on temperature with s a l t water as t h e flowing fluid. It a l s o showed t h a t both the flow l i n e s i n s i d e t h e a i r bath and t h e pump i t s e l f were c o r r o d i n g w i t h t h e s a l t water flow. The p r o g r e s s of t h e c o r r o s i o n produced p a r t i c u l a t e cold be delayed by use of f i l t e r s but t h e r e would s t i l l be permanent damage t o t h e pump and flow lines.

During t h e d i s t i l l e d w a t e r experiments and t h e i n i t i a l s a l t water experiment, a 15 micron f i l t e r was used i n t h e flow l i n e o u t s i d e t h e a i r bath. During t h e f i r s t s a l t water experiment t h i s f i l t e r plugged badly and t h e r e was a 20% r e d u c t i o n i n p e r m e a b i l i t y d u r i n g t h e f i r s t 95 pore volumes of throughput ( t h i s is f a r h i g h e r than would be expected merely due t o s e t t l i n g a s w i l l be seen l a t e r ) . A t t h i s p o i n t t h e c o r e w a s a t 150°F and t h e experiment was stopped. The 1 5 micron f i l t e r o u t s i d e t h e a i r bath w a s r e p l a c e d w i t h a 2 micron f i l t e r and a second 2 micron f i l t e r was i n s t a l l e d i n the a i r bath immediately p r i o r t o the c o r e h o l d e r .

A t t h i s p o i n t a s o l u t i o n ( i n s t e a d of a d e l a y i n g approach) was sought. A f t e r phone c a l l s t o numerous major o i l company r e s e a r c h l a b o r a t o r i e s , it was determined t h a t 100 p a r t s per m i l l i o n of sodium s u l f i t e (Na2S03) would remove t h e d i s s o l v e d oxygen from t h e s a l t water. A l t e r n a t e m a t e r i a l s such a s t i t a n i u m and m n e l were considered as replacements f o r t h e 316 s t a i n l e s s steel t u b i n g , b u t , due t o time and c o s t l i m i t a t i o n s , were not used. A system of check v a l v e s and c y l i n d e r s which allowed flow of mercury i n t h e pump and water i n t h e system was implemented. This d e s i g n is shown s c h e m a t i c a l l y i n Figure 6. A f t e r s e v e r a l a b o r t i v e experiments i n which mercury leaked i n t o t h e flow system, t h i s d e s i g n seemed t o work s u c c e s s f u l l y .

The r e s u l t s of t h e second experiment a r e shown i n Figure 2. Here p e r m e a b i l i t y is graphed v e r s u s temperature. There was a l o o s e wire t o one of t h e a i r bath h e a t i n g elements and t h e maximum a t t a i n a b l e temperat u r e was 284°F. These r e s u l t s show no s i g n i f i c a n t r e d u c t i o n i n a b s o l u t e permeab i l i t y d u r i n g t h e h e a t i n g p o r t i o n of t h e

During t h e f i n a l experiment t h e r e was a continued d e c r e a s e i n p e r m e a b i l i t y w i t h

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Derjaguin, B.V. and Churaev, N.V. (1973), "Nature of 'Anomalous Water'," Nature, Aug. 17, 1973, 430-431.

throughput ( a l t h o u g h not of t h e m g n i t u d e t h a t would be caused by mercury in t h e core). The low f l o w r a t e measurements yielded permeability values s i g n i f i c a n t l y lower than t h e h i g h rate values. This experiment w a s stopped w h i l e h e a t i n g t o 250°F when t h e d i f f e r e n t i a l p r e s s u r e a c r o s s t h e c o r e kept i n c r e a s i n g . This is i n d i c a t i v e of flow of t h e c o n f i n i n g o i l i n t o t h e core.

F a t t , I. (1953), "The E f f e c t of Overburden P r e s s u r e on R e l a t i v e P e r m e a b i l i t y , " Trans. , AIME, 325-326.

198,

F a t t , I. and Davis, D.H. (1952), "Reduction i n P e r m e a b i l i t y w i t h Overburden P r e s s u r e , " Trans., AIME, 329.

195,

Discussion I n h i s work, Mr. P o t t e r found c o r r o s i o n of h i s system (316 and 304 s t a i n l e s s s t e e l ) w i t h d i s t i l l e d water flow a t e l e v a t e d temperatures. This was due t o o x i d a t i o n which produced Fe+3 ions. When t h e system was changed t o t i t a n i u m , no more c o r r o s i o n problems were encountered. During the d i s c u s s i o n f o l l o w i n g h i s p r e s e n t a t i o n , i t was suggested t h a t t h e 304 s t a i n l e s s s t e e l components caused t h e problems and t h a t t h e 316 s t a i n l e s s would have no problems. This has been borne o u t by two r e c e n t s t u d i e s (Gobran, Sageev). However, systems c o n s t r u c t e d of 316 s t a i n l e s s s t e e l cannot be used with salt water flow at e l e v a t e d temperatures. Baaed on t h e r e s u l t s of t h i s work and o t h e r s t u d i e s on s t a i n l e s s steel (Gordon), s t a i n l e s s s t e e l and s a l t water a r e not compatible above 170'F. Therefore materials such as i n c o n e l , t i t a n i u m o r m n e l are needed f o r such systems.

Gobran, B.D. (1981), "The E f f e c t s of Confining P r e s s u r e , Pore P r e s s u r e and Temperature on A b s o l u t e P e r m e a b i l i t y , " Ph. D. D i s s e r t a t i o n , Stanford University. Gordon, B.M. (1980), "The E f f e c t of C h l o r i d e and Oxygen on t h e S t r e s s Corrosion Crackine of S t a i n l e s s S t e e l s : Review of L i t e r a t u r e , " Materials Performance, 2, A p r i l , 1980, 29-38.

-

F a t t , I. and Bergamini, G. Gray, D.H., (1963), "The E f f e c t of S t r e s s on P e r m e a b i l i t y of Sandstone Cores," SOC. Pet. Eng. J. , June, 1963, 95-100. Poston, S.W. , Ysrael, S., Hossain, A.K.M.S. , Montgomery, E.F. , 111, and Ramey, H.J. , Jr. (1970), "The E f f e c t of Temperature on I r r e d u c i b l e Water S a t u r a t i o n and R e l a t i v e P e r m e a b i l i t y o f Unconsolidated Sands," sot. Pet. Eng. J., June, 1970, 171-180.

Conclusions Two c o n c l u s i o n s can be made from t h i s study: 1 ) Absolute p e r m e a b i l i t y of sandpacks t o s a l t water is not a f u n c t i o n of temperature. 2 ) Materials o t h e r t h a n 316 s t a i n l e s s s t e e l must be used f o r s a l t water systems a t e l e v a t e d temperatures.

Nur, A. and Dibble, W.E., Jr. P o t t e r , J.M., (1980), " E f f e c t of Temperature and S o l u t i o n Composition on t h e P e r m e a b i l i t y of S t . P e t e r s Sandstone: Role of I r o n ( I I I ) , " Proceedings: S i x t h Workshop o n Geothermal R e s e r v o i r Engineering, 316-321.

Acknowledgement I would l i k e t o acknowledge t h e f i n a n c i a l s u p p o r t of t h e United S t a t e s Department of Energy who funded t h i s work through c o n t r a c t DE-CO3-76ET12056 a t t h e S t a n f o r d U n i v e r s i t y Petroleum Research Institute. I would a l s o l i k e t o thank ARC0 O i l and Gas Company f o r a l l t h e i r a s s i s t a n c e i n t h e p r e p a r a t i o n of t h i s paper and f o r a l l o w i n g me t o p r e s e n t it.

Sageev, A. (1980), "The Design and C o n s t r u c t i o n of a n Absolute Permeameter t o Measure t h e E f f e c t of E l e v a t e d Temperature on t h e Absolute P e r m e a b i l i t y t o D i s t i l l e d Water o f Unconsolidated Sand C o r e s , " M.S. Report S t a n f o r d U n i v e r s i t y .

,

References

Weinbrandt, R.M. (1972), "The E f f e c t of Temperature on R e l a t i v e P e r m e a b i l i t y , " Ph.D. D i s s e r t a t i o n , Stanford University.

Aruna, M. (1976), "The E f f e c t s of Temperat u r e and P r e s s u r e on Absolute P e r m e a b i l i t y of Sandstones," Ph.D. D i s s e r t a t i o n , S t a n f o r d University

Wyble, D.O. (1958), " E f f e c t of Applied P r e s s u r e on t h e C o n d u c t i v i t.y., P o r o s i t y and P e r m e a b i l i t y of Sandstones," Trans., AIME, 213, 430-432.

.

-

Casse, F.J. (1974), "The E f f e c t of Temperat u r e and Confining P r e s s u r e on F l u i d Flow P r o p e r t i e s of C o n s o l i d a t e d Rocks," Ph.D. D i s s e r t a t i o n , Stanford University,

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RELIEF

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Figure 1.

Schematic diagram of the experimental apparatus.

o

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m

m

THRWGHPUT (PORE VDLWS)

Figure 2.

Salt water permeability

Figure 3.

versus temperature.

Salt water permeability

versus throughput.

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fo I Figure 4 .

D i s t i l l e d water

Figure 5 .

p e r m e a b i l i t y v e r s u s throughput.

HATER SUPPLY

D i s t i l l e d water p e r m e a b i l i t y

v e r s u s temperature.

UECK VNE

CHECK VALVE

+

Figure 6 .

Mercury flow modifications.

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Si02 PRECIPITATION ACCOMPANYING FLUID FLOW THROUGH GRANITE HELD I N A TEMPERATURE GRADIENT

D. E. Moore,

U.S.

C. A. Morrow, and J.

D. Byerlee

Geological Survey, Menlo Park, C a l i f o r n i a 94025

A b s t r a c t I n experiments s i m u l a t i n g t h e r i s e y d r o t h e r m a l f l u i d s from a geothermal r e s e r v o i r , water was flowed a t a l o w r a t e down a temperature g r a d i e n t through f r a c t u r e d and i n t a c t c y l i n d e r s o f B a r r e and Westerly Granite. Temperatures ranged from 80 t o 105°C a t t h e o u t e r edges o f t h e c y l i n d e r s t o 250 t o 300°C along a c e n t r a l borehole which housed t h e h e a t i n g c o i l . As a r e s u l t o f m i n e r a l deposition, p a r t i c u l a r l y Si02 , a t l o w temperatures i n t h e g r a n i t e samples, p e r m e a b i l i t i e s were reduced 10- t o 1 0 0 - f o l d i n p e r i o d s o f 1 t o 3 weeks. Chemical analyses were made o f t h e low- temperature f l u i d s discharged f r o m t h e c y l i n d e r s . Early- sampled f l u i d s were supersaturated w i t h r e s p e c t t o several m i n e r a l s a t low temperatures i n t h e g r a n i t e s . O f t h e o v e r s a t u r a t e d species, Si02 showed t h e most r a p i d decrease w i t h time, and i n an experiment w i t h l o w i n i t i a l f l o w r a t e , t h e s o l u t i o n reached e q u i l i b r i u m w i t h q u a r t z a t t h e low- temperature edge o f t h e c y l i n d e r w i t h i n about 6 days. I n c r e a s i n g t h e maximum temperature o f t h e g r a d i e n t a t c o n s t a n t c o n f i n i n g pressure l e d t o h i g h e r Si02 c o n c e n t r a t i o n s i n t h e discharged f l u i d s . However, i n c r e a s i n g c o n f i n i n g pressure along w i t h maximum temperature r e s u l t e d i n lower d i s s o l v e d Si02 contents, because o f enhanced r e a c t i o n a t t h e reduced f l o w r a t e s accompanying t h e pressure increase. The behavior o f Si02 c o n t r a s t e d w i t h most o t h e r d i s s o l v e d species, which were a f f e c t e d by changes i n temperature b u t n o t f l o w r a t e i n t h e time o f t h e experiments.

f r o m those o f t h e o t h e r species i n s o l u t i o n . The d i s t i n c t i v e behavior o f s i l i c a and i t s e f f e c t on t h e p e r m e a b i l i t y o f t h e g r a n i t e s form t h e s u b j e c t o f t h i s sumnary paper. Ex eriments The experimental design i s 7e-Ts own i n i g u r e 1. C y l i n d r i c a l samples o f g r a n i t e 3" i n diameter and 3-1/2" l o n g c o n t a i n e d a 1/2"-diameter borehole i n which a c o i l e d r e s i s t a n c e heater produced a temperature g r a d i e n t between t h e c e n t e r and .outer edge. D i s t i l l e d water was f l o w e d r a d i a l l y through t h e g r a n i t e from h i g h t o low temperatures. Gold shims were p l a c e d

THERMOCOUPLE WIRES

PORE PRESSURE INLET

,,,,

PORE PRESSURE CUTLET

w

Introduction A series o f permeability experiments was conducted t o model t h e f l o w o f f l u i d s f r o m h i g h t o low temperatures across g r a n i t i c rocks. As a r e s u l t o f several days o f slow flow, t h e permeabi 1ity o f each g r a n i t e specimen was reduced t o 1 t o 10% o f i t s value p r i o r t o heating (Morrow and others, 1981). As an a i d i n determining t h e causes o f t h e p e r m e a b i l i t y reductions, t h e f l u i d s discharged f r o m t h e g r a n i t e specimens were c o l l e c t e d f o r chemical a n a l y s i s (Moore and others, i n press). An unexpected f i n d i n g o f t h e f l u i d analyses was t h a t t h e v a r i a t i o n s i n dissolved s i l i c a concentrations w i t h changing experimental c o n d i t i o n s d i f f e r e d

FUSED SILICA

STAINLESSSTEEL MESH

Cu-Ni PLATED JACKET Ni- PLATED STEEL PLUG

F i g u r e 1 Schematic sample assembly.

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c o n t e n t was determined from measurements o f t o t a l i n o r g a n i c carbon, made w i t h a carbon analyzer. The anions C1, F, and s u l f a t e were determined using an i o n chromatograph w i t h i n t e g r a t o r attachment.

a t t h e ends o f the g r a n i t e samples t o seal themi from t h e a d j o i n i n g fused s i l i c a cy1 inders t h a t served as thermal insuilators. Other exposed metal around t h e borelhole was g o l d - p l a t e d t o prevent contamination o f t h e f l u i d s . The sample asselmbly was separated from t h e c o n t a i n i n g jack.et by a s t a i n l e s s s t e e l mesh t h a t allowed drainage o f t h e discharged, low- temperature f l u i d s away from t h e r o c k cy1 inder.

Some o f t h e f l u i d compositions were analyzed w i t h t h e SOLMNEQ computer program o f Kharaka and Barnes (1973). which determines t h e s t a t e s o f r e a c t i o n (aGR)of the s o l u t i o n s a t a g i v e n temperature w i t h r e s p e c t t o up t o 158 mineral species. I n o r d e r t o determine t h e d i f f e r e n c e s i n r e a c t i v i t y o f a g i v e n f l u i d across t h e sample, most o f t h e s o l u t i o n compositions analyzed by SOLMNEQ were r u n a t b o t h t h e high- and low- temperature extremes o f t h e granite cylinders.

The experiments conducted a r e sumnarized i n Table 1. C y l i n d e r s o f Barre and Westerly Granite, b o t h b i o t i t e - m u s c o v i t e grariodiori tes, were used i n d i f f e r e n t experiments. Temperatures w i t h i n t h e boreholes ranged from 250 t o 300°C, and tholie a t t h e o u t e r edges were 80 t o 105°C. C o n l i n i n g pressures o f 300 bars were coupled w i t h pore pressures o f 100 bars, corresponding t o 1.2 km thickness o f overburden w i t h f 1u i d pressure r e s u l t i n g from t h e h y d r o s t a t i c head. Confining pre!;sures o f 600 bars and pore pressures o f 200 bars were a l s o used. A 5 t o 10 bar port! pressure d i f f e r e n t i a l produced a low r a t e o f f l u i d flow. D i s t i l l e d water was t h e s t a r t i n g f l u i d i n t h e experiments l i s t e d i n Table 1. An a d d i t i o n a l experiment using a NaHCOyCaC12 s o l u t i o n r a t h e r than d i s t i l l e d water r e s u l t e d i n s i m i l a r p e r m e a b i l i t y reductions.

P e r m e a b i l i t y Changes The v a r i a t i o n i n p e r m e a b i l i t y w i t h t i m e was determined from measured changes i n mass f l o w r a t e over t h e constant pore pressure d i f f e r e n t i a l , using the r a d i a l f l o w model o f Darcy’s Law. As an example o f t h e t r e n d o f p e r m e a b i l i t y decrease, t h e normal i z e d permeabi 1it y changes w i t h time f o r NWD22 and 24 a r e shown i n F i g u r e 2. The g r a n i t e c y l i n d e r s i n each experiment showed a r a p i d i n i t i a l p e r m e a b i l i t y drop f o l l o w e d by lower r a t e s o f decrease. A t constant c o n f i n i n g presures, an increase i n borehole temperature r e s u l t e d i n a more r a p i d r a t e o f p e r m e a b i l i t y reduction. I n a d d i t i o n , t h e l a r g e r t h e volume o f f l u i d flowed through t h e c y l i n d e r s , the g r e a t e r t h e permeabi 1ity drop.

N i nc? d i ssol ved species were analyzed from 1.5 m l f l u i d samples, c o l l e c t e d as separate 1.2!i and 0.25 m l samples. D e t a i l s o f t h e a n a l y t i c a l techniques and many f l u i d compositions are contained i n Moore and o t h e r s ( i n press). The 0.25 m l sample was analyzed f o r Si02 using t h e molybdate- blue method o f spectrophotometric From a n a l y s i s (ASTM, 1974; p. 401-402). t h e l a r g e r f l u i d sample, t h e c a t i o n s Ca, Na, K, and Mg were determined using atomic absorption t e c hn iques The bicarbonate

T e x t u r a l Evidence o f Reaction Thin- section and SEM examinations y i e l d e d some evidence of mineral r e a c t i o n and p r e c i p l t a t l o n i n t h e g r a n i t e c y l i n d e r s d u r i n g t h e experiments. Observations o f high- temperature mineral r e a c t i o n were c o n f i n e d t o narrow c o n c e n t r i c zones around t h e boreholes. There, c a l c i t e showed pronounced e t c h i n g along cleavage

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and f r a c t u r e t r a c e s and p h y l l o s i l i c a t e a l t e r a t i o n minerals i n p l a g i o c l a s e had changed c o l o r from green t o yellowish- brown. A few e a r l y experiments were conducted using samples c o n t a i n i n g a through- going f r a c t u r e (Morrow and others, 1981). A f t e r the experiments, t h e f r a c t u r e surfaces showed t r a c e s o f mineral d e p o s i t i o n near t h e low-temperature sides. Nost o f t h e deposits consisted o f patchy masses and f i b e r s o f s i l i c a which formed on exposed quartz g r a i n s ( F i g . 3 ) . I n addition, some Ca- rich f i b e r s grew on plagioclase crystals.

Table 2. Temperatures a t which the Si02 contents and Na, K, and Ca concentrations f o r selected f l u i d samples would be equi 1ibrium values. Sample

20 28

170 167

198 171 218 208

NWD22

63 69 70

104 99 72

250 263 225

186

265 229 221

NWD23

7 7 83 84

171

154

NWD24

93

iNa, Ca, K, Mg, F , and HCO3 U n l i k e C 1 and ' s u l f a t e t h e concentrations o f these 6 species are r e l a t e d t o mineral so I ubi 1it i e s , w i t h temperature the major icontrol on s o l u t i o n concentrations. The 'nigher Na contents an Na/Ca r a t i o s i n F l u i d s d e r i v e d from Barre G r a n i t e ( F i g . 4 ) (dre c o n s i s t e n t w i t h the occurrence o f a inore Na- rich p l a g i o c l a s e and a separate ( n l D i t i c a l t e r a t i o n phase i n Barre. I n t h e same way, the s l i g h t l y Mg-enriched j u i u t i o n s from Westerly a r e r e l a t e d t o t h e i i g h e r Mg contents o f m i n e r a l s i n t h a t g r a n o d i o r i t e . The amounts o f Na, K, and Ca s o l u t i o n doubled o r t r i p l e d w i t h a 50°C :e:nperatur-e increase ( F i g . 4 ) . The 1

Na-K-Ca

NWD20

C h l o r i d e and S u l f a t e N e i t h e r C1 nor S are inajor c o n s t i t u e n t s o f any mineral i n N e s t e r l y o r Barre; instead, they are concentrated i n i n t e r g r a n u l a r f l u i d s and 'adsorbed onto g r a i n surfaces ( E l l i s and IYahon, 1964). Considerable amounts o f b o t h i o n s were removed by f l u s h i n g c o l d water through the rock c y l i n d e r s f o r 3-5 days 3 r i o r t o heating. Nevertheless, the concentrations o f both species i n s o l u t i o n rose again upon i n i t i a l heatins w i t h the higher- temperature water t h e more e f f e c t i v e leaching agent ( F i g . 4). Subsequently, both i o n s dropped r a p i d l y t o low values.

1

Quartz

99

119 142

100

130

237 217 208

bicarbonate c o n c e n t r a t i o n a1 so doubled, which c o n t r a s t s w i t h t h e negative temperature dependence o f c a l c i t e s o l u b i l i t y b u t which can be explained by d i f f e r e n c e s i n s o l u t i o n pH. The Mg and F contents were c o n t r o l l e d by t h e s o l u b i l i t y o f mafic minerals and o f accessory f l u o r i t e , r e s p e c t i v e l y . The concentrations o f these 2 i o n s d i d n o t increase w i t h temperature, because minerals such as c h l o r i t e and f l u o r i t e show s l i g h t l y negative s o l u b i l i t y r e l a t i o n s h i p s i n t h a t temperature range ( E l l i s , 1971; Mahon, 1964). I n the same way, the s l i g h t l y Mg-enriched s o l u t i o n s from Westerly are r e l a t e d t o t h e h i g h e r Mg contents o f minerals i n t h a t granodiorite.

~

Changes i n the i o n i c concentrations w i t h time may r e f l e c t p a r t i a l r e a c t i o n o f the s o l u t i o n s a t 1ower temperatures i n t h e g r a n i t e s . Equi 1ib r i m temperatures o f selected f l u i d compositions c a l c u l a t e d using t h e Na-K-Ca geothermometer o f F o u r n i e r and Truesdell (1973) are l i s t e d i n Table 2, along w i t h quartz temperatures. Nearly a l l o f the i n i t i a l Na-K-Ca temperatures were below t h e borehole temperatures and they a1 so decreased w i t h time. The gradual decreases i n temperature indicate increasing reaction o f the s o l u t i o n s a t low temperatures as f l o w r a t e s decreased. Throughout most o f t h e 250°C experiments the s o l u t i o n s were undersaturated w i t h respect t o c a l c i t e a t t h e lowest temperatures i n the c y l i n d e r s , and bicarbonate concentrations increased w i t h

F i g u r e 3 Low-temperature d e p o s i t i o n o f s i l i c d f i b e r s on a quartz g r a i n exposed along a f r a c t u r e surface. SEM photomicrograph, 30,000 x indgnification. -130-

time. I n c o n t r a s t , t h e s o l u t i o n s were oversaturated w i h c a l c i t e a t t h e low- temperature edge f o r most o f the 300'C experiments, and t h e bicarbonate concentrations decreased w i t h time i n an apparent attempt t o reach e q u i l i b r i u m concentrations.

p r e c i p i t a t i o n o f t h e f l u i d volume and the r e l a t i v e i n t e r f a c i a l area between s o l i d and aqueous phases. W i t h t h e very small channel size, l a r g e c r y s t a l surface areas, and low and p r o g r e s s i v e l y d e c l i n i n g f l o w rates, t h e c o n d i t i o n s o f these experiments were h i g h l y f a v o r a b l e f o r quartz p r e c i p i t a t i on.

Ca concentrations decreased s t e a d i l y during t h e experiments, p a r t i c u l a r l y those a t 300'C. The r a t i o Ca/HC03 a l s o decreased w i t h time. The lowered Ca contents r e l a t i v e t o HC03 suggests t h a t some l i m i t i n g mineral s o l u b i l i t y o t h e r than c a l c i t e , such as t h e p r e c i p i t a t i o n o f a Ca-zeol it e o r Ca-montmori 1l o n i te, may have caused Ca concentrations t o decrease.

Thus, although t h e higher- T f l u i d s w i l l acquire g r e a t e r amounts o f d i ssol ved s i 1i c a near t h e borehole, t h e correspondingly decreased f l o w r a t e s a t t h e higher c o n f i n i n g pressures w i 11 enhance r e p r e c i p i t a t i o n of t h e s i l i c a a t low temperatures i n t h e g r a n i t e s . Concluding Remarks As i n d i c a t e d by f l u i d chemistry and petrographic examination o f f r a c t u r e surfaces, quartz p r e c i p i t a t i o n along g r a i n boundaries and m i c r o f r a c t u r e s may have been the p r i n c i p a l source o f t h e observed permeabi 1it y decreases i n t h e g r a n o d i o r i t e s . The s i l i c a c o n t e n t o f t h e discharged f l u i d s i s p a r t i c u l a r l y s e n s i t i v e t o reductions i n f l o w rate, i n contrast t o the o t h e r d i s s o l v e d species. Other minerals, such as c a l c i t e and probably some a l u m i n o s i l i c a t e s were a l s o supersaturated i n some o f t h e f l u i d s . These m a t e r i a l s may have been deposited as w e l l , b u t a t low r a t e s compared t o quartz.

Silica

The presence o f s i l i c a i n t h e i s c o n t r o l l e d most s t r o n g l y by t h e d i s s o l u t i o n o f quartz, which shows marked increases i n s o l u b i l i t y w i t h i n c r e a s i n g temperature ( f o r example, Kennedy, 1950) and i s by f a r t h e most r e a c t i v e mineral i n g r a n o d i o r i t e subjected t o f l o w o f hydrothermal f l u i d s (Charles, 1978). F o r experiments r u n a t t h e same c o n f i n i n g pressure w i t h t h e same rock type (NWDPO and 231, a 30°C increase i n temperature l e d t o somewhat h i g h e r d i s s o l v e d s i l i c a Concentrations. However, i n c r e a s i n g c o n f i n i n g pressure as w e l l as temperature r e s u l t e d i n much lower s i l i c a contents ( F i g . 4 ) and correspondingly lower e q u i l i b r i u m temperatures (Table 2 ) . The lower concentrations a t higher temperatures and c o n f i n i n g pressures c o n t r a s t markedly w i t h t h e behavior o f t h e o t h e r d i s s o l v e d species t h a t show p o s i t i v e temperaturedependent s o l u b i l i t i e s ( F i g . 4).

The e f f e c t s o f temperattire and f l u i d volume on t h e r a t e o f p e r m e a b i l i t y decrease are e x p l i c a b l e i n terms o f t h e amount o f m a t e r i a l t r a n s p o r t e d from h i g h t o low temperatures across the g r a n i t e s . Increasing temperature r a i s e d t h e concentrations o f t h e s o l u t i o n s around the borehole, thus i n c r e a s i n g t h e amount of m a t e r i a l t h a t was t r a n s p o r t e d and redeposited a t low temperatures i n t h e c y l i n d e r s . However, decreasing f l o w r a t e reduced t h e amount o f mass t r a n s f e r i n a g i v e n time; and i f t h e f l u i d volume was s u f f i c i e n t l y low, p e r m e a b i l i t y decreases occurred a t a lower r a t e d e s p i t e i n c r e a s i n g temperature.

None o f t h e discharged f l u i d s was s a t u r a t e d w i t h r e s p e c t t o quartz a t t h e borehole temperatures o f t h e g r a n i t e c y l i n d e r s . However, s i l i c a phases such as quartz, chalcedony and a - c r i stobal it e were oversaturated i n a t l e a s t some f l u i d s a t t h e low- temperature side. The values o f AGR f o r s i l i c a species decreased w i t h time i n an experiment, c o n s i s t e n t w i t h s i l i c a d e p o s i t i o n from t h e low- temperature, oversaturated s o l u t i o n s . Q u a r t z r a t h e r than some o t h e r s i l i c a species may be t h e c r y s t a l l i z e d phase, because t h e s i l i c a contents o f t h e s o l u t i o n s continued t o decrease t o values below t h e s o l u b i l i t i e s o f chalcedony and a - c r i stobal ite. I n a d d i t i o n , t h e f i n a l f l u i d samples o f t h e 300°C Westerly G r a n i t e experiment (NWD22) were i n e q u i l i b r i u m w i t h q u a r t z a t t h e low- temperature, o u t e r edge o f t h e c y l i n d e r (Table 2 ) .

REFERENCES American Society F o r T e s t i n g and M a t e r i a l s (1974), "Annual Book o f ASTM, p a r t 31 [water]," Philadelphia, Pa., 902 pp. Charles, R. W. (1978) , "Experimental geothermal loop: 1, 295°C study," Los Alamos S c i e n t i f i c Laboratory Informal Rept. LA-7334-MSI 44pp. E l l i s , A. J . (19711, "Magnesium i o n concentrations i n t h e presence o f magnesium c h l o r i t e , c a l c i t e , carbon dioxide, quartz," Am. J . Sci. 271, 48 1-489.

The reduced d i s s o l v e d s i 1i c a concentrations i n h i g h e r temperature and pressure experiments ( F i g . 41 may be a f u n c t i o n o f t h e competing e f f e c t s of temperature and f l o w r a t e on s i l i c a s o l u t i o n and redeposition. R i m s t i d t and Barnes (1980) have demonstrated t h e importance t o q u a r t z

E l l i s , A. J. and Mahon, W. A. J . (1964), " Natural hydrothermal systems and -131-

Mahon, W. A. J . (19641, " F l o u r i n e i n t h e n a t u r a l thermal waters o f New Zealand," N. Z. J. Sci. 7, 3-28.

experimental h o t - w a t e r h o c k i n t e r a c t i o n s , " Geoc him. Cosmoc him. Acta 28, 1323-1357.

Moore, D. E., Morrow C., and Byerlee, J. D., "Chemical r e a c t i o n s accompanying f l u i d f l o w through g r a n i t e h e l d i n a Temperature g r a d i e n t , " Geochim. Cosmochim. Acta, i n press.

I'ournier, R. 0. and Truesdell, A. H. (19731, "An e m p i r i c a l Na-K-:a geothermometer f o r n a t u r a l waters, Geochim. Cosmochim. Acta 37, 1255-1275. Kennedy, G. C. (19501, "A p o r t i o n o f t h e system s i l i c a - w a t e r , " Econ. Geol. 45, 629-653.

Morrow, C., Lockner, D., Moore, D., and Byerlee, J. (1981 1, "Permeabi 1ity o f g r a n i t e i n a temperature gradient," J. Geophys. Res. 86, 3002-3008.

Kharaka, Y. K. and Barnes, I . (19731, "SOLMNEQ: Solution- mineral equi 1i b r i u m computations," NTIS, U.S. Oept. Comnerce, PB-215 899, 8 1 pp.

R i m s t i d t , J. D. and Barnes, H. L. (19801, "The k i n e t i c s o f s i l i c a - w a t e r reactions," Geochim. Gosmochim. Acta 44, 1683-1699.

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STRESS INDUCED RELEASE OF RnZz2 AND CHI, TO PERCOLATING WATER I N GRANITIC ROCK

C. G. Sammis,

M. Banerdt, and D. E. Hammond

Department o f Geological Sciences U n i v e r s i t y o f Southern C a l i f o r n i a Los Angeles, Cal if o r n i a 90007 may change t h e ground water radon concentrat i o n s observed a t a sampling s i t e i n two ways; (1) d i r e c t l y , by changing t h e number o f radon atoms which e n t e r t h e groundwater, and ( 2 ) i n d i r e c t l y by changing the t r a n s p o r t o f radon from t h e s i t e a t which i t i s created t o the sampling s i t e .

A b s t r a c t The radon, methane, and carbon d i oxide concentrations have been measured i n water which has p e r c o l a t e d through g r a n i t i c rock under t r i a x i a l stresses ranging from 0.1 t o 0.95 o f t h e f r a c t u r e s t r e s s . We simultaneously measured t h e p e r m e a b i l i t y and poro s i t y (hence h y d r a u l i c r a d i u s ) o f each sample. The f i r s t s e r i e s o f experiments were on seventeen i n i t i a l l y d r y rock samples. The radon concentration i n t h e f i r s t 3 gin o f water c o l l e c t e d v a r i e d by a f a c t o r o f 10, b u t was n o t c o r r e l a t e d w i t h s t r e s s . A good c o r r e l a t i o n was found between radon and p e r m e a b i l i t y ; rocks having h i g h p e r m e a b i l i t i e s tend t o release l e s s radon. I n a second s e r i e s o f experiments, rock samples were s a t u r a t e d under stress, equi 1ib r a t e d f o r one month, then restressed and measured. These samples produced between 2 and 10 times more radon than i n i t i a l l y d r y rocks having t h e same permea b i l i t y . I n a t h i r d s e r i e s o f experiments, m u l t i p l e successive w a t e r f r a c t i o n s were sampled. We found t h a t most o f t h e radon i s r e moved w i t h t h e f i r s t pore volume c o l l e c t e d , w h i l e methane e x t r a c t i o n r e q u i r e s several pore volumes. An experiment i n which the s t r e s s was changed d u r i n g a run produced an increase i n CHI, b u t no increase i n RnZz2. These r e s u l t s are i n t e r p r e t e d i n terms o f a numerical model f o r f l o w and gas e x t r a c t i o n from a m i crocrack network.

The r e l a t i v e l y s h o r t h a l f - l i f e o f RnZz2 ( T t = 3.825 days) assures a s h o r t term causal r e f a t i o n s h i p between changes i n t h e f r a c t u r e network morphology and the r e s u l t a n t v a r i a t i o n i n groundwater radon. Because radon i s an i n e r t gas, i t does n o t r e a d i l y e n t e r i n t o chemical combination and i t s h i g h s o l u b i l i t y i n water (22.4 cm3 p e r 100 cm3 H20 a t 25OC and one atmosphere) means t h a t , i n t h e presence o f groundwater, most radon w i l l be dissolved r a t h e r than adsorbed t o t h e w a l l s o f the f r a c t u r e network. The p h y s i c a l mechanisms by which radon i s generated and transported are discussed i n more d e t a i l by Tanner (1980). Two a p p l i c a t i o n s i n which groundwater radon has been monitored are earthquake p r e d i c t i o n and geothermal r e s e r v o i r engineering. The b u i l d - u p o f t e c t o n i c s t r e s s preceding an earthquake might be expected t o change t h e morphology o f a f r a c t u r e network by c r e a t i n g and opening cracks p a r a l l e l t o t h e maximum p r i n c i p a l s t r e s s w h i l e c l o s i n g orthogonal cracks. I n t h e l a b o r a t o r y , the crack opening process dominates when t h e d i f f e r e n t i a1 stress, UI-U~ i s g r e a t e r than approximately one- half o f t h e f r a c t u r e s t r e s s and t h e rock a c t u a l l y increases i n volume. Laboratory s t u d i e s o f t h i s " d i l a t a n c y " phenomenon are reviewed by Brace (1978) and Byerlee (1978). F i e l d observations o f groundwater radon v a r i a t i o n s associated w i t h earthquakes are summarized by Hauksson (1981 ) and Teng (1 980). The withdrawal o f water and heat from a geothermal r e s e r v o i r might a l s o be expected t o change t h e morphology o f t h e f r a c t u r e network through t h e combined e f f e c t s o f thermal cont r a c t i o n and v a r i a t i o n s i n pore pressure. The associated change i n radon concentration could serve as a u s e f u l t o o l f o r n o n i t o r i n g reserv o i r development. Radon measurements i n geothermal systems a r e discussed by Stoker and Kruger (1975).

I n t r o d u c t i o n The p h y s i c a l p r o p e r t i e s o f radon gas (Rn"') make i t an i d e a l n a t u r a l t r a c e r f o r d e t e c t i n g changes i n t h e f r a c t u r e networks which permeate n a t u r a l rock masses. As an i n t e r m e d i a t e daughter i n t h e U238 decay s e r i e s , Rn222 i s c o n s t a n t l y being generated i n uranium b e a r i n rocks. Formed by alpha-decay o f t h e radium QRaZz6) parent, t h e radon atom has a r e c o i l energy o f about 100 KeV; s u f f i c i e n t energy t o t r a v e l hundreds o f l a t t i c e spacings upon formation. While most o f these atoms lodge w i t h i n the i n t e r i o r o f a g r a i n , some end up i n t h e network o f microcracks (and macrocracks) which permeate a r o c k mass, and may thereby e n t e r the groundwater. The amount o f radon d i s s o l v e d i n groundwater i s thus p r i m a r i l y a f u n c t i o n o f the c o n c e n t r a t i o n and s p a t i a l d i s t r i b u t i o n o f radium w i t h i n the rock, and the p o r o s i t y and p e r m e a b i l i t y o f the f r a c t u r e network. Changes i n the f r a c t u r e network

-133-

Interpretation of f i e l d anomalies requires three basic s e t s of information: ( 1 ) a thorough understanding of t h e subsurface hydrology (reservoirs , permeabi 1i t i e s , and press u r e s ) , ( 2 ) a detailed knowledge of the subsurface geology (rock types and uranium concentrations), and (3) empirical relationships between changes i n s t r e s s , the resulting changes i n fracture permeability, and the associated release of radon (and other gases) i n t o the groundwater ( a s a function of rocktype). The laboratory measurements reported below were designed t o explore relationships required in s t e p ( 3 ) above. We have focused on the relationships between d i f f e r e n t i a l s t r e s s , permeability, and the release of radon and methane t o water percolating through the natural fracture network i n g r a n i t i c rock.

J:

L..J

A cylindrical granite sample (two inches in diameter by four inches long) is loaded i n the t r i a x i a l c e l l which i s placed between the anv i l s of a 160,000 pound hydraulic press. This uniaxial s t r e s s i s s u f f i c i e n t t o cause f a i l u r e of the samples under our nominal 200 bar confining pressure.

Pi-evi ous Laboratory Studies of Radon Emanation arid Transport

..

1.11,

fig. I

There have been several labor-

Confining pressure, u3, and flow pressure, p f , are generated by air- driven hydraulic pumps w i t h special s t a i n l e s s s t e e l valve isolation chambers t o maintain water purity. Radon f r e e d i s t i l l e d water i s flowed through the sample a t constant flow pressure pf of 100 bars. A one millimeter thick layer o f 200 mesh Z r C powder spreads the water a t pf over the e n t i r e t o p surface area, A , of the sample. The lower surface of the sample i s maintained a t p=o since we are sampling i n t o a vacuum. The base-plate has a pattern of radial and circumferential grooves which channel the flow water i n t o the collection outlet hole. By measuring the amount of water as a function of time, the flow r a t e , q(cm3/sec) , i s determined. Darcy's law may then be used t o calculate the sample permeability, k (cm2), according t o

story studies of radon emanation from rock

surfaces. Chiang e t a l . (1978) demonstrated t h a t radon emanation is proportional t o surface area by measuring the radon emanated from a number of rock samples having the same volume b u t d i f f e r e n t surface areas. The relation between s t r e s s and radon emanation has been studied by the Group of Hydrochemistry o f the Peking Seismological Brigade (1977), Holub and Brady (1981), and Jiang and L1 ('1981). In each of these experiments, a i r was circulated around t h e rock sample ( o r throu h holes bored i n the sample in the 1977 study? a!; uniaxial s t r e s s was applied. Radon levels in t h i s c i r c u l a t i n g a i r were continuously monitored. In t h e two Chinese experiments only small increases in radon were observed d u r i n the uniaxial loading. Holub and Brady ( 'I981 observed a 1 arge (50%) temporary i n ci-ease in radon a t about half the breaking strength of the sample, presumably due t o t h e opening of axial microfractures a t t h e onset o f dilatancy. In a l l three experiments, the rddon level increased by a f a c t o r between two and ten upon f a i l u r e , probably r e f l e c t i n g the large increase i n surface area d i r e c t l y access.ible t o the c i r c u l a t i n g gas.

4

k=-qpl /APf

(1 1

where 1 i s the sample length i n cm and p i s the water viscosity i n dynes sec/cm2. We have

The fundamental difference between these studies and our experimental work described below is t h a t the above studies measured radon emanation from t h e surface of rock samples while we measured the radon released t o water percolating through the microfracture system. We are t h u s able t o look f o r quantitative correlations between radon re1 ease and s t r e s s , permeability, porosity, hydraulic radius, and crack surface area. Experimental Apparatus

The experimental apparatus is shown schematically i n Figure 1 below. The t r i a x i a l pressure vessel i s standard except f o r an o u t l e t a t the base t o allow sampling of the pore water.

-134-

been working a t low values of the confining pressure ( u 3 nominally 200 bars). The functions of the confining pressure are ( 1 ) t o seal the rubber sample jacket thus confining the f r a c t u r e f l u i d within the sample, and ( 2 ) t o maintain sample i n t e g r i t y a t h i g h values of uniaxial s t r e s s . Granite Samples Approximately twenty f e e t of granite core was supplied by the U.S.G.S. The core i s from the depth range 425-445 f e e t i n the Sierra Nevada batholith, i s very fresh, and appears both compositionally and mechanic a l l y uniform over i t s e n t i r e length. Radiogenic analysis of six samples (chosen as representing extremes i n observed radon production as discussed below) showed U238 concentrations between 2.50 and 3.10 ppm. Uranium concentration was uncorrelated w i t h Rn222 concentrations observed i n the percolating water.

Rn222 Measurement Techniques Water samples were c o l l e c t e d i n evacuated pyrex test- tubes sketched below. Typical water samples were between 2 and 10 m l .

-lllk7

,*LEI

s e r i e s the c o n c e n t r a t i o n o f CH,I measured i n a d d i t i o n t o Rn222.

I n t h e i n i t i a l s e r i e s o f experiments, a dry r o c k was jacketed and s t r e s s e d a x i a l l y t o 01, under a c o n f i n i n g pressure o f u3. An evacua t e d sample c o l l e c t i o n vessel was then attached t o the c o l l e c t i o n p o r t i n the baseplate, and a flow pressure, pf, was e s t a b l i s h e d a t t h e upper end o f the sample a t time t=o. The time a t which t h e f i r s t drop of water appeared a t t h e baseplate, tf, was recorded, as was the time, ts, when the sample b o t t l e was sealed and removed. As discussed above, A t = t s - t f and t h e t o t a l volume were used t o f i n d t h e sample p e r m e a b i l i t y . We show below t h a t tf may be used t o f i n d the sample p o r o s i t y once t h e p e r m e a b i l i t y i s known.

Oll/rl 'RY"

and C02 were

ell

fig. 2

Radon was e x t r a c t e d from t h e water u s i n g r e c i r c u l a t i n g helium as a c a r r i e r (e.g. Key g a l . , 1979). Radon was separated from the He c a r r i e r by two stages o f li u i d n i t r o g e n coldtrapping. A d r y i n g column qCaSOs+Ascari t e ) was used t o separate COP and H20 from RnPP2 which was subsequently t r a n s f e r r e d t o a s c i n ti l l a t i o n c e l l . Radon gas l e v e l s were measured by c o u n t i n g s c i n t i l l a t i o n s associated w i t h i t s a decay. The o v e r a l l d e t e c t i o n e f f i c i e n c y i s about 80% as e s t a b l i s h e d by analyzing RaPP6 standard s o l u t i o n s . The background l e v e l i s 0.7 cpm (counts p e r minute)*; our experimental measurements are always a t l e a s t a f a c t o r o f f o u r above t h i s l i m i t . Note t h a t radon dpm ( d i s i n t e g r a t i o n s p e r minute) r e p o r t e d below are l e s s than the t o t a l observed c m s i n c e t h e two a1 ha e m i t t i n g daughters o f RngP2 (Po218 and are detected w i t h the same e f f i c i ency as t h e i r parent.

The sealed sample vessel was then attached d i r e c t l y t o the radon e x t r a c t i o n apparatus and t h e radon concentration determined as discussed above. Following t h e radon e x t r a c t i o n , u 1 was increased s l o w l y (40 bars/min) and the f r a c t u r e stress, uf, was recorded. The r e s u l t s o f t h i s f i r s t s e r i e s o f e x p e r i m n t s are p l o t t e d as open c i r c l e s i n Figures 3-5.

CHI, and CO? Measurement Techniques Before radon was e x t r a c t e d from sane water samples, small (Q 1 cc) a l i q u o t s o f the gas phase i n the sample c o n t a i n e r were drawn i n t o glass syringes through a rubber septum. These samp l e s were i n j e c t e d i n t o a gas chromatagraph equipped w i t h flame i o n i z a t i o n and thermal conduct iv i t y detectors. CHI, was i s o l a t e d from o t h e r gases on a molecular sieve #5A column and COP was i s o l a t e d on a s i l i c a gel column. T h i s sampling technique r e s u l t e d i n contamin a t i o n o f t h e sample w i t h a i r , and t y p i c a l l y 50-70% o f t h e CHI, and C02 peaks observed were due t o t h i s contamination. However, c l e a r l y detectable amounts o f these two gases were released i n most experiments. A n a l y t i c a l prec i s i o n f o r d u p l i c a t e analyses was about 8%. Blanks were run on the water b e f o r e i t passed through t h e rock t o ensure t h a t t h i s was n o t a s i g n i f i c a n t source o f contamination. The t o t a l q u a n t i t y o f gas released from each samp l e was c a l c u l a t e d by assuming e q u i l i b r i u m e x i s t s between l i q u i d and gas i n t h e sample container, knowing t h e volume o f t h e gas and l i q u i d phases i n t h e container, and c o r r e c t i n g f o r contamination from a i r (which was e s t i mated from t h e O2 c o n t e n t ) .

&- - -

fig. 3

DIFFERENTIAL

STRESS

UP0

Figure 3 shows sample p e r m e a b i l i t y , k, as a f u n c t i o n o f s t r e s s . Although the data s c a t t e r , they are c o n s i s t e n t w i t h a s l i g h t decrease i n k w i t h i n c r e a s i n g u1 t o approximately ul=uf/2, f o l l o w e d by a more r a p i d increase i n k as u1 approaches c y (as documented by Zoback and Byerlee (1975) a t h i g h e r c o n f i n i n g pressures). Our s c a t t e r i s due t o the f a c t t h a t each data p o i n t represents a d i f f e r e n t rock, whereas Zoback and Byerlee measured k as a f u n c t i o n o f uJ1 on one sample. Figure 4 shows radon conc e n t r a t i o n i n t h e f i r s t t h r e e grams o f water c o l l e c t e d as a function o f the d i f f e r e n t i a l Note t h a t t h e r e i s no obvious s t r e s s , UI-U~. r e l a t i o n between s t r e s s and radon. I f t h e radon c o n c e n t r a t i o n i s p l o t t e d as a f u n c t i o n o f the d i f f e r e n t i a l s t r e s s normalized t o the f r a c t u r e s t r e s s , (u1-u3)/uf, t h e r e i s s t i l l no obvi ous co r r e 1ati on.

Experimental Procedures and Results Two s e r i e s o f experiments were run: experiments on i n i t i a l l y d r y rocks where a s i n g l e water sample was c o l l e c t e d , and experiments on d r y and i n i t i a l l y s a t u r a t e d rocks where sequential water samples were c o l l e c t e d . I n t h i s second *ICurie=Z.Z x 1Ol2 cpm

-135-

The r e s u l t s of the sequential sampling experiments are given i n Figures 6 and 7 f o r radon and Figure 8 f o r nethane. In Figure 6 , the t o t a l radon i s plotted as a function of the total water collected. The s o l i d curves are calculated from a crack model discussed below. The dashed curves are sketched through data s e t s which could not be f i t t o the model.

fig. 4

fig. 7

fig. 5

Figure 7 shows the radon concentration which would be measured i f the experiment were stopped a f t e r the t o t a l amount of water shown on the abscissa had been collected.

Figure 5 shows t h a t radon concentration tends t o decrease w i t h increasing permeability. The second s e t of experiments were designed t o see i f saturated rocks y i e l d more radon t h a n tha'se which a r e i n i t i a l l y dry. Several rock sarriples were stressed and saturated as above until the first drop of water was observed. They were then removed from the apparatus and s t o r e d under water f o r approximately one month t o allow radon t o e s t a b l i s h an equilibrium concentration. They were then restressed and several successive water samples were collected. Sequential sampling experiments were a l s o performed on two i n i t i a l l y dry rocks f o r comparison.

Note t h a t four of these multiple sample runs were on the same rock (435-3D). This rock was i n i t i a l l y saturated, equilibrated, and then restressed t o a d i f f e r e n t i a l s t r e s s of 141 MPa. Three successive water samples were collected. The s t r e s s was then increased to 193 MPa and a fourth sample was collected. This fourth point, the s o l i d t r i a n g l e i n Figures 6 and 7 , i s on trend w i t h the previous three points and shows no s i g n i f i c a n t increase i n radon following the s t r e s s change. The rock was then removed from the apparatus, reequilibrated under water, and restressed t o 193 MPa. The four, sequential, water samples collected a t t h i s s t r e s s contained s l i g h t l y more radon (much l e s s than a f a c t o r of two) than the 141 MPa run. The rock was removed, reequilibrated, and then stressed to 244 MPa. Four samples were again taken, b u t t h i s time an unusually low radon concentration was measured. The sample was observed t o have devel oped a through-going, extensive fracture zone. Because the low radon level f o r t h i s r u n i s due almost e n t i r e l y t o the low concent r a t i o n in the f i r s t sample collected, the rock was re-equilibrated and rerun a t 244 MPa as a check. Again, the radon concentration i n the f i r s t sample was anomalously low when compared w i t h the other multiple sample runs.

The radon concentrations measured i n the f i r s t three grams of water collected d u r i n g these runs are plotted as open t r i a n g l e s in Figures 3-5. In Figure 5 i t i s apparent t h a t , a t a given permeability, the presence of water i n the fracture network increases the radon release by a f a c t o r between two and ten. fig.

In Figure 8 , the t o t a l methane released i s plotted as a function of the t o t a l water col1ected. TOTAL

WATER

COLLECTED, p m

-136-

Once t h e p o r o s i t y and the h y d r a u l i c radius, defined as the volume t h e i r surface area). (1978) gives

f

fig. 8

p e r m e a b i l i t y a r e known, m y may be found (m i s o f the cracks d i v i d e d by For i n t a c t rock, Brace

k = m2q3/k,

(5 1

where k, i s a geometrical f a c t o r between 2 and 3, w h i l e f o r d i l a t a n t microcracks i n low pomsi t y rock, p e r m e a b i l i t y i n the d i r e c t i o n of the microcracks has the form (6 1

k = m2n/k, Note t h a t the general shape o f these curves i s s i m i l a r t o the radon curves i n Figure 6, b u t t h e r e are s i g n i f i c a n t d i f f e r e n c e s . More t o t a l water f l o w was r e q u i r e d t o e x t r a c t a l l t h e a v a i l a b l e CH4r i n f a c t , some rocks released almost no methane t o t h e f i r s t few grams o f flow water. Also, t h e rock which was run a t t h r e e d i f f e r e n t stresses (435-3D) released more methane a t h i g h e r stresses. Even t h e s t r e s s change from 141 MPa t o 193 MPa d u r i n g t h e f i r s t run produced an increase i n CHI, concen t r a t ion , whi 1e t h e Rn222 concentration was unaffected.

For our samples the above a n a l y s i s y i e l d e d p o r o s i t i e s i n t h e range 0.2% t o 10%. P l o t t i n g p o r o s i t y as a f u n c t i o n o f p e r m e a b i l i t y y i e l d s k p r o p o r t i o n a l t o n 2 , between the n 3 dependence of (5) and t h e l i n e a r dependence o f (6). The i m p l i c a t i o n i s t h a t the microcrack o r i e n t a t i o n s a r e somewhere between random and vert i c a l . The observed radon release d i d n o t c o r r e l a t e w i t h pore volume , hydraul i c r a d i u s , o r crack surface area. Models f o r Radon Release We consider two extreme-case models f o r the radon e x t r a c t i o n process: ( 1 ) a well- mixed r e s e r v o i r model, and ( 2 ) a p i p e - f l o w model.

Carbon d i o x i d e was more e r r a t i c than methane o r radon when p l o t t e d as a f u n c t i o n o f t o t a l water c o l l e c t e d . There was no ap a r e n t corr e l a t i o n between CH4, C02, and RnE22. Our i n t e r p r e t a t i o n o f these r e s u l t s i s t h a t each o f these gases must occupy d i f f e r e n t p o s i t i o n s i n t h e rock.

I n t h e r e s e r v o i r model, we assume t h a t dnr/dt =

Sample P o r o s i t y and H y d r a u l i c Radius The p o r o s i t y o f t h e sample under s t r e s s may be c a l c u l a t e d from tf, the time i t takes t o f i l l the i n i t i a l l y d r y rock w i t h water. Following Brace e t a l . (1968) we approximate the f l o w law as F p n x 2 = 0. Hence dp/dx = f ( t ) , & t h e pressure g r a d i e n t i s approximated by a l i n e a r g r a d i e n t which changes w i t h time. Brace e t a l . (1968) have shown t h a t t r a n s i e n t s due t o %eneglected terms are on the order o f 10-30 sec. Since i t takes a t l e a s t an hour f o r our samp l e s t o f i l l , t h e l i n e a r g r a d i e n t should be a good f i r s t approximation. As water i s pumped i n t o t h e rock a t a constant back-pressure, pf, the water f r o n t w i l l advance i n the axial z direction a t a r a t e dz/dt = q/qA where n i s t h e p o r o s i t y . ( 1 ) f o r q gives dZ/dt =

-

(2)

Using Darcyls law kPf/pnz

(3)

which may be i n t e g r a t e d t o y i e l d

n

=

2kpftf/v12

(4)

-137-

-

qnr/Vc

( 7)

where nr i s the number o f accessible radon atoms i n the rock and Vc i s the crack volume. Using q dV/dt and nr + nw = N, where No i s t h e t o t a l accessible radon and nw i s the numb e r o f accessible radon atoms i n the f l o w water, ( 2 ) may be r e w r i t t e n and solved t o g i v e nw = N,[l-exp(-V/Vc)l I n terms of radon a c t i v i t y a=xn (h=1.26 x min 1 ) a, = A,[l-exp(l-V/Vc)]

(8)

lo-'' (9)

Only our d r y samples, 435-4D and 435-1Db could be f i t t o t h i s model ( s o l i d l i n e s i n Figure 6). They both l i e on t h e same curve which i s def i n e d by the parameters A, = 4.6 dpm and V = 9.0 cm3. The t o t a l radon a c t i v i t y i n the& rocks should be 1.2 x l o 3 dpm (based on 3 ppm uranium). Hence we are removing about 0.4% o f t h e a v a i l a b l e radon. A crack volume o f 9 cm3 corresponds t o p o r o s i t y (under s t r e s s ) o f about 4.5%, which seems a b i t high. None o f the wet rocks c o u l d be f i t t o t h i s model because the t o t a l radon curves i n Figure 6 have too sharp a "knee" f o r the exponential form i n ( 9 ) . Such behavior i s more t y p i c a l o f a p i p e model which assumes a l l the radon i s i n s o l u t i o n and i s simply pushed o u t by t h e advancing f l o w water w i t h no mixing. I n t h i s case, Figures 6 and 7 should appear as sketched

below.

I

I R) /----

L'watc r

I

( h\

The hypothesis which i s c o n s i s t e n t w i t h a l l our observations i s t h a t most of the observed radon i s d e r i v e d from radium which has been deposited on the w a l l s o f o l d microcracks. We are curr e n t l y performing a s e r i e s o f experiments t o t e s t t h i s hypothesis.

_-/--

Vwaier

The i n i t i a l l y s a t u r a t e d samples i n Figure 6 l o o k more l i k e ( a ) above than t h e smooth curve p r e d i c t e d by (9). The t r u e s i t u a t i o n probably 1i e s between these two extreme models , & the water moves through as i n a p i p e model, b u t i s c o n t i n u a l l y being f e d by s i d e channels i n the crack network thus rounding the sharp corners i n (a) and (b) above.

References

Summary o f Results We can b r i e f l y sumnarize our i n t e r p r e t a t i o n o f t h e r e s u l t s presented i n t h e previous section. a. There was no c o r r e l a t i o n between & released and s t r e s s a p p l i e d m i n i t i a l l y d r Sam l e s . Among several p o s s i b l e explana-$It i o n s of t i s observation are: (1) t h e new m i c r o f r a c t u r e s opened by t h e h i g h e r stresses do n o t connect w i t h t h e network o f f r a c t u r e s which are c a r r y i n g t h e water, and/or they do n o t c a r r y a s i g n i f i c a n t f r a c t i o n o f t h e water o r ( 2 ) these new m i c r o f r a c t u r e s c a r r y a s i g n i f i c a n t f r a c t i o n o f t h e p e r c o l a t i n g water b u t they do n o t c o n t a i n s i g n i f i c a n t radon. They might n o t c o n t a i n radon e i t h e r because only p r e - e x i s t i n g cracks have had t i m e t o concentrate and t r a p s i g n i f i c a n t radon, o r because radium i s p r e f e r e n t i a l l y l o c a t e d as secondary surface coatings on e x i s t i n g cracks and t h i s radium i s responsible f o r v i r t u a l l y a l l o f t h e observed Rn emanation (Tanner, 1980).

-5

Explanation ( 1 ) i s c o n s i s t e n t w i t h microscopic observations (Brace, 1977), b u t n o t w i t h t h e r e s u l t s o f t h e s t r e s s change experiment on sample 435-3D. The observation t h a t Rn d i d n o t change w h i l e methane increased i n response t o a s t r e s s change d u r i n g t h e f i r s t r u n on t h i s sample i m p l i e s t h a t t h e f l o w water "saw" t h e new microcracks and removed the methane, b u t t h a t t h e r e was no radon t o be removed. b. Samples which were s a t u r a t e d w i t h water released more radon than those which were --------

i n i t i a l l y dry. These observations are n o t surp r i s i n g i n l i g h t o f Tanner's (1980) d i s c u s s i o n o f t h e r o l e o f water as an absorber o f t h e radon's r e c o i l energy. Thus, t h e amount o f water and t h e d i s t r i b u t i o n o f t h i s water i n t h e r o c k are c r i t i c a l f a c t o r s c o n t r o l l i n g radon re1ease. c. Permeability and radon release a r e i n v e r s e l y c o r r e l a t e d . This may be i n t e r p r e t e d i n two ways. E i t h e r water flows through t h e rock s o q u i c k l y t h a t radon does n o t have t i m e t o d i f fuse from s i d e channels, o r most of t h e water flows through a few, large, main channels ( r a t h e r than many small channels) and thus r e moves a s m a l l e r f r a c t i o n o f t h e accessible radon atoms produced i n t h e rock.

-138-

Brace, W. F. , J. B. Walsh, and W. T. Frangos, " Permeability o f g r a n i t e under h i g h pressure," J. Geophys. Res., 73,2225-2236. Brace, W. F. (1977), " Permeability from r e s i s t i v i t y and pore shape," J. Geophys. Res., 82, 334313349.' Brace, W. F. (1978), " A note on p e r m e a b i l i t y changes i n geological m a t e r i a l due t o stress," Pageoph,, 116, 627-633. Chiang, J. H., W. 5 . Moore, and P. Talwani (1977) , " Laboratory studies o f the r e l a t i o n s h i p between surface area and radon release i n Henderson gneiss ,I' Trans. Am. Geophys. , 58, 434. Hazsson, E. (1981), "Radon content o f groundwater as an earthquake precursor: e v a l u a t i o n o f worldwide data and p h y s i c a l basis," J. Geophys . Res. , 86, 9397-9410. Holub, R. F., a n d x . T. Brady (1981), "The e f f e c t o f s t r e s s on radon emanation from rock," J. Geophys. Res., 86, 1776-1784. Jiang, F . , and G. L i ( 1 9 8 1 a z "The a p p l i c a t i o n o f geochemical methods i n earthquake pred i c t i o n i n China," Geophys. Res. L e t t e r s , 8, 469-472. Key, R. M., R. L. Brewer, J. H. Stockwell, N. L. Guinasso Jr., and D. R. Schink (1979), "Some improved techniques f o r measuring radon and radium i n marine sediments and i n seawater," Marine Chemistry, 251-264. Stoker, A. K . , and P. Kruger (1975), "Radon measurements i n geothermal systems ,I' Stanford Geothermal Program , Technical Report #4. Tanner, A. B. (1980), "Radon m i g r a t i o n i n t h e ground: a supplementary review," Natural Radiation Environment, v o l . 3, 5. Teng, T. L. (1980), "Some recent s t u d i e s on groundwater radon content as an earthquake precursor," J. Geophys. Res., 85, 3089-3099. The Group of Hydro-Chemi s t r y , The Sei smol ogi c a l Brigade of Peking (1977) , "An experimental study o f t h e r e l a t i o n between rock r u p t u r e and v a r i a t i o n of radon content," Acta Geo h s i c a Sinica, 20, No. 4, 2 7 7 m . .&OZ , and 3. D. Byerlee (1975), "The e f f e c t o f microcrack d i l a t a n c y on t h e permea b i l i t y o f w e s t e r l y granite," J. Geophys. Res., 80, 752.

l,


INTERSTITIAL FLUID PRESSURE SIGNAL PROPAGATION ALONG FRACTURE LADDERS

Gunnar Bodvarsson

School o f Oceanography Oregon S t a t e U n i v e r s i t y Corval 1i s , Oregon 97331 A b s t r a c t Arrays o f interconnected permeable f r a c t u r e spaces t h a t form f r a c t u r e ladders propagate small amplitude f l u i d pressure s i g n a l s i n much the same way as slabs o f porous format i o n s . Data from geothermal f i e l d s i n I c e l a n d indicate t h a t the fracture width there i s o f the o r d e r o f 1 t o 2 mm and the s i g n a l d i f f u s i v i t y 20 t o 100 m2/s. Well i n t e r f e r e n c e t e s t s a r e n o t l i k e l y t o f u r n i s h data t o d i s t i n g u i s h between f r a c t u r e l a d d e r s and e q u i v a l e n t porous slabs. I n t r o d u c t i o n The m a j o r i t y o f medium t o hightemperature geothermal systems are embedded i n formations o f igneous o r i g i n t h a t g e n e r a l l y a r e c h a r a c t e r i z e d by a f r a c t u r e dominated f l u i d c o n d u c t i v i t y . The f r a c t u r e s a r e o f e l a s t o mechanical/tectonic and/or thermoelastic o r p o s s i b l y chemoelastic o r i g i n . The f r a c t u r e c o n d u c t i v i t y i s i n v a r i a b l y h i g h l y heterogeneous, a n i s o t r o p i c and i s q u i t e o f t e n confined t o f l a t s h e e t - l i k e s t r u c t u r e s such as f a u l t zones and v o l c a n i c dikes. Q u a n t i t a t i v e r e l a t i o n s r e l e v a n t t o axisymmetric Darcy t y p e f l o w i n homogeneous/isotropic porous media generall y do n o t apply t o such s i t u a t i o n s and an unc r i t i c a l standard t y p e i n t e r p r e t a t i o n o f w e l l t e s t data from f r a c t u r e d r e s e r v o i r s i s theref o r e l i k e l y t o l e a d t o f a u l t y conclusions. Unfortunately, s i n c e l i t t l e i s known about t h e dimensions and d i s t r i b u t i o n o f f r a c t u r e s i n the various types o f n a t u r a l s e t t i n g s , i t i s d i f f i c u l t o r even impossible t o d e r i v e r e l e vant q u a n t i t a t i v e r e l a t i o n s . I t i s , nevertheless, o f considerable i n t e r e s t t o o b t a i n some measure o f t h e discrepancy t h a t would r e s u l t from an a p p l i c a t i o n o f t h e standard i n t e r p r e t a t i o n a l procedures. The purpose o f t h i s s h o r t note i s t o discuss a few very simple concepts and r e l a t i o n s t h a t a r e u s e f u l i n t h e present context. The F r a c t u r e Ladder For t h e present purpose, we w i l l consider a s p e c i f i c case o f a composi t e f r a c t u r e conductor c o n s i s t i n g o f a l i n e a r a r r a y o f interconnected i n d i v i d u a l f r a c t u r e spaces o f s i m i l a r dimensions as d i s p l a y e d i n Fig. 1. We w i l l r e f e r t o t h i s system as a f r a c t u r e ladder. The i n d i v i d u a l f r a c t u r e s o r ladder-elements a r e assumed t o have a quasir e c t a n g u l a r shape w i t h a c h a r a c t e r i s t i c edge l e n g t h L. The two surfaces a r e welded togethe r a t t h e edges. The w i d t h o f t h e open f r a c t u r e space may vary over t h e L x L element

area, and t h e r e may even be some a s p e r i t i e s where the opposite surfaces meet, b u t we assume t h a t they touch w i t h o u t a s o l i d weld. From t h e elastomechanical p o i n t o f view, the f r a c t u r e element a c t s as an open space o f an edge l e n g t h L. Moreover, we assume t h a t w i t h regard t o f l u i d flow, t h e f r a c t u r e element has a w e l l d e f i n e d average f l o w w i d t h h. No s p e c i f i c assumptions have t o be made as t o t h e t y p e o f i n t e r c o n n e c t i o n except t h a t t h e f l u i d can f l o w f r e e l y between adjacent l a d d e r elements and t h a t the s p e c i f i c f l o w conductance can be taken t o be approximately u n i f o r m over 'the l e n g t h o f t h e ladder. Obviously, i t i s p o s s i b l e t o generalize t h i s model by e n v i s i o n i n g a system o f p a r a l l e l ladders t h a t are interconnected along t h e i r e n t i r e l e n g t h and form a f r a c t u r e sheet. The p r i n c i p a l p h y s i c a l parameters o f the l i n ear l a d d e r c o n s i s t i n g o f one s t r a n d o f e l e ments a r e e a s i l y defined. A t steady- state laminar f l o w c o n d i t i o n s , t h e v e r t i c a l l y i n t e g r a t e d f l u i d c o n d u c t i v i t y o f a f r a c t u r e space o f w i d t h h between two p a r a l l e l planes t h a t would be r e f e r r e d t o as t h e t r a n s m i s s i v i t y CF i s obtained on t h e b a s i s o f t h e w e l l known cubic law (Lamb, 1932) cF = h3/12v where v i s the kinematic v i s c o s i t y o f t h e f l u i d . A t these c o n d i t i o n s , the mass f l o w through a u n i t l e n g t h o f t h e f r a c t u r e i s q = CFVP where vp i s t h e pressure gradient, and hence the l o c a l f l o w over the l a d d e r i s Q = LCFVP.

Figure 1 The Fracture Ladder

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l o obtain the hydraulic c a p a c i t i v i t y o r stor-

F i n a l l y assuming equal d i f f u s i v i t i e s , the rat i o o f the transmissivities i s

age c o e f f i c i e n t o f t h e ladder, we assume t h a t the element w a l l s a r e e l a s t i c Hookean w i t h a r i g i d i t y p . Moreover, l e t the volume e l a s tance o f a f r a c t u r e space o f volume V be def i n e d by e = dV/dp where p i s t h e i n t e r n a l pressure t h a t i s assumed t o be uniform, Since no a n a l y t i c a l expression i s a v a i l a b l e f o r t h e elastance o f a r e c t a n g u l a r f r a c t u r e element, we w i l l r e s o r t t o approximating t h e element by a c i r c u l a r o r penny-shaped element o f equal The area such t h a t t h e diameter i s 1.12L. elastance o f t h e penny-shaped c a v i t y o f diamet e r d has been obtained by Sneddon (1346) as e = d3/4p where Poisson’s r e l a t i o n o f equal Lame parameters has been assumed. Based on t h i s r e s u l t the elastance o f a f r a c t u r e e l e ment would approximately be e = ( 1 . 1 2 L ) 3 / 4 ~ o r about L3/3p. Hence, t h e elastance per u n i t area, t h a t i s , t h e c a p a c i t i v i t y i s

sF = (L/3u) + hK

(7)

cS/cF = 3psH/L

Borehole/Fracture Contacts It i s q u i t e e v i dent t h a t because o f t h e small cross sections a v a i l a b l e , t h e l o c a l f l o w - v e l o c i t i e s from f r a c t u r e spaces i n t o boreholes may be q u i t e h i g h and t h e f l o w regime t h e r e f o r e h i g h l y t u r bulent. The above r e l a t i o n f o r t h e laminar type t r a n s m i s s i v i t y i s then i n v a l i d and has t o be revised. The r e s u l t i n g r e l a t i v e l y l a r g e f r a c t u r e l b o r e h o l e c o n t a c t r e s i s t a n c e can be d e r i v e d as follows. Consider a borehole o f diameter D which cuts a h o r i z o n t a l f r a c t u r e o f w i d t h h as shown i n Fig. 2. L e t t h e f l u i d be incompressible, o f d e n s i t y p and the mass f l o w o u t o f t h e f r a c t u r e be M. Moreover, l e t t h e f l u i d pressure a t a distance r from t h e c e n t e r o f t h e h o l e be p ( r ) and the f l u i d v e l o c i t y t h e r e be v ( r ) .

(2)

where K i s t h e c o m p r e s s i b i l i t y o f t h e f l u i d . We can u s u a l l y take t h a t v i s o f t h e order o f l D l o t o 2 x lo1’ Pa and assuming t h a t the f l u i d i s l i q u i d water w i t h K = 5 x lo-’’ Pa-’, tlhe product KU = 5 t o 10. The second term on tlie r i g h t o f ( 2 ) can then be neglected when L>>30h. I n general, t h i s c o n d i t i o n holds and we w i l l t h e r e f o r e simp1 i f y t h e expression f o r s: by n e g l e c t i n g the f l u i d c o m p r e s s i b i l i t y term. I t i s t o be noted t h a t t h e above expression f o r S F neglects t h e p o s s i b l e presence of‘ s a t e l l i t e f r a c t u r e t h a t may c o n t r i b u t e t o the c a p a c i t i v i t y .

Borehole

F

O n t h e b a s i s o f ( 1 ) and t h e s i m p l i f i e d v e r s i o n o f ( 2 ) f o l l o w s the laminar f l o w d i f f u s i v i t y o f the l a d d e r

Figure 2

aF = cF/psF = h3u/4Ln

(3) where r~ i s t h e absolute v i s c o s i t y o f the f l u i d . Moreover, t h e r e may be leakage from t h e f r a c t u r e l a d d e r i n t o t h e adjacent formation. On a 1.inear laminar f l o w model, t h e f l u i d l o s s p e r u n i t area o f t h e l a d d e r would be c h a r a c t e r i z e d by a c o e f f i c i e n t b such t h a t t h e leakage i s bp where p i s t h e f l u i d pressure i n t h e ladder. W e have no way o f a r r i v i n g a t any expressions f o r t h i s c o e f f i c i e n t t h a t has t o be t r e a t e d as a p u r e l y experimental parameter.

We have then M = 2arhpv

(8)

(9)

the w a l l f r i c t i o n i s represented by t h e second term on t h e r i g h t o f t h i s equation t h a t i s der i v e d i n the same manner as f o r t h e case of pipes and where f i s t h e f r i c t i o n c o e f f i c i e n t o f t h e f r a c t u r e . Assuming t h a t the formation pressure a t a l a r g e d i s t a n c e from t h e borehole i s po t h i s equation i s e a s i l y i n t e g r a t e d f o r p and we o b t a i n p = po

-

(M2/8r2h2p)[(l/r2)

+ (f/hr)]

(10)

I f t h e pressure i n the borehole i s pb, the f o l l o w i n g expression i s obtained f o r t h e mass flow i n t o the hole

(4)

M = ad? hD [ p ( ~ b - ? ~ ) / ( l + ( f D / 2 h ) ) ] ’ / ’

(11) Abbreviating (Po-Pb) = Ap and ( 1 / a f i ) = 0.23, we d e f i n e t h e c o n t a c t r e s i s t a n c e o f t h e borehole

H = h3/12k (5) Mclreover, assuming equal t r a n s m i s s i v i t y , t h e r a t i o o f the d i f f u s i v i t i e s i s aS/aF = L/3psH

v = M/2arhp

dp = -pvdv t (fpv2dr/2h)

The Ladder and the Porous Slab I t i s i n t e r e s t i n g t o compare t h e parameters o f t h e f r a c t u r e ladder t o those o f a homogeneous/isotropi c porous s l a b w i t h Darcy type f l o w o f t h e t h i c k ness H, p e r m e a b i l i t y k and h y d r a u l i c capacit i v i t y (storage c o e f f i c i e n t ) s. The thickness o f the s l a b o f equal t r a n s m i s s i v i t y i s obt a i ned by kH/v = h3/12v

or

The pressure loss over t h e distance d r i s due t o the conversion o f p o t e n t i a l energy i n t o k i n e t i c energy and f r i c t i o n heat, viz.

I

such t h a t

Borehole/Fracture Contact

(6)

R = ap/M = ( 0 , 2 3 / h D ) [ ( A p / ~ ) ( l + ( f D / 2 h ) ) ] ~ / ~ (12)

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C l e a r l y , these r e s u l t s h o l d o n l y f o r t h e t u r b u l e n t r e g i o n around the borehole. L i t t l e data i s a v a i l a b l e on the values o f the f r i c t i o n coefficient f f o r natural fractures, but based on experimental data f o r pipes w i t h rough w a l l s , we can expect t h a t f ?r 0.05 t o 0.10 (see, f o r example Moody, 1947). I t should be p o i n t e d o u t t h a t because o f t h e quad r a t i c terms i n (9), t h e mass f l o w M i s n o t a l i n e a r f u n c t i o n o f d p and R t h e r e f o r e depends on AP. The p r i n c i p a l a p p l i c a t i o n o f equation (11) i s f o r t h e e s t i m a t i n g o f t h e f r a c t u r e w i d t h h i n f i e l d cases where M and Ap a r e known.

now a r r i v e a t the b a s i c equation f o r the propagation of pressure s i g n a l s along a f r a c t u r e ladder. Assuming a homogeneous/isotropic f l a t l a d d e r and n e g l e c t i n g i n e r t i a forces, t h e basi c equation i s t h e d i f f u s i o n equation i n two s p a t i a l dimensions f o r the f l u i d pressure p, viz. , PsFatp + bp + c p 2 p = m

(13)

where P i s t h e d e n s i t y o f t h e f l u i d , 112 = -V2 i s t h e Laplacian i n two dimensions and m i s a source density. The leakage term on t h e l e f t can be e l i m i n a t e d by a t r a n s f o r m a t i o n p = u exp(-bt) where u i s a new dependent v a r i a b l e . The p r i n c i p a l small amplitude propagation parameters, t h e p e n e t r a t i o n depth and t h e s k i n depth (assuming b = 0) d = (aFt)1/2 and ds = (2aF/w)l/' (14) P where t i s time and w the angular frequency, f o l l o w then i n t h e usual way.

.

F i e l d Data L i t t l e i n f o r m a t i o n i s a v a i l a b l e on t h e dimensions o f f r a c t u r e s i n nature. Perhaps t h e most accessible extensive data i s on some o f t h e geothermal r e s e r v o i r s i n I c e l a n d (Thorsteinsson, 1976, Bjornsson, 1979). I t i s we1 1 known t h a t t h e h y d r o l o g i c a l systems o f I c e l a n d a r e embedded i n f r a c t u r e dominated flood- basalts o f l a t e T e r t i a r y t o Pleistocene age. This m a t e r i a l enables us t o make a t tempts a t e s t i m a t i n g f r a c t u r e widths i n some o f t h e I c e l a n d r e s e r v o i r s . Very b r i e f l y , we can proceed as f o l l o w s .

The pressure s i g n a l d i f f u s i v i t i e s i n d i c a t e d by t h e I c e l a n d data are q u i t e h i g h and s i g n a l propagation t h e r e f o r e rapid. For example, a t aF = 50 n?/s t h e p e n e t r a t i o n depth f o r a peri o d o f 10% i s about 700 m. I t i s i n t e r e s t i n g t o note t h a t the f r a c t u r e s t r u c t u r e s simulate porous slabs o f H = 20 t o 60 m and permeabilit i e s o f t h e o r d e r o f k = lo-" = 10 darcy. The g l o b a l p e r m e a b i l i t i e s o f t h e h o s t systems appear, nevertheless, t o be orders o f magnitude s m a l l e r (Bodvarsson and Zais, 1978).

( 1 ) Borehole p r o d u c t i o n data i n various geothermal f i e l d s i n I c e l a n d i n d i c a t e t h a t major f r a c t u r e conductors can produce mass flows from a few up t o a few tens o f kg/s a t pressure d i f f e r e n t i a l s of a few lo5 Pa. A f i g u r e o f M = 10 kg/s a t AP = 4 x l o 5 Pa i s q u i t e r e p r e s e n t a t i v e o f t h e performance o f a product i v e i n d i v i d u a l f r a c t u r e i n a borehole o f D = 0.22 m. Assuming t h a t t h e c o n d i t i o n s s e t f o r t h i n t h e previous s e c t i o n hold, equation (11) w i t h f = 0.06 gives then an estimate o f h = 1.3 mm.

Under t h e circumstances assumed here, an i n t e r f e r e n c e t e s t would n o t p r o v i d e data t o d i s t i n g u i s h between t h e two models, t h e l a d d e r and t h e slab, and an i n t e r p r e t a t i o n on t h e basis o f slabs o n l y can t h e r e f o r e l e a d t o erroneous conclusions.

( 2 ) Well i n t e r f e r e n c e t e s t i n g i n 4 geothermal f i e l d s i n Southwestern I c e l a n d have y i e l d e d t r a n s m i s s i v i t i e s o f CF = 2.6 x 10-4 t o 25 x 10-4 ms. R e i n t e r p r e t i n g these r e s u l t s i n terms o f s i n g l e f r a c t u r e systems f l o w i n g water a t 100°C w i t h = 3 x 10-7 m2/s, we o b t a i n w i t h t h e h e l p o f equation ( 1 ) above t h e e s t i mate o f h = 1 t o 2 mm. Moreover, d u r i n g t h e same t e s t s , u n i t area c a p a c i t i v i t i e s (storage c o e f f i c i e n t s ) o f SF = 1 x 10-8 t o 4 x 10-8 m/Pa were obtained. Equation (2) then y i e l d s estimates o f L = 400 t o 1600 in and t h e r e s u l t i n g d i f f u s i v i t i e s are aF = 20 t o 100 m2/s.

References Bjornsson, A. , 1979, ( e d i t o r ) , "The Akureyri D i s t r i c t Heating System," Report OS JHD 7827 by t h e National Energy A u t h o r i t y , Reykjavik. Bodvarsson, G. and E. Zais, 1978, " A F i e l d Example o f Free Surface Testing," Workshop on Geothermal Reservoir Engineering, December, 1978, S t a n f o r d U n i v e r s i t y , Stanford, CA. Lamb, H., 1932, "Hydrodynamics," 6 t h ed. e r P u b l i c a t i o n s , New York, N.Y., 738 pp.

( 3 ) I t i s o f i n t e r e s t t o note t h a t f r a c t u r e widths can a l s o be estimated on t h e b a s i s o f the o v e r a l l f l o w r e s i s t a n c e i n i n d i v i d u a l geothermal systems. Knowing the d i s t a n c e o f r e charge, t h e a v a i l a b l e pressure d i f f e r e n t i a l and o t h e r parameters, i t i s p o s s i b l e t o a r r i v e a t estimates o f an average h. The present w r i t e r has obtained along these l i n e s r e s u l t s t h a t compare w e l l w i t h t h e above estimates. Unfortunately, space does n o t p e r m i t a discuss i o n o f t h i s method.

, Dov-

Moody, L. F., "An approximate formula f o r Pipe F r i c t i o n Factors," Mech. Eng. 69 (1947) 1005. Sneddon, I.N., 1946, "The D i s t r i b u t i o n o f Stress i n t h e Neighbourhood o f a Crack i n an E l a s t i c Solid," Proc. Roy. SOC., 187, 229. Thorsteinsson, T. , 1976, 'Redevelopment o f the Reykir Hydrothermal System i n Southwestern Iceland,'' Second U.N. Symposium on the Development and Use o f Geothermal Resources, San Francisco, 1975.

Signal Propagation On t h e above premises, we

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ME4SURED ENTHALPY COMBINED WIm CHEMICAL CONCENTRATION IlATA TO DIAGNOSE RESERVOIR BEHAVIOUR

M. SALTUKLAROGLU

ELC

-

EECTROCONSULT (ITALY)

ABSTRACT: Concentrations of chemicals such as C1 and N a i n a geotherml w a t e r s e p m t e d at atmspheric pressure (at a s i l e n c e r ) , or any other pressure, when related t o the measured enthalpy of the produced s t e d w a t e r mixture are indicative of reservoir production conditions. Achieving this r e l a t i o n by means of a m t h e m t i c a l l y meaningful graphical method, conditions in t h e reservoir such as boiling, steam gain or loss, heating o r cooling (by cop duction or mixing) can be inferred, and the f l u i d s of different w e l l s can be related. The method is i l l u s t r a t e d with data obtained f r o m Los Azufres Geotherrrdl Field i n Mexico.

INTRODUCTION: The r e l a t i o n between t h e enthaL py of a geothermdl f l u i d and t h e concentratio= of chemicals within t h e separated water has been long recognised and m d e use o f . A s early as 1 9 6 7 Wilsonet a l l 1 1 used t h e changes i n chloride concentrations of waters fm Wairakg wells t o calculate t h e underground changes i n enthalpy, and by comparing t h e i r figures with the actually measured values they could comment on t h e production conditions within tk reservoir. Later on Wilson (2) again dealt with t h e idea and published a graphical method by wich d i l u t i o n of reservoir water o r heat gain or loss by t h e reservoir f l u i d by means of conduction could be inferred.

The idea has been a l s o applied by Truesdell and Fournier ( 3 , 4 ) and others ( 5 ) t o r e l a t i n g lmiling spring waters of arl area and t o calcul a t i o n of t h e i r temperatures a t depth, by mking use of a graphical method, i n wich enthalpy i s plotted against chloride concentrations in spring waters with cartesian coordinates. The enthalpy i s considered as t h a t of water at an underground temperature indicated by geothemmeters (silica or Na-K-Ca). The method presented here and applied t o Los Azufres w e l l s has a similar graphical form t o t h a t used by Truesdell and Fournier, but with -11 modifications t h a t M e the form of t h e graph matherratically meaningful, and t h e enthalpy used i s t h a t of t h e stedwater mixture, as measured a t t h e surface.

METHOD

: When a geothermal f l u i d with an enthai py of ho and C1 or N a concentrations of Co i s produced and separated a t a m s p h e r i c condit i o n s (at silencer), t h e concentration of t h e

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water fraction would be (SeeAppendix f o r derivation and see nomenclature a t t h e end): ( hsa - hwa

\ hsa

-

ho

1co /

...............

(1)

This mtherratical expression can be represented i n graphical form as shown i n Fig. 1. If t h e fluid i s water at reservoir conditions, Co would be t h e concentration i n the reservoir water. Starting f r o m t h i s initial condition defined as So i n Fig. 1, we can examine what would happen t o the enthalpy concentration re15 tionship i n different cases of reservoir behaviour : CASE 1. Evaporation i n Reservoir (Gain of heat f r o m reservoir rocks). This normally happens when t h e pressure i n t h e reservoir decreases s u f f i c i e n t l y , due t o drawdown, f o r t h e water t o b o i l . Say the enthalpy of the produced f l u i d increases t o hl. If a l l t h e fraction of t h e steam formed as a result of t h i s i s produced with the associated fraction of water, the state of t h e f l u i d would be represented by p i n t S immediately above So, as t h e concentrg t i o n o 6 t h e f l u i d i n t h e reservoir (steadwater mixture) would stay t h e same. However, now, because of t h e increased enthalpy t h e concentr5 t i o n i n t h e w a t e r s e p a t e d a t a m s p h e r i c cond i t i o n s ( m a l ) would be higher (Cwal Cwa) .Also due t o t h e decrease i n pressure, t h e temperature would decrease (assuming boiling i n the form t i o n ) , and t h e water f r a c t i o n w u l d have an enthalpy hwrl and a concentration Cwrl (where hwrl( ho and C w r l ) Co).

>

However ,

Co=(l-xrl) Cwrl=(l-xal) cwal

.............( 2 )

Therefore, evolution of a w e l l with the points plotted on a diagram l i k e i n Fig. 2 , and moving along l i n e SoSl would be indicative of this pat i c u l a r case. However, because of t h e r e l a t i v e permeability e f f e c t s steam and water m y not enter i n t o t h e flow i n t h e same fractions as they m y exist i n t h e reservoir around t h e wellbottom. In other words, although due t o evaporation in t h e reser voir an average steam fraction of x r l can e x i s t

arcund t h e wellbottom, corresponding t o t h e en talpy hl, the f l u i d which enters t h e w e l l m y have a different steam q d i t e ( X r l ' ) Then:

Xrl')

S2 i s mving f r o m So u p a d s along apro&tel y t h e hsa - Cwa line (See Fig. 3 ) .

Therefore evolution of a w e l l from the initial condition (So) along aproximtely the i n i t i a l condition l i n e hsa - Cwa would mean steam gain f r o m a d i f f e r e n t l e v e l with negligible or no additional steam forming i n the f o m t i o n f r o m which t h e k n w a t e r production mmes. However, steam gain f r o m evaporation i n t h e f o m t i o n would result in a path e i t h e r l i k e SoSlSI' or mre l i k e l y m v e along SoS1' as the resultant of two compnentprocesses. It i s nevertheless obvious t h a t it m y be difficult t o diferentiate between this case and t h e case 1.1, i f t h e i n i t i a l conditions of the w e l l are not

i f there i s steam gain ( r e l a t i v e l y mre steam enters the flow)

Xrl,

i f there i s steam loss, (relatively mre w a t e r enters t h e flow, t h a t i s some steam i s l o s t t o t h e f o m t i o n s a b v e before it can enter i n t o t h e flow).

Xrl"< Xrl,

>

CASE 1.1. Steam Gain (Xrl' Xrl) (See Fig.2) In this case t h e entalpy of t h e production m u l d be h l ' , with t h e state being defined by the p i n t S1' Then,

.

N')

hOwn.

and it can be shown t h a t (See

N ) ho,

I n t h e case of steam loss t h e condition p i n t s would m v e downwards along t h e hsa - Cwa l i n e .

Appendix) :

Cwal

hsa - hwa ; h s r l h s r l - hwrl, hsa

=Cwrll

-

N' \ hl' * ' ' ' ' ' * ( 4 )

CASE 3. Conductive Heating or Cooling Evam r a t i o n in t h e reservoir i s r e a l l v a svecial form of the conductive heating case. However, i f the conditions do not permit evaporation and i f there i s conductive heating t h e condit i o n of t h e w e l l would evolve along So Sly with an enthalpy h l , where:

But , f r o m equation ( 3 ) :

However,

h s r l - N' hsa - N'

Then

Cwal'

C1' -Also Cwal'

hsrl - N hsa - h l

N_

N

..........( 5 )

Cwal

hsa - N ' hsa - hwa

~

and

C1' Cwal * * * ' ' . ' ' ( 6 )

This condition is similar t o the case where steam enters the production f r o m another l e v e l and yet no boiling m y take place in t h e fomt i c n , which i s considered below as case 2.

W E 1.2.

Steam Loss (Xrl"( Xrl) Similary it can be shown i n t h i s case t h a t the p i n t s muld rrove down the hsa - Cwal l i n e , l i k e t h e point Sl". The enthalpy N" w u l d be d l e r than hl.

W E 2.

Steam Gain o r Loss without Boiling i n Reservoir A s pointed out above this i s very similar t o Case 1.1. If the vroduction en thaipy i s h2 (h2 ho) it can beLshown t h a t (assuming the temperature difference between the steam and water flows i s negligible):

the

>

(See Appendix f o r derivation)

(

and (hsa hsa

- h"') jhsr2 - h2 1 ( h s r 2 -

Therefore,

Cwa2

5

)

(equation 1)

h2) ho

Cwa

1,

Cwrl

Co

C1,

Cwal

> Cwa

A s t h e r e l a t i v e permeability effects are n o m l l y present, a pure case of evaporation

Therefore, it can be seen t h a t Sl' i s approximgt e l y on t h e hsa - Cwal l i n e and movement of plctted p i n t s f r o m S1 t o S1' along t h i s l i n e m u l d indicate steam gain.

But, Cwa = Co hsa hsa - hwa ho

h w r l ) ho

......... ( 8 ) ......... ( 9 )

This m u l d mean t h a t , the new condition p i n t

i n the reservoir with condition points mving along So S 1 m u l d be unlikely. I f , however, t h i s behaviour exists it i s mre l i k e l y t o p i n t out t o a conductive heating case without boiling taking place in t h e reservoir (However, this should a l s o be checked by t h e enthalpy value). The mvement of the condition p i n t s i n t h e opposite direction (with decreasing enthalpy) would indicate conductive cooling which m y happen i f a f l u i d has t o t r a v e l acmss a cooler t e r r a i n t o get t o the w e l l , as a r e s u l t of t h e evolution of t h e f i e l d conditions. (See p i n t S3 i n Fig. 4 ) .

CASE 4. Heating or Cooling by Mixing Depending on t h e enthalpy of the mixing fluids, heating or cooling m y result. If t h e i n i t i a l conditions of the mixing (contaminating), f l u i d are defined by a p i n t M, the p i n t s defining t h e condition of the mixture would m v e along the line SoM, t h e i r exact positions depending on the mixing r a t i o . Here again evolut i o n of t h e condition points (enthalpy - concenirdon relationship) would be diagnostic of what i s happening i n the reservoir (See p i n t S4 i n Fig. 4 ) . It i s abvious t h a t presence of other i n f o m t i o n would help t o corroborate the deduction or inferences obtained by t h i s method. Where possible t h i s should be looked f o r i n order t o have mre confidence i n the results.

Also, it w i l l be noted t h a t when t h e initial

-144-

conditions of different wells are plotted on the same diagram, the conditions of production existing around the bottom of different wells can be identified with respect to the wells which seem to be producing the originally exis ting hot water. In this way, wells within the same field, but producing with apparently different enthalpy concentration relationships m y be related.

(at a silencer), or any other pressure, when related to the measured enthalpy of steam/wa@r mixtures are indicative of reservoir conditim Basis of this relationship at different reservoir conditions, a methodology for interpretation and examples of it from a geothermdl field in Central Mexico have also been presented. ACKNOThe author expresses his gMtidue to C.F.E. (Comisi6n Federal de Electricidad) of Mexico for permitting the use of the data.

EXAMPLES: The figures 5 through 9 show data plotted, as explained above, of several wells of Los Azufres Geothermal Field which is 10% ted in the state of Mickoacan in Central Western Mexico. The geothermal reservoir is forred by volcanic rocks (minly andesites) and seems to have a steam cap which at some locations is thick and results in dry steam p m duction. The &un temperature encountered is in the order of 300OC.

The author also states that the opinions expressed are his own and not necessarily t h E of his company or of the C.F.E. l.-WILSON, S.H.; MAHON, W.A.J., and ALWUS K.J. "The Calculation of Undergr0u-d Changes in Enthalpy from Changes in Chloride Concentration of Waters from Wairakei Drillhole Discharges During 1965" New Zealand Journal of Science Vol. 10, No. 3. Sept. 1967. EFERENCES

Figure 5 illustrates the relation betwcen the initial production conditions at different wells. It appears that different wells display different degrees of steam gain or loss. The wells 2, 3, 4 and 18 indicate condition near to that of the hot water. However, the wells 1, 8, 13 and 19 show definite steam gain and the wells 7 and 15 indicate steam loss after boiling in the formation. Depending on the actual condition taken as representative of the undiluted hot water, pssible conductive heating (4,18,5) or cooling (2,3) can also be inferred, but the data are not accurate enough to say anytking definitly at this stage. In figure 6 the behaviour of well 3 can be sea Itwill be noted that the well produced a water which m s t probably was diluted with a water of similar temperature, but of low chemical concentration, possibly condensed steam. Over a period of two mnths the dilution gradually decreased and the well started producing the undiluted water with a smll gain of steam. Figure 7 shows the behaviour of well 5. It can be said that as the production rate increases the behaviour of this well is dominated by steam gain due to evaporation in the formation and probably also from a steam cap. This is confirmed by the production characteristics of the well presented in Fig. 10.

2.- WILSON, S.H.; "Statisrlcal Interpretation of Chemical Results from Drillholes as an Aid to G e o t h m l Prospecting and Exploration" Geothermics, Special Issue 2, 1970. (U.N.Sympsium on the Development and Utilization of Geothermal Resources Pisa 1970, Vol. 2 Part. 2).

3.- TRUESDELL, A.H, and FOURNIER, R.O. "Calculation of Deep Tanperatures in Geothermal Systems from the Chemistry of Boiling Spring Waters of Mixed Origin" U.N. Sympsim San Francisco 1975, Vol. 1. 4.- - - - - - - -"Convective Heat Flow in Yellowstone National Park" U.N. Symposium, San Francisco, 1975, Vol. 1.

5.- CUSICANQLJI, H.; MAHON, W.A.J. and ELLIS,A. J. "The Geochemistry of the El Tatio Geothend Field, Northem Chile" U.N. Symposium, S a n Francisco 1975. Vol. 1.

NOMENCLATURE C h

The behaviour of well 13 presented in Fig. 8 is similar, but indicates that the mjor part of the steam gain is coming from a steam level. The relatively high enthalpy also reflects this. Its production curves are shown in Fig. 11 and confirm these observations. Finally, Fig. 9 show the behaviour of well 15 which originally produced a fluid which had lost steam. It seems that as the production t h e increased the well was producing a fluid which was gradually nearing to original reservoir water conditions (less steam loss).

x

Concentration (ppm) of a chemical like C1 or Na. Enthalpy Steam fraction

Subscripts: s Steam w Water ws Evaporation o Initial a At atmspheric conditions r At reservoir conditions 1, 2, 3, 4 cases considered

CONCLUSIONS: It has been shown that concentrations of chemicalssuch as C1 or Na in a geothz 11131 water separated at abmspheric conditions

- 14 5-

APF'ENDIX

Water fraction at abmspheric conditions: 1-Xa ho - hwa . . (A.1) hwsa

......

As a l l t h e C1 concentration i n t i a l l y i n a u n i t mass of f l u i d muld now be concentred within this water fraction: (l-Xa)

Cwa

cwa

1-Xa

CO

- - - - - . (A.3) *

= ( hsa - hWa' hsa - ho c0

i

. . (A.4)

CASE 1.1. I f t h e concentration m u d t h e w e l l bottom within t h e reservoir is now Cl', with the same reasoning: C1f=Cwrll f hsrl - hl'

', h s r l - hwrl

Cwall/hsa - hl' \hsa - hwa

I

. . . . . . (A.5) The concentration within t h e w a t e r f r a c t i o n does not change.

Cwrl.

ThereforeCwrl'

CASE 2. -C2

I

&a2 (1-Xa2)

Cwr2 (l-Xr2)

. . . . (A.7)

However i f no boiling o r negligible boiling i n the formation, then

co

Cwr2

. . . . . . . . (A.8)

Then :

=

'hsa

-

( hsa -

h2 ) hwa /

> ' h s r 2- h2 co jhsr2 - hc

-146-

e

$

P

hwa.

L Cwentrotion in Reservoir. Fig: I W C O I Rewesentotion of the MdhM..

Fig. 2 E v w o t i o n in Reservoir

hso

h2

-t f

ho \ \

W

hp'

S2'

0

/

I

M

hso c2

co

Cp'

cwo cwap

Concentration in Reservoir

Fig. 3

Steam goln or laas without boiling in Ihe nsewoir

Cmcentratim in Rmrvoir .

-147-

Fig: 4

Heating and codmg by conduction and mixing.

J 0

a

c

f

2

/

n

e

-148-

/

/

w z

$

ill

I;

m

Y

!

n VI W

s 0

?

Y

0

n

P

c

il

0

j

0

;

-

/II

-

-'

4

I W

m I I I I

-

@

e

a

e

b 0

B

E

e

2

-15c-

m


HEAT TRANSFER I N FRACTURED GEOTHERMAL RESERVOIRS W I T H BOILING

Karsten P r u e s s

Earth Sciences Division Lawrence Berkeley Laboratory Berkeley, C a l i f o r n i a 94720

I n t r o d u c t i o n Most high- temperature geothermal r e s e r v o i r s a r e h i g h l y f r a c t u r e d systems. The f r a c t u r e s p r o v i d e c o n d u i t s through which f l u i d ( a n d heat) can flow a t sufficiently large rates to a t t r a c t commercial i n t e r e s t . The rock m a t r i x h a s a low flow c a p a c i t y , b u t it s t o r e s most of t h e h e a t and f l u i d r e s e r v e s . The f r a c t u r e s r e p r e s e n t a v e r y s m a l l f r a c t i o n of t h e v o i d volume, and p r o b a b l y c o n t a i n less t h a n 1% of t o t a l f l u i d and h e a t r e s e r v e s i n r e a l i s t i c cases. S u s t a i n e d p r o d u c t i o n from a fractured reservoir i s only possible i f t h e d e p l e t i o n of t h e f r a c t u r e s can be r e p l e n i s h e d by l e a k a g e from t h e m a t r i x . The rate a t which h e a t and f l u i d can be t r a n s f e r r e d from t h e m a t r i x t o t h e f r a c t u r e s i s t h e r e f o r e of c r u c i a l importance f o r an assessment of r e s e r v o i r l o n g e v i t y and e n e r g y recovery. Y e t most work i n t h e a r e a of geothermal r e s e r v o i r e v a l u a t i o n and a n a l y s i s h a s employed a " porous medium"-approximation, which amounts t o assuming i n s t a n t a n e o u s ( t h e r m a l and hydrologic) e q u i l i b r a t i o n between f r a c t u r e s and m a t r i x . E f f e c t s of m a t r i x / f r a c t u r e i n t e r a c t i o n have been i n v e s t i g a t e d by Bodvarsson and Tsang ( 1981) f o r s i n g l e - p h a s e r e s e r v o i r s , and by Moench and coworkers ( 1 9 7 8 , 1980) f o r b o i l i n g r e s e r v o i r s . Moench's work a d d r e s s e s t h e q u e s t i o n of p r e s s u r e t r a n s i e n t s d u r i n g drawdown and buildup t e s t s i n f r a c t u r e d r e s e r v o i r s . The p r e s e n t paper f o c u s e s on a complementary a s p e c t , namely, e n t h a l p y t r a n s i e n t s . W e use simple a n a l y t i c a l e x p r e s s i o n s t o a n a l y z e f l u i d and h e a t t r a n s f e r between rock m a t r i x and f r a c t u r e s . I t i s shown t h a t h e a t cond u c t i o n i n a m a t r i x w i t h low p e r m e a b i l i t y can s u b s t a n t i a l l y i n c r e a s e flowing enthalpy i n t h e f r a c t u r e s . This a f f e c t s f l u i d mobility, and h a s i m p o r t a n t consequences f o r energy r e c o v e r y and r e s e r v o i r l o n g e v i t y . W e p r e s e n t r e s u l t s of numerical s i m u l a t i o n s which i l l u s t r a t e t h e s e e f f e c t s and show t h e i r dependence upon m a t r i x p e r m e a b i l i t y and f r a c t u r e spacing.

vapor Here (Vp), i s t h e normal component of pressure gradient a t the matrix/fracture interface. In a boiling reservoir, pressure g r a d i e n t s a r e accompanied by t e m p e r a t u r e gradients. I d e a l i z i n g t h e reservoir f l u i d a s p u r e water s u b s t a n c e , t h e t e m p e r a t u r e g r a d i e n t i s g i v e n by t h e Clapeyron e q u a t i o n : (Vv

-

vQ)( T + 273.15)

T h e r e f o r e , t h e r e i s a one-to-one correspondence between mass f l u x and t h e c o n d u c t i v e h e a t f l u x which i s given by

9 =

-

KVT

(3)

The t o t a l h e a t f l u x d i s c h a r g e d i n t o t h e f r a c t u r e system i s (4)

I n t h e m a t r i x , h e a t i s s t o r e d i n r o c k s and fluids. In t h e fractures, heat resides s o l e l y i n t h e f l u i d f i l l i n g t h e v o i d space. The h e a t f l u x g i v e n by ( 4 ) h a s t o be c a r r i e d , t h e r e f o r e , by t h e m a s s f l u x given by ( 2 ) . Upon e n t e r i n g t h e f r a c t u r e system, t h e f l u i d h e a t c o n t e n t is enhanced by t h e a b s o r p t i o n of t h e c o n d u c t i v e h e a t f l u x . From G, = hF w e o b t a i n t h e e f f e c t i v e flowing enthalpy of t h e f l u i d e n t e r i n g t h e f r a c t u r e s :

kt Conductive Enhancement of Flowing E n t h a l p y P r o d u c t i o n from t h e f r a c t u r e s c a u s e s p r e s s u r e s t o d e c l i n e , and i n i t i a t e s f l u i d flow from t h e m a t r i x i n t o t h e f r a c t u r e s . Assuming D a r c y ' s law t o h o l d i n t h e m a t r i x , and n e g l e c t i n g g r a v i t y e f f e c t s f o r t h e moment, t h e mass f l u x b e i n g d i s c h a r g e d i n t o t h e f r a c t u r e system can be w r i t t e n :

-151-

r

klim

H e m w e have d e f i n e d a l i m i t i n g e f f e c t i v e p e r m e a b i l i t y , dependent upon h e a t conduct i v i t y and t e m p e r a t u r e , a s : I . !

klhn(K,T)

a va ( vv

- va ) ( T + 273.15)

= V K

9

Numerical S i m u l a t i o n s of F r a c t u r e d R e s e r v o i r Behavior The above c o n s i d e r a t i o n s are borne o u t by numerical s i m u l a t i o n s . W e employ an i d e a l i z e d model of a f r a c t u r e d r e s e r v o i r , w i t h t h r e e p e r p e n d i c u l a r sets of i n f i n i t e , p l a n e , p a r a l l e l f r a c t u r e s of e q u a l a p e r t u r e 6 and s p a c i n g D (see F i g u r e 3 ) . Modeling of t r a n s i e n t f l u i d and h e a t flow i s accomplished with t h e " multiple i n t e r a c t i n g continua" method ( M I N C ) . T h i s method i s c o n c e p t u a l l y s i m i l a r t o , and i s a g e n e r a l i z a t i o n o f , t h e well-known d o u b l e - p o r o s i t y approach (Barenb l a t t e t a l . , 1960; Warren and Root, 1963). A d e t a i l e d d i s c u s s i o n of t h e MINC-method i s given i n P r u e s s and Narasimhan (1982). T h i s method can be e a s i l y implemented w i t h any s i m u l a t o r whose f o r m u l a t i o n i s based on t h e " i n t e g r a l f i n i t e d i f f e r e n c e " method. The c a l c u l a t i o n s p r e s e n t e d below were made w i t h LBL's geothermal s i m u l a t o r s SHAFT79 and MULKOM, w i t h p a r a m e t e r s given i n Table 1.

(hv

- ha) 2

(6)

I n t h e absence of g r a v i t y e f f e c t s , w e have := 1. The enhancement of f l o w i n g e n t h a l p y occiirs because t h e c o n d u c t i v e h e a t f l u x v a p o r i z e s p a r t ( o r a l l ) of t h e l i q u i d which i s Idischarged i n t o t h e f r a c t u r e s . The e f f e c t depends upon t h e r a t i o of h e a t c o n d u c t i v i t y K t o e f f e c t i v e permeability f o r t h e l i q u i d phase, kmka. The s m a l l e r t h e p e r m e a b i l i t y of t h e m a t r i x , t h e smaller i s t h e mass f l u x which h a s t o a b s o r b t h e c o n d u c t i v e h e a t f l u x , r e s u l t i n g i n a s t r o n g e r enhancement of f l o w i n g e n t h a l p y . From ( 5 ) it can be s e e n t h a t f o r kmki = k l i m a l l l i q u i d i s v a p o r i z e d , g i v i n g rise t o d i s c h a r g e of s a t u r a t e d steam from t h e matrix ( h = hv) L a r g e r permeab i l i t y (kmka > klim) r e s u l t s i n d i s c h a r g e of two-phase f l u i d , w h i l e f o r kmki < k l i m s u p e r h e a t e d steam i s produced even i f a mobile l i q u i d p h a s e is p r e s e n t i n t h e m a t r i x .

Vg

.

( i )R a d i a l Flow The r a d i a l f l o w problem u s e s parameters a p p l i c a b l e t o a t y p i c a l w e l l a t The Geysers, and assumes a mobile l i q u i d phase (ka = .143 a t Sa = . 7 0 ) . The c a l c u l a t i o n s show t h a t f o r kmka < k l i m , t h e produced e n t h a l p y rises t o above 2.8 MJ/kg ( s u p e r h e a t e d steam) w i t h i n m i n u t e s , whereas f o r h k a > k l i m , a two-phase steam/water m i x t u r e i s produced ( P r u e s s and Narasimhan, 1981).

Gravity e f f e c t s diminish t h e l i m i t i n g e f f e c t i v e p e r m e a b i l i t y , hence t h e c o n d u c t i v e enhancement of f l o w i n g e n t h a l p y . T h i s i s e a s i l y understood by n o t i n g t h a t g r a v i t y d r i v e n flow does n o t r e q u i r e non-zero p r e s s u r e g r a d i e n t s , and t h e r e f o r e need n o t be accompanied by c o n d u c t i v e h e a t t r a n s f e r . P r u o s s and Narasimhan (1981) have shown t h a t i n c l u s i o n of g r a v i t y r e d u c e s t h e l i m i t i n g e f f e c t i v e p e r m e a b i l i t y by a f a c t o r

( i i ) Areal D e p l e t i o n Problem W e have s t u d i e d t h e d e p l e t i o n of an areal 7 km x 3 km r e s e r v o i r , w i t h p a r a m e t e r s s i m i l a r t o t h o s e used by Bodvarsson e t a l . (1980) i n t h e i r assessment of t h e geothermal r e s e r v o i r a t Baca, N e w Mexico. R e s u l t s f o r e n t h a l p i e s and p r e s s u r e s are g i v e n i n F i g u r e s 4 and 5. Two b a s i c d e p l e t i o n p a t t e r n s are o b s e r v e d , depending upon whether m a t r i x p e r m e a b i l i t y i s ( r e l a t i v e l y ) "high" or " l o w " . For low km = 10-17 m2, t h e b o i l i n g p r o c e s s i s l o c a l i z e d i n t h e v i c i n i t y of t h e f r a c t u r e s , w i t h vapor saturations i n t h e matrix decreasing a s a f u n c t i o n of d i s t a n c e from t h e f r a c t u r e s . The o p p o s i t e p a t t e r n i s observed f o r "high" km = m2, where t h e d e p l e t i o n p r o c e s s 9x10-17 m2, c a u s e s a b o i l i n g f r o n t t o r a p i d l y move i n t o t h e m a t r i x , g i v i n g rise t o l a r g e s t vapor s a t u r a t i o n s i n t h e i n t e r i o r of t h e m a t r i x , away from t h e fractures.

G r a v i t y e f f e c t s w i l l be s m a l l i f ( i )v e r t i c a l p r e s s u r e g r a d i e n t s are c l o s e t o h y d r o s t a t i c (ap/'az =: Pag) , ( i i ) i f v e r t i c a l p e r m e a b i l i t y i s s m a l l ( k , < < k,) , or ( i i i ) i f p r e s s u r e gradients a t t h e matrix/fracture interface a r e s i g n i f i c a n t l y l a r g e r than hydrostatic.

-

I t i s a p p a r e n t from F i g u r e 4 t h a t produced e n t h a l p y depends much more s t r o n g l y upon m a t r i x p e r m e a b i l i t y t h a n upon f r a c t u r e spacing. Enthalpy i n c r e a s e s w i t h d e c r e a s i n g m a t r i x p e r m e a b i l i t y , i n agreement w i t h t h e d i s c u s s i o n g i v e n above. From F i g u r e 5 it can be n o t e d t h a t p r e s s u r e d e c l i n e i s more r a p i d i n c a s e of h i g h e r e n t h a l p y , due t o t h e f a c t t h a t t h e m o b i l i t y of two-phase f l u i d gene r a l l y decreases with i n c r e a s i n g enthalpy. The c r u c i a l importance of m a t r i x p e r m e a b i l i t y i s p a r t i c u l a r l y e v i d e n t i n t h e case of km = 10-17 m2, where f r a c t u r e s p a c i n g s of 5 m and 50 m, r e s p e c t i v e l y , r e s u l t i n v i r t u a l l y

k l i n l i s p l o t t e d a s a f u n c t i o n of t e m p e r a t u r e i n F i g u r e 1 f o r t h e n o - g r a v i t y case ( V 9 = 1 ) . F i g u r e 2 shows t h e e f f e c t i v e f l o w i n g e n t h a l p y of f l u i d d i s c h a r g e d i n t o t h e f r a c t u r e s as a f u n c t i o n of m a t r i x p e r m e a b i l i t y c a l c u l a t e d from ( 5 ) . Curves a r e g i v e n f o r d i f f e r e n t v a l u e s of vapor s a t u r a t i o n ; i n t h e s e c a l c u l a t i o n s t h e r e l a t i v e p e r m e a b i l i t i e s were assruned t o be g i v e n by C o r e y ' s e q u a t i o n w i t h Sar = 0.3, S,, = 0.05. It is seen t h a t condiuctive enhancement of f l o w i n g e n t h a l p y becomes s i g n i f i c a n t f o r k < 10-15 m2, and becomes v e r y l a r g e f o r s m a l l e r p e r m e a b i l i t y .

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identical enthalpy and pressure response, even though the matrix/fracture contact areas differ by a factor of 10. It might appear from Figures 4 and 5 that porous-medium type reservoirs will have greater longevity than equivalent fractured reservoirs. This is not generally true, however, and is caused by discretization effects in this case. Below we present calculations with better spatial resolution, which show that fractured reservoirs may in some cases have greater longevity than equivalent porous medium reservoirs. It should be pointed out that the reservoir longevities predicted from this study are probably too pessimistic, due to the use of Corey-type relative permeability functions. The well test analysis from which the (permeability) x (thickness) product is derived uses Grant's relative permeabilities, which, for the sake of consistency, should also be employed in the analysis of reservoir depletion (Grant, 1977). Substantially greater .reservoir longevities would then be expected (Garg, 1981). (iii) Five Spot For a more realistic assessment of the depletion of a naturally fractured boiling reservoir, we investigated a five-spot production/injection strategy for the reservoir discussed in the previous example. The basic mesh as given in Figure 6 takes advantage of flow symmetry. The production/injection rate was fixed at 30 kg/s, which corresponds to the more productive wells in the Baca reservoir. Our results show that without injection, pressures will decline rapidly in all cases. The times after which production-well pressure declines below 0.5 MPa are: 1.49 yrs for a porous medium, 2.70 yrs for a fractured reservoir with D = 150 m, km = 9 ~ 1 0 - ld~ , and 0.44 yrs for D = 50 m, km = lx10-17 m2. Note that the fractured reservoir with large k, (9x10-l7 m2) has a greater longevity than a porous reservoir. The reason for this is that the large matrix permeability provides good fluid supply to the fractures, while conductive heat supply is limited. Therefore, vapor saturation in the fractures remains relatively low, giving good mobility and a more rapid expansion of the drained volume.

the three cases studied after 36.5 years of simulated time. The temperatures of the porous-medium case and the fractured reservoir with D = 50 m agree remarkably well, indicating an excellent thermal sweep for the latter (see also Bodvarsson and Tsang, 1981). The temperature differences AT = Tm Tf between matrix and fractures are very small: after 36.5 years, we have AT = .2 OC near the production well, .001 OC near the injection well, and less than 5 OC in between. In the D = 50 m case, produced enthalpy remains essentially constant at h = 1.345 MJ/kg. It is interesting to note that this value is equal to the enthalpy of single-phase water at original reservoir temperature T = 300 OC. Thus, there is an approximately quasi-steady heat flow between the hydrodynamic front at T I 300 OC and the production well, with most of the produced heat being supplied by the thermally depleting zone around the injector.

-

At the larger fracture spacing of D = 250 m, the contact area between matrix and fractures is reduced, and portions of the matrix are at larger distance from the fractures. This slows thermal and hydrologic communication between matrix and fractures, causing the reservoir to respond quite differently to injection. After 36.5 years, thermal sweep is much less complete, with temperature differences between matrix and fractures amounting to 16 OC, 118 OC, and 60 OC, respectively, near producer, near injector, and in between. For the particular production and injection rates employed in this study, thermal depletion is slow enough that even at a large fracture spacing of D E 250 m, most of the heat reserves in the matrix can be produced. We are presently investigating energy recovery in the presence of a prominent short-circuiting fault or fracture between production and injection wells, under which conditions less favorable thermal sweeps are expected. Acknowledgement The author expresses his gratitude to T.N. Narasimhan for many stimulating discussions throughout the course of this study. Thanks are due T.N. Narasimhan and J. Wang for a critical review of the manuscript. This work was supported by the Assistant Secretary for Conservation and Renewable Energy, Office of Renewable Technology, Division of Geothermal and Hydropower Technologies of the U . S . Department of mergy under Contract no. W-7405-ENG-48.

The results obtained with 100% injection demonstrate the great importance of injection for pressure maintenance in fractured reservoirs with low permeability. Simulation of 90 years for the porous medium case, and 42 and 103 years, respectively, for fractured reservoirs with D = 50 m and D = 250 m (km = m2), showed no catastrophic thermal depletion or pressure decline in either case. These times are significantly in excess of the 30.5 years needed to inject one pore volume of fluid. Figure 7 shows temperature and pressure profiles along the line connecting production and injection wells for

-153-

References

Nomenclature

B a r e n b l a t t , G.E., Zheltov, I.P., and Kochina, I.N. (19601, "Basic Concepts i n t h e Theory of Homogeneous L i q u i d s i n F i s s u r e d Rocks," J o u r n a l of Applied Mathematics (USSR), V. 24, n. 5, p. 1286-1303:

D:

F: w g: G:

u

h: k: K:

Bodvarsson, G.S., and Tsang, C.F. ( 1 9 8 1 ) , " I n j e c t i o n and Thermal Breakthrough i n F r a c t u r e d Geothermal R e s e r v o i r s , " LBL-12698, Lawrence Berkeley L a b o r a t o r y , Berkeley, C a l i f o r n i a (May) ( t o be p u b l i s h e d i n t h e J o u r n a l of Geophysical Research)

G r a v i t a t i o n a l a c c e l e r a t i o n (9.81 m/s2) H e a t f l u x (W/m2) S p e c i f i c e n t h a l p y (J/kg) Absolute p e r m e a b i l i t y (m2) Heat c o n d u c t i v i t y (W/m°C)

L i m i t i n g e f f e c t i v e p e r m e a b i l i t y ( m2) R e l a t i v e p e r m e a b i l i t y f o r p h a s e 6, dimensionless P: P r e s s u r e (Pa) 9: Conductive h e a t f l u x (W/m2) r: R a d i a l c o o r d i n a t e ( m ) S: S a t u r a t i o n ( v o i d f r a c t i o n ) , dimensionless Sir: Irreducible liquid saturation, dimensionless Ssr: I r r e d u c i b l e vapor s a t u r a t i o n , dimensionless Temperature (OC) T: S p e c i f i c volume (m3/kg) V: Z: V e r t i c a l c o o r d i n a t e (m) vg: G r a v i t y r e d u c t i o n f a c t o r f o r k l b , dimensionless V i s c o s i t y of p h a s e 6 (Pa.6) 6' : D e n s i t y of phase 6 (kg/m3)

klb: ks:

.

Bodvarsson, G.S., Vonder H a a r . S., W i l t , M., and Tsang, C.F. (1980) , " P r e l i m i n a r y E s t i m a t i o n of t h e Reservoir C a p a c i t y and t h e Longevity of t h e Baca Geothermal F i e l d , New Mexico," SPE-9273, p r e s e n t e d a t t h e 5 5 t h Annual F a l l T e c h n i c a l c o n f e r e n c e and Exhib i t i o n of t h e SPE, D a l l a s , Texas (September). Garg, S.K.

F r a c t u r e s p a c i n g (m) Mass f l u x (kg/m2.s)

( 1 9 8 l ) , p e r s o n a l communication.

Grant, M.A. ( 1977), " P e r m e a b i l i t y Reduction F a c t o r s a t Wairakei," P a p e r 77-HT-52, p r e s e n t e d a t AICHE-ASME Heat T r a n s f e r Conference, S a l t Lake C i t y , Utah ( A u g u s t ) . Moench, A.F. ( 1 9 7 8 ) , "The E f f e c t of Thermal Conduction upon P r e s s u r e Drawdown and Buildup i n F i s s u r e d , Vapor-Dominated R e s e r v o i r s , " P r o c e e d i n g s , F o u r t h Workshop on Geothermal R e s e r v o i r Engineering, S t a n f o r d U n i v e r s i t y , Stanford, California.

ws:

Subscripts f: II: m: n: v:

Moench, A.F., and D e n l i n g e r , R. (19801, "Fissure- Block Model f o r T r a n s i e n t P r e s s u r e A n a l y s i s i n Geothermal Steam R e s e r v o i r s , " P r o c e e d i n g s , S i x t h Workshop on Reservoir Engineering, Stanford University, Stanford, C a l i f o r n i a (December).

0:

Fracture Liquid Matrix Normal component vapor Phase ( 6 = l i q u i d , vapor)

P r u e s s , K., and Narasimhan, T.N. ( 1981) , "On F l u i d Reserves and t h e P r o d u c t i o n of Superh e a t e d Steam from F r a c t u r e d , Vapor-Dominated Geothermal Reservoirs," LBL-12921, Lawrence Berkeley L a b o r a t o r y , Berkeley, C a l i f o r n i a ( J u l y ) ( s u b m i t t e d t o J o u r n a l of Geophysical Research). P r u e s s , K., and Narasimhan, T.N. ( 1 9 8 2 ) , "A P r a c t i c a l Method f o r Modeling F l u i d and Heat Flow i n F r a c t u r e d Porous Media", SPE-10509, t o be p r e s e n t e d a t t h e S i x t h SPE Symposium on Reservoir Simulation, New Orleans, Louisiana (January)

.

150

Warren, J . E . , and Root, P.J. ( 1 9 6 3 ) , "The Behavior of N a t u r a l l y F r a c t u r e d R e s e r v o i r s , " S o c i e t y of Petroleum E n g i n e e r s J o u r n a l ( S e p t e m b e r ) , p. 245-255.

200 250 Temperature ('C)

XK)

3!

F i g u r e 1: L i m i t i n g e f f e c t i v e p e r m e a b i l i t y .

-154-

K=2Wlm°C

-13

Absolute permeobility ( m Z ) XMU4-3UO

Figure 5 : Pressure d e c l i n e f o r a r e a l d e p l e t i o n problem.

Figure 2 : E f f e c t i v e flowing enthalpy.

I I I I

I I

'

Matrix'

t

Fract u res

?

t Production well injection well "8, BoI0-1IYl

XBL 813- 2725

Figure 6 : Five- spot computational mesh.

Figure 3: I d e a l i z e d model of fractured reservoir.

3.0

-!g

-e

8.0

Y)

7.0

5.0

6

L

j

B

10

12

14

16

18

Time(vaorr1

Figure 4: Produced enthalpy f o r a r e a l d e p l e t i o n problem.

Figure 7: Temperature and pressure p r o f i l e s f o r f i v e - s p o t a f t e r 3 6 . 5 years.

-155-

E

TABLE

I:

Parameters U s e d i n S i m u l a t i o n s D e p l e t i o n Problem

Radial Flow Problem

Formation rock grain density r o c k specific h e a t rock h e a t conductivity porosity permeability x thickness reservoir thickness matrix permeability

PR = 2400 kg/m3 CR = 960 J / k p C

K

=

4 w/mOc

4 = .08 kh = 1 3 . 4 ~ 1 0 - m3 ~~

h = 500 m kl = m2,

m2,

&

2600 kg/m3 950 J / k g OC 2.22 w/m OC .10 1 . 8 3 ~ 1 0 - l ~m3 305 m 10-15 m2, 9x10'17

m2, 10-17 m2

Fractures t h r e e o r t h o g o n a l sets aperture spacing p e r m e a b i l i t y per f r a c t u r e e q u i v a l e n t continuum . permeability e q u i v a l e n t continuum porosity

6

= 2x10-4 m

=50m kf = 62/12 = 3 . 3 ~ 1 0 - 9 m2

D

( a) 5 m, 50 m, 150 m ( a)

k2

Z

2kf6/D = 2 6 . 8 ~ l O - l ~m2

6x1 0-15 m2

02

%

36/D = l.2X10-5

.10

Relative Permeability Corey- curves

S i r = .30, Ssr = .05

I n i t i a l Conditions temperature liquid saturation

T

= 243 OC

300 OC 99 %

S i = 70%

Production wellbore radius

r, = .112 m

skin e f f e c t i v e w e l l b o r e radius

= -5.18 r&= rwe'S = 20.0 m Vw = 27.24 m3 q = 20 k g / s

w e l l b o r e s t o r a g e volume p r o d u c t i o n rate

8

Injection

rate

30 kg/s(') 5x105 J / k g

enthalpy

( a ) f r a c t u r e s modeled a s extended r e g i o n s o f h i g h p e r m e a b i l i t y , w i t h a w i d t y o f n .2 m (b) rectangular r e s e r v o i r (c) f i v e s p o t

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SIMULATION OF FLOW IN FRACTURED POROUS MEDIA

A. M. Shapiro*and G . F. Pinder Department of Civil Engineering Princeton University Princeton, New Jersey 08544 studying the relationship between them. The outstanding question i s whether the continuum model can adequately represent the mathematical-physical behavior of the discrete system. To address this problem, we have constructed a discrete fracture model and a corresponding continuum model. The discrete fracture model i s shown i n figure 1 . I t consists of a s e t of i n f i n i t e l y long prisms w i t h square crosssections.

Introduction While flow in fractured porous media i s a phenomenon often encountered i n reservoir simulation, there exists no genera l l y accepted simulation methodology. One can catalogue existing approaches as e i t h e r discrete fracture or continuum. As the name implies the discrete fracture mode2 considers each fracture as a geometrically well-defined ,entity wherein the f l u i d behavior i s described u s i n g some variant of classical f l u i d mechanics. The geometry of the porous blocks i s also assumed known and the pore f l u i d behavior i s determined via the equations describing the physics of flow through porous media. The two systems a r e coupled through conservation constraints along the fractureporous block interface. Discrete fracture models have been popular f o r some time. Early work was conducted by Berman (1953) and Crawford and Col 1 ins (1 954) ; recently Grisak and Pickens (1980) used t h i s approach to examine mass transport.

The equations describing f l u i d flow in the fractures are, f o r the x coordinate direction

(mass conservation) and (2)

The continuum model, sometimes referred t o a s the double porosity model , does not attempt to describe the behavior in each porous block or fracture explicitly. Rather one abandons t h i s detailed level of observation and a l t e r natively examines the physical phenomenon from a more d i s t a n t perspective. A t t h i s higher level of observation, one considers only the average properties of the pores and fractures. These properties a r e in turn represented by functions which a r e assumed to s a t i s f y certain smoothness conditions cons i s t e n t with the fundamental postulates of continuum mechanics. This approach r e l i e s more heavily on constitutive theory t o establish meaningful experiments to determine these property functions. The concept of the continuum model, as applied to fractured reservoirs, i s generally attributed to Barenblatt and Zhel tov (1 960). Only recently however have the mathematical-physical underpinnings of t h i s approach been carefully examined. Duguid and Lee (1977) were the f i r s t t o recognize the necessity of adhering to continuum principles i n equation formulation A recent sumnary of work in t h i s area can be found in Shapiro (1981). The Model Problem A1 though both model1 ing approaches have received considerable attention, l i t t l e e f f o r t has been expended i n

(DtCx + GxDxCx) + DxP - ( T4 P

+

h ) D;Cx

(momentum conserv t i o n ) where p

i s f l u i d density,

P

i s f l u i d pressure, X

i s the average fluid velocity,

P

is the shear fluid viscosity,

A

i s the bulk f l u i d viscosity,

II

i s the fracture thickness, and

Dt(*)

and D x ( - ) a r e partial derivatives

i n time and the x coordinate direction respectively .

A similar s e t of equations can be written for the y direction. *Now a t Wenner Gren Center, Stockholm, Sweden.

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Comparison w i t h t h e e a r l i e r continuum representation o f Barenblatt indicates t h a t h i s f o r m u l a t i o n generates a s o l u t i o n somewhat d i f f e r e n t than e i t h e r t h e d i s c r e t e f r a c t u r e o r continuum f o r m u l a t i o n s presented h e r e i n .

The equation d e s c r i b i n g f l o w i n t h e porous blocks i s (3)

k 2 -P; V P

( c f $ + PB)DtP

where

-

- 0

cf

i s medium c o m p r e s s i b i l i t y ,

B

i s f l u i d compressibility,

k

i s m a t r i x p e r m e a b i l i t y , and

$

i s porosity.

References B a r e n b l a t t , G.I. and Yu. P. Zheltov (1960), "Fundamental Equations o f F i l t r a t i o n o f Homogeneous L i q u i d s i n F i s s u r e d Rocks", S o v i e t Physics-Doklady, 5, 1960, pp. 522-525.

The continuum equations f o r t h e porous medium and f r a c t u r e s a r e g i v e n by (Shapiro, 1981).

(4)

Dt(P

k 4k 1

- ,pkp p

Berman, A.S. (1953), "Laminar Flow i n Channels w i t h Porous Wall s" , Journal o f Appl i e d Physics, 24, 1953, pp. 1232-1235.

v2pp

IJ

Crawford, P.B. and R.E. C o l l i n s (1954), " Estimated E f f e c t o f V e r t i c a l F r a c t u r e s on Secondary Recovery" , Trans. AIME, 201, 1954, pp. 192-196.

(porous medium)

+

Duguid, J.O. and P.C.Y. Lee (1977), "Flow i n F r a c t u r e d Porous Media", Water Resour. 1977, pp. 558-566. Res.,

u,

f f

(5)

(pf)

-

v2Pf

Grisak, G.E. and J.F. Pickens (1980), " S o l u t e T r a n s p o r t through F r a c t u r e d Media: 1. The E f f e c t o f M a t r i x D i f f u s i o n " , Water Resour. Res., 16, 1980, pp. 719-730.

U

=

-

pfqf

C 5=f,p

(aBcfP B - A B & B B

V2P6)

u 4 Shapiro, A.M. (1981), " F r a c t u r e d Porous Media: Equation Development and Parameter I dent if ic a t io n " , Ph D . d is s e r t a tion , P r i n c e t o n U n i v e r s i t y , 1981 , p. 373.

( f r a c t u r e d medium)

.

where a B and a r e c o e f f i c i e n t s in t h e mass exchange f u n c t i o n . The terms on t h e r i g h t hand s i d e o f ( 4 ) and ( 5 ) r e p r e s e n t t h e i n t e r a c t i o n between t h e b l o c k s and t h e f r a c t u r e s . Parameter E s t i m a t i o n and A n a l y s i s The imnediate o b j e c t i v e i s t o determine t h e a b i l i t y o f equations ( 4 ) and ( 5 ) t o d e s c r i b e t h e p h y s i c a l response o f t h e f r a c t u r e d porous medi un system represented by equations (1 ) , ( 2 ) and ( 3 ) . To examine t h i s hypothesis, w determine t h e unknown parameters and A u s i n g t h e s o l u t i o n t o (I), (2) and (3). I n o t h e r words we use t h e d i s c r e t e f r a c t u r e ( 2 ) and ( 3 ) as o u r model and equations (l), experimental o b s e r v a t i o n s and s o l v e f o r t h e unknown parameters. TO e s t a b l i s h t h e robustness o f t h e continuum model we subsequently compare t h e s o l u t i o n s o b t a i n e d u s i n g t h e two approaches. The parameters used i n t h i s mathematical experiment a r e l i s t e d i n t a b l e 1. The two s o l u t i o n s a r e presented i n f i g u r e 2.

t3

I t i s apparent from f i g u r e 2 t h a t t h e continuum model generated a s o l u t i o n q u a l i t a t i v e l y s i m i l a r t o t h a t generated by t h e d i s c r e t e f r a c t u r e model. Experiments conducted u s i n g REVS o f d i f f e r e n t s i z e s i n d i c a t e t h a t t h e continuum model s o l u t i o n i s r e l a t i v e l y unaffected by t h i s parameter and t h a t t h e continuum parameters a r e t e m p o r a l l y s t a b l e .

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Table 1 Experimental Properties Property

1)

2)

3)

Value

Symbol

Porous medi um properties Porosity Permeabi 1i ty Matrix and Fluid Compressibility

@ k ($cf t

Fracture properties Thickness Fracture spacing Fluid velocity a t i n l e t

p ~ )

L

L "0

F l u i d properties Compressi bil i ty Viscosity Reference density Reference pressure

Cf

v

0.20 1 .OE-08cm2 1 .OE-1 1gmldyne-cm

0.01 cm 8.0 EOlcm 1 .O cm/sec 0.48 E - 1 Ocm2/ dyne 1.3E-02 dyne/cm2-sec 1 .O gm/cm3 0.0 dyne/cm2

X

I Dy

=O

P = P (x,t)

P'P,

Figure 1 : Diagrammatic representation of a discrete fracture system.

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10'

lo3

FRACTURES

g IO2

DISCRETE FRACTURE MODEL

% 0

\

V

MODEL OF SHAPIRO

Y

;

w

IO'

w a a IO0

IO':

A

I

1

100

200

1

I

I

400 500 DISTANCE ( cm)

300

lo3 -

I

600

7 1 0

POROUS MEDIUM

DISCRETE FRACTURE MODEL CONTINUUM MODEL OF SHAPIRO

REN-TT

I

100

Figure 2:

1

200

I

I

300

400 DISTANCE ( cm)

I

1

500

600

0 7(1

Pressure response calculated using discrete fracture and continuum models by Shapiro and Barenblatt.

-160-


RESERVOIR ENGINEERING OF SHALLOW FAULT-CHARGED HYDROTHERMAL SYSTEMS

S.M. Benson, G.S.

Bodvarsson, and D.C. Mangold

Earth Sciences Division Lawrence Berkeley Laboratory Berkeley, California 94720

Introduction Many of the low-to-moderate temperature ( < 15OOC) hydrothermal resources being developed in the United States occur in near-surface aquifers. These shallow thermal anomalies, typical of the Basin and Range and Cascades are attributed to hydrothermal circulation. The aquifers are often associated with faults, fractures, and highly complex geological settings; they are often very limited in size and display anomalous temperature reversals with depth. Because of the shallow depth and often very warm temperatures of these resources, they are attractive for development of direct-use hydrothermal energy projects. However, development of the resources is hindered by their complexity, the often limited manifestation of the resource, and lack of established reservoir engineering and assessment methodology. In this paper a conceptual model of these systems is postulated, a computational model is developed, and reservoir engineering methods (including reservoir longevity, pressure transient analysis, and well siting) are reevaluated to include the reservoir dynamics necessary to explain such systems. Finally, the techniques are applied to the Sumnville, California hydrothermal anomally.

lbl

-Z III

Ill

calculate the temperature distribution of the system as a function of the flowrate into the aquifer, the temperature of the water entering the aquifer, initial linear temperature profile, system geometry, rock properties, and time (Bodvarsson et al., 1981). The primary assumptions are listed below:

( 2 ) The rock matrix above and below the aquifer is impermeable. Horizontal conduction in the rock matrix is neglected. ( 3 ) The energy resistance at the contact between the aquifer and the rock matrix is negligible (infinite heat transfer coefficient).

t b

ROCK 2

A semi-analytic model has been developed to

(1) The mass flow is steady in the aquifer, horizontal conduction is neglected, and tern perature is uniform in the vertical direction (thin aquifer). Thermal equilibrium between the fluid and the solids is instantaneous.

Thermal Model Figure 1 shows a schematic of the conceptual model developed to explain the occurrence of near-surface hot water aquifers. GROUND SURFACE ROCK 1

Heated fluids rise along a fault until a highly permeable aquifer is intersected. Fluid then enters the aquifer and with time, replaces the existing fluid with hot water. As the water moves away from the fault it is cooled by equilibration with surrounding rock and conductive heat transfer to the overlying and underlying rock units. The model discussed in this paper is most applicable to thin aquifers as vertical temperature variations in the aquifer are not considered.

ROCK 2

I

111

(4) The thermal properties of the formations above and below the aquifer may be different, but all thermal parameters of the liquid and the rocks are constant. The differential equation governing the temperature in the aquifer at any time (t) can be readily derived by performing an energy balance on a control volume in the aquifer:

z = o :

Figure 1 Schematic of a conceptual model for a fault-charged hydrothermal system. -161-

--

2

The symbols are d e f i n e d i n t h e nomenclature. I n t h e caprock and t h e bedrock t h e onedimensional heat- conduction e q u a t i o n c o n t r o l s t h e temperature:

z > 0:

A

A

z < o :

aT1

a2T1

2 aZ

=

at

PIC1

n

Proceedings on the Seventh Workshop on

Proceedings on the Seventh Workshop on

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