Coalbed Methane in the Sydney Basin- Australia

Proceedings of the 1993 International Coalbed Methane Symposium The University of Alabama/fuscaloosa May 17-21, 1993

9304

Coalbed Methane in the Sydney BasinAustralia M. A. Bocking and C. R. Weber, Pacific Power, New South Wales,Austraiia

The rates of desorption, which are somewhat low, are comparabla 10 those of North Appalachian coals.

ABSTRACT The Sydney Basin covers , 1,500 sq.mi. (30 ,OOO km2 ) of the slate of New South Wales in eastern Australia. The BaSin contains approximately 1,200 biUion tons of in situ bituminous coal at depths less than 6000 ft (1800 m). Some 50% of this lies at deplhs less than 3000 ft (900 m). The cumulative thickness of coal is about 110 It (30 m) over most of the basin. however this increases 10 330 II (100 m) in the north east and

The Ultimate usefulness of the resource may be moderated by the generally low permeability of the coal seams. Permeabilitles of 0.1 to 1.0 md (ml!lidarcies) are common, substantially reduced in places by authigenic mineralisation. PermeabiJily has been found 10 be governed by cleat distribution and in situ stress. Anisotropic horizontal stress fiek:ls have been found to facilitate the propagation of vertical hydraulic fractures.

reduces to 30 ft (9 m) on the southern and south western edges. The coal is present within the Late Permian Singleton Coal Measures and their lateral equivatents and the underlying Early Pennian Greta Coat Measures. The potential methane gas in place is some 130 Tef (130,000 PJ) and the basin is adjacent to the major urbanlindustrial market of the SydneyNewcas[[e region.

INTRODUCTION PacifiC Power Is the largest power utility in Australia and operates as an Integrated energy corporation with total assets of $A10A billion. lis generation capacity is 12,090 MW of which 11.460 MW is conventional pu lverised ·coal.lired plant. This plant is supplied with 20 Mt of bituminous coal of which 10 Mt is sourced from underground mines operated by wholly oW-ned subsidiary mining companies. Total electricity sales in 1991-92 were 49,100 GWh. Growth in demand is 2.5"1'0 p.i (Pacific Power i 992).

Of the Six exp l or~tion permits in the Sydney Basin, th ree are held and operated by Pacifi c Power who currently operate 12,OdO MW of thermal power plant and own 8 coal mines. Pacific Power anticipates the significant utilisation of gas for power generation by the year 2000. Al ter a state wide appraisal of the coal bed methane potential In 1989, a major project to investigate the coal bed methane resources in the Sydney Basin cbmmenced in 1990. A complex picture of coal bed methane distribution and resource potential has emerged.

Options for future gene ration capacity include gas fired plant, for peaking and base load duty. Since 1988 Pacilic Powe r has vigorously pursued the investigation of coal bed methane in the slate of New South Wales. It currently holds three Petroleum E)(ploralion licences in the Sydney Basin, PEl's 278. 279 and 284 , which cover a total of 3000 square miles (7500 km').

The high to tow volatile bituminous coals of the Sydney Basin vary In rank from 0.6% to 2.0% vilrinite reflectance (Ro max). By comparison with Nonh American Pennsylvanian age coals they contain much more inertinite. Vitrinite conient may be as little as 10% or as much as 90% (mml). The vitrinile rich coals have been found to have a methane sorptive capability as much as 20% greater than for inertinite rich coals of the same rank. .

LOCATION The Sydney-Bowen Basin stretches through eastern Australia from Wollongong to Collinsville (Figure 1). The Sy.dney Basin covering 11,500 square miles (30,000 k m~) extends soma 200 miles (320 km) north west of Sydney. The adjacent Gunnedah Basin extends to Southern Queensland where its junction with the .Sowen Basin Is overlain by the Jurassic Surat Basin (Figure 2) .

Gas saturation Is governed by topography, hydrologic regimes and the distance to o ~tc rop or sub crop. Coal seam ga6 composition Is typically methane with minor CO2 but areas of hig h CO 2 oonoentratlons are present.

15

COAL BED METHANE IN TH E SYDNeV BASI N _ Al,.tSTRAJJA

common, weak, tuffaceous claystone marker beds. The impact Ihese will have on gas production is unclear.

BASIN HISTORY

The Sydney Basin is at the southern end of a continuous belt of Pe rmian basins some 1100 miles (1Z99 ,,",~'!I) long. A number of models have been proposed for the origin of the belt of basins; the most popu lar Is that it formed as a back·arc basin, between a generally stable craton to the west and an active Permian volcanic arc to the east.

The principal stratigraphic units are identified in Figure 5 (Herbert, 1980). Although marine rocks occur within the coal measures, the limestone and dolomite present within the Carboniferous coal measures of North America and Europe are not found in the Sydney Basin.

The Sydney Basin was fHled with lIuvial, deltaic and near shore marine sediments throughout the Permian and Triassic. In general, the Permian and Triassic delta system prograded from nol1h to south. Total thickness is around 7,000 ft (2,000 m).

RANK Sydney Basin coals are sub·bituminous and coal rank, as measured by vitrinite re fl ectance (Rn max) , ranges from 0.7% in shallow coats in the north, to 2.0% tn deep coals in the more central parts of the basin (Middleton, 1988).

The basin is separated from the Gunnedah Basin by a structural high, known as the Mt Coricudgy Anticline. The basin deepens to the south east, where the Permlan·Triassic boundary 'reaches depths in excess of 2500 ft (800 m) (Figure 3). The sequences crop out along the western and north westem margins of the basin. Overall dips are generally lass than 5°.

The rank variations are betleved to be .aue to depth of burial, rather than to Increases o f heat flow arising from tectonism. l ocal inc;reases in rank are commonly due to the effects of igneous intrusions, which took place in the Jurassic (alkaline inlermediate plugs and sills) and, more common ly, in Tertiary limes (dolerite).

The basin structure has been modified by gentle folding and faulting, mostly along northerly axes. SO me areas of the basin have been covered by flood basalts of Tertiary age. Tertiary and Jurassic dykes and si lls are present in some areas and have cindered coal seams.

In anyone region , thaore is generally a modest increase in rank with, depth. For example, in the north" rank increases from ' 0.7% to '0.9% within a 2000 ft (600 m) interval. variation is substantial : the rank of the time interval at the top of the Permian increases. from 0.7% In the north to almost 1.5% in the centre of. 1he basin (Figure 6).

Latera(

STRATIGRAPHY In the Sydney Basin, coal seam development is restricted 10 deltaic sequences in the Permian strata. Two sets of coal measures are widespread, each underlain by a marine sequence. Basement consists of a folded Devonian marine sequence and the lowermost members of the basin sequence are Early Pe rmian basic volcanics. These are overlain by a sandy marine sequence. The earliest extensive coal occurs in the Early Permian Greta Coal Measures which extend for some 120 miles (190 km) along the northern margit) of the basin and reach a thickness of some 50 ft (15 'm). These extend southwards towards the cenlre of the basin progressively deteriorating to less than 10ft (3 m) of coal. An overlying marine sequence is some 1600 It (500 rn) thick and is variously known as the upper part of the Shoalhaven Group in the south and west and tha Maitland Group In the north.

GAS RESOURCES

In situ gas content in coal" varies across the basin but reaches 630cubi9 fElet per ton ( 18m~l1) in a number of areas. In thecentral -and southern parts of the basin, where cOver is typically 1500 to 3000 ft (450 to g'oo m), theccial has 'a rank >1% {Ro max} and gas contents range from 350 to 630 cu.ftlt. In the north west and north east where the coaJ rank is generally lower and cover varies from 0 to 3000 It (900 m) , gas cOntent varies from 0 to 400 cu.fVt. It has· been found that boreholes drilled on the western side· of this basin can yield zero gas contents to depths of 1800 ft (550 m). Selected results are given in Table 1. The composition of the coal seam gas is often almost pure rnethanebut CO2 Is partfcularly abundant in the southern end of the basin and is dominant at scattered locations throug hout the basin. The reason for this CO2 distribution is unclear. Igneous activity is believed responsible in the south. To the. north, in the Hunter Valley however, Intruded ancf unintruded seams may contain pure methane, whilst nearby holes exhibiting a minimum of 19neousactivlty ·contain In excess of 80% CO~ In all seams. Many gas analyses have also recorded algoilleant (5 to 200/0) nitrogen.

In the late Permian , a more continuous sequence of coal measuras extended across the entire basin, known as the Illawarra Coal Measures in the south and wast, the Wittingham and Wollombi Coal Measures in the north west and the Tomago and Newcastle Coal Measures in the north east. They contain 280 It (85 rn) 01 coat in the north which reduces to 30 ft (9 m) in the. south. The average nat coal across the basin Is 100 'f t (30 m) (Figure 4). The coals seams are characterised by the Inclusion 01

16

,

,

BOCKING MlD WEBER

I ::nI!lng IS curren.lly 100 sparse 10 determine the Sydney Basin's in situ gas resources with much accuracy but the 30 or so deep gas investigation boreholes indicate some 130 Tcf of methane is likely in the lilawarra Coal Measures and their lateral equivalents. This compares very favourably to the 5 Tcf natural gas resource in the Cooper basin, that currently supplies Sydney.

Gas consumption is currently 88 Bcf per annum and gas usage for power generation is nil , gas consumption is conservatively projected to grow to 150 Bcf per annum by 2010 without providing gas f.or power generation. In addition gas for power generation could grow to between 20 and 200 Bcf per annum depending on price and possible carbon taxes (Figure 8).

HISTORY OF COAL BED METHANE

INVESTIGATION STRATEGY AND RESULTS

Early coal mining in the Southern Sydney Basin was plagued with gas outbursts and explosions. The worst occurred in 1887 and 1902. More recently, since long horizontal hole drilling has been available, in seam drainage has been widely practised in the Southern Coalfield. Since the mid 1960's the evaluation of gas contents for mine safety purposes has been a standard part of exploration drilling.

Pacific Power is undertaking a series of investigation wells at sites chosen on the basis of current, often detailed, knowledge of the coal measures. The coal bearing strata is fully cored and the coal cores thus obtained are the subject of gas desorption , gas analyses, lost and residual gas determinatlon and chemical analyses. In addition to proximate testing and the determination of coal quality distribution by floaVsink testing, rank determinations and maceral composition is determined by petrographic methods. Representative core is selected for sorption isotherm and laboratory permeability studies.

In 1902 coal mining commenced beneath Sydney Harbour at a depth of 2900 ft (900 m). Mining was carried out by an advancing manual longwall technique until 1931 but was intermittent due to ventilation difficulties in the face of high methane gas levels. The mine was sealed, but from 1942 to 1950 methane gas from the mine was compressed and sold as a petrol substitute. (Sayers and Bashford, 1988)

The wells are geophysically and geotechnically logged prior to well testing on the principal seams, carried out by straddle packer isolation. In addition in situ stress determinations are made above and below target seams. All core is photographed and descriptively logged in detail. The results of all the above testing are compiled into well completion reports. Selected data is loaded into Vulcan databases for basin modelling on Silicon Graphics workstations.

During the early 1980's AGL explored. widely for coal bed melhane in PEL's 255 and 260 (Figure 7). AGL sought to apply the "Occidental" method of sinking shafts with radial boreholes to extract coal bed methane. At the same time BHP trialled hydraulic fracturing at Appin Colliery south of Sydney. Both efforts yielded only limited success.

Production Simulation will be undertaken commercial 3D simulators in the near future.

By 1988 news, of the commercially successful application of hydraulic fracturing to Alabama coal seams reached Pacific Power. Exploration licences were sought, and building upon a long history of coal exploration, Pacific Power began its investigation of coal bed methane in the Sydney Basin. The first investigation wells were drilled in 1991 and since that time Amoco has likewise commenced investigations in the acreage p(eviously held by AGL (Figure 7). Most recently, Pacific Power in conjunction with the CSIRO has undertaken hyd rau lic-fracture-mine-through experiments at Munmorah colliery north of Sydney.

on

Gas Distribution A complex pattern of gas distribution is emerging and other parameters relevant to gas extraction are being studied in detail. Collaborative R and 0 is being undertaken with the CSIRO on a number of topics. Gas content has been found to be influenced by the height above the lowest level of the regional water table. In the Sydney Basin Permian strata, the coal seams are the aquifers. The coal seams are completely depleted of gas to depths in excess of 1600 ft (500 m) where such seams lie above the height of existing or ancient stream levels.

MARKETS FOR GAS The market for gas in the Sydney-NewcastleWollongong area (population 4 million) is met by pipeline gas from conventional reservoirs in the Cooper Basin The Cooper Basin is a 5 Tcf (SOOO PJ) field located 800 miles (1300 km) to the north west of Sydney in the state of South Australia. At this time a heads of agreement governs supply until 2006 and gas currently costs about $A3.00 per mef to the city gate west of Sydney. Industrial bulk supply tariff Is about $A9.00 mcf.

This is well illustrated at Mt Arthur in PEL 278 (Figure 9) but also occurs in the western Sydney Basin and in the Newcastle Coalfield (Creech,1992). The unexpectedly J.ow levels of gas, preseI"!UQ_§~ams at the base of the coal measures at Mt Arthur, may be associated with the presence cif 011 in that part of the sequence (Levine, 1992).

17

COAL BEO METHANE IN THe SYONEV BASIN • AJJSTRALIA

CONCLUSION

Sorplive Capacity The sorptlve capacity of Sydney Basin coals has been observed to vary for differing coal maceral compositions as well as rank. Vitrinite rich coals have been found to have a sorptlve capacity up to 20% greater thim inertinite rich coal of the same rank from the same iqcation (Figure 10). This is particularly Important in the Sydney Basin where seams contain a very high proportion of inertinite . up 10 90%. This feature makes the coal seams unlike the Carboniferous coals of North America. like north Appalachian coals, Sydney Basin coals are allen slow to desorb, particularly inertinite rich seams.

Pacific Power has identified a number of promiSing prospects in PEL's 278 and 279 but is keen to identify the best, prior to developing a trial production facifity by 1995. This year a 'further four sites will be Investigated in the Sydney Basin and activities will also commence in the adjacent Gloucester Basin, within PEL 285. In addition to directing its own studies Pacific Power seeks opportunities to join with others to develop the coal bed methane industry in the state of New South Wales and 10 assist in the understanding 01 coal bed methane phenomena.

The usual patterns of sorptive capacity variance with rank and tempe rature still exist and in the Hunter Valley a number of areas of high heat flow lead to specu lation that overpressured regions may be found.

ACKNOWLEDGEMENT

'

REFERENCES

Permeability Permeability in the Sydney Basin has been found by Pa cific Power 10 vary from 5 microdarcies 10 50 millidarcies. The 'coals are quite strong (10 • 20 MPa UCS) and cleat development is proportional to the vitrinite content, which is often low. As a result natural cleat deve lopment provides only low permeability, typically 0.1 - 0.3 millidarcies. However the dull coals are prone to fracturing and consistently give higher permeabilities, typica!ly 1 - 10 mill idarcies. Selected permeability results are compared with relevant cleat and fracture indexes in Table 1.

Bocking, M.A. , Odins. P.A., Smith. B.C.. 1992. MWel1 Completion Reper! - ELecom Hunter Uanillo DOH 1." Pacijjc Power. "The BocKing, M.A. and Weber. C.A., 1991. Electricity Commission's Involvement in Coal Bed Methane with early results from the Sydney Basin." in Bamberry W.J. and Deepers, '(Ed.) 1991. "Gas in Australian Coals·. Gao!' Soc of Australia SympoSium Proceedings 2 Brakel, A.T. 1989. "Sydney Basin: Permian Coal Measures - Stratigraphy and Sedimentation.-, in H.J. Harrington (Ed.), 1989. ~Permian Coals of Eastern Australia: SMA Bull 231 Australian p ublishing Servjc'i! Canberra .

Authigenic mineralisation has been found 10 have a dramatic effect on the permeability on seams. Calcite Is the most common offender and can reduce permeability by a factor of 100. The distribution of such minerals appears to be localised and due to nearby igneous activity.

CreeCh, M., 1992. "Geological Controls on Gas Distribution in the . Newcastle Coalfield." in "Twenty Sixth Newcastle Symposium Proceedings, , 992." Dept Of Geology UniyerSlty of Newcastle.

Futu re production may benefit from unbalanced stress fi elds . identified in parts of the Sydney Basin, which have been foun.d to assist the propagation of vertical fractures and may enhance natural permeability due to stress relief tracturing in proximity 10 the well bore.

Gas Council of New South Wales and Ofrlce of Energy, 1992. MJoint Study on the long term 'supply of natural gas for New South Wales - A Public Discussion Paper". Pub, Rep, NSW Office of Energy'"

Paciric Power routinely unde rtakes straddle packer well testing and utilises low vo lume injection and production techniques to ascertain in situ permeability (Koenig el ai, 1992). Comparing the results of this testing with coal properties determined from 61 mm continuous drill core indicates the following relationships.

· · •

The authors acknowledge with thanks the permission o(Pacific Power to present this paper.

Herbert, C., 1980. "Depositional Development of the Sydney Basin." in Herbert, Coo and Helby, R., 1980. "A Guide to the Sydney Bas in." Bu ll Geol SUN, of NSW , 26 11 - 52 , Koenig, R. A, Bocking, M. A., Forster, I., Enever, J. R. and Casey, D. A., 1992. "Experience with well testing and in situ stress measurements in the Sydney Basin for evaluation of Coalbed . Methane prospectivity,· Proc, Coalbed Methane Symposium, TownsyHle. 1992 PO 75 • 95

Permeability increases with increased fraCluring. FraCluring is inversely proportional 10 Ihe amount 01 bright coal. Permeability is inversely proportional 10 Ihe amount of mineralisation .

18

BOCKlNG AND WEBEI'!

Levine, J .A. , 1992. "OverSimplification Can Lead to Faulty Coalbed Gas Reservoir Analysis." Oil and Gas Journal Noy 1992 pg 63·69 Middleton, M.F., 1989 -Coal·Rank Trends of Eastern Australia in Permian Coal basins." in H.J. Harrington (Ed.), 1989. "Permian Coals of Eastern Australia: 8MB Bull. 231 Aystralian publishiOO Service. Canberra.

Odins, PA and Bocking , M.A., 1993. "Well Completion Report · PacifiC Power Hunter Coricudgy DOH 1." Pacific power. Pacific Power, 1992. "Electricity Development and Fuel Sourcing Plan .- June 1992". Pub Rep, Pacific

EllYillL Pinczewskl , W.V., Steve nson, M.D., 1993. "Measure ment of Sorption Isotherms, Hunter Valley Coal , February , 993 - Co nfidential Report APCRC014." Australlao Petroleum Cooperative Research Centre University of New South Wales, Rehfisch, M.W. and Socking , M.A., 1992. ·Well Completion Repon • Elecom Hawkesbury Usarow DDH 1." pacific pqwer. Sayers, P. and BashfOrd A., 1988. -Sydney Harbour COlliery, Balmain" in "Second Heritage Seminar, Sydney, 1 988.~ p roc. Aug!. Inst. Min, & Meta l, PO 37~

Smith, B.C. , Odins, P.A. and Bocking, M.A., 1992. ' Well Completion Report· Elecom Hunter Randwick Park DOH 1.· ?acHjc power. Stuntz, J., 1972. "The subsurface distribution of the 'Upper Coal Measures', Sydney Basin, New South Wales." pap Coot. Australas, los!' Min, MetalL Newcastle, 1972 pp 1 • 9,

19

•

COAL BED MnHAN E IN THE SYDNEY BASIN· AUSTRALIA

980 1086 1408 1451 1801

1 1 1

EHL 1

NA 560 1100

340 1000 84

15 18 39 13

64 160 1500 460 1000 31 16 16

jll

(1) (2) (3) (4) •

16 5 32

45 52 61 76 91 13

NA NA NA NA

EHKL 1

54

4

NA

EMA52

8D

NA 1630

PHCYl ELN 84

18 16 59

1002

12000

1307 1632 2186 2289 2490

10000 19000 69 73 370

2600

81

"

18

45 59 8 6 9 19 41

13.8 19.3

0.2 0.9 1.0 0.4 0.1 0.2 0.1 0.4 0.5 1.0 0.4 0.9 1.1 0.4 0.6 1.0 1.0 0.2 0.2 0.1 0.2

9.7 9.7

NA 11.0 16.2 22.1

23 23 29

NA 14 17 1 22 6 6 0 9 0 0 3

18

NA NA NA NA 27 28.5

31 33

Cleat Indax .. % Dull coal + % Dull, minor bright coal. Fracture Index _ Total natUfal fracture lengHvcoal seam thicknass Minarallndex .. % of Coal pl1li1s containing Ylsible minera~satlon Minimum Horizontal Stms!> (MPa) determined beneath seam. Indicates projected value

TABLE t. SELECTED PERMEABILITY RESULTS. GAS CONTENTS AND PHYSICAL PROPERTIES FROM THE SYDNEY BASIN

20

222 212

335

215 335 116 152 159 131 134 2" 0 0 0 0 0

400 500

155 77 459 364 614 660

0 '

,

BOCK1NG AND WEBER

N

N

-

300 mllErl ~ ~

,

.... ., ..

·· '.'.

\

V

;~~fS'GUNNEDAH

,, /

BASIN

· .. ... ....... .. . . . '\' .. .

500 km

~

300 miles RGURE 1. LOCATlOHOF THE SYDNEY · 8CM'EN BolSIN IN EASTERN AlISTRAlIA.

FIGURE ' 2. STfiIUCTURAl ELEMENTS OF RElEVANCE

TO THE SYOOEY BASIN

,. --." ' to

,w

,..

,..

3Q miles

FIGIJ1.E 4. NET COAl. (IN FEET]

IQURE 3. STRUCTURE CONTotF.S (IN FEET) ON TOP OF

PEA~I,I,N

COAL MEASURES

IN PERMIAN COAL MeASURES

(AFTEA BRAl