PROCEEDINGS, Twenty-Ninth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 26-28, 2004 SGP-TR-175
REINJECTION EXPERIENCE IN GONEN FIELD OF TURKEY Umran Serpen and Niyazi Aksoy* Dept. of Petroleum and Natural Gas Eng. of Istanbul Technical University Maslak, Istanbul, 80626, Turkey e-mail:
[email protected] *Torbali Technical Institute of University of Sept 9th Torbali Izmir, 35860, Turkey e-mail:
[email protected]
ABSTRACT Gonen geothermal system can be defined as low temperature fracture zone system that was formed on NE-SW oriented Yenice-Gonen fault, which was created by the North Anatolian Fault. The geothermal system seems to be locally delimited by smaller faults that were identified by geophysical surveys. Partial reinjection conducted in the field did not stop pressure decline and caused cooling in the geothermal reservoir. After quadrupling the capacity of the district heating system in 16 years of exploitation, a pressure drop of about 6 bars and a cooling of 10-15oC were observed in the reservoir. In this study, by analyzing the geological, geophysical surveys and the available tests conducted, problems arisen from the existing reservoir management are stated, and solutions are proposed for improving utilization of the geothermal system on the way to sustainability. INTRODUCTION Gonen geothermal field is a part of apparently larger hydrothermal system situated in the northwestern part of Turkey. Hot Springs of Gonen have been known since Byzantine era. The geothermal field was first studied in 1962, and the first geophysical survey was conducted the next year, and later first two wells G-1 and G-2 were drilled in 1976 to 133 and 534 m, respectively. Hydrothermal system originally had the well known Gonen Hot Springs in the outflowing region, which have long disappeared, and nowadays, it feeds a health spa resort and district heating system. The hydrothermal system was on its natural state prior to 1976 and used to feed only a small heath spa. The
district heating facility for heating 600 households was installed in 1987. The district heating system was first enlarged to 1200 households by 1994, and then, was expanded to 2500 households by 2000. As the system is enlarged, the number of wells used for production and reeinjection is also increased to 16 in a small delimited area by a low resistivity anomaly. By 2001, first signs of insufficient heat supply were observed. The situation was so deteriorated that the district heating system was shut down for a month in the midst of winter 2002. The problem was thought to be arisen due to both cooling effect reinjected water and at the same time paradoxically declining reservoir pressure. This study looks for the reasons of this problem by analyzing first geological, geophysical and geochemical data, then temperature and pressure surveys run in the wells, production-reinjection data and finally the reservoir management approach. The pressure and initial and late temperature distributions of the field were built to observe flow patterns and the effect of reinjection operations. On the other hand, the pressure decline in the reservoir is also investigated to explain the behavior of the reservoir. Finally, solutions to overcome the existing problems are reported. GEOLOGICAL SETTING Geology around Gonen Hot Springs has been extensively studied by Serruya, (1962), Urgun, (1963), Kartal, (1973) and Okay et al., (1990). Paleozoic, Mesozoic and Cenozoic aged units outcrop in the region. The basement is consisted of metamorphic rocks, and marbles are found in the upper part of this unit. Permian aged arkose, greywacke and quarzschist series together with limestone olistoliths represent formations related to
the Paleozoic era. Overlain Jurassic aged karstic limestones are generally crystallized and partially brecciate. Miocene aged several detritals, tuffs and agglomerates overlay Jurassic formations. At the top of the stratigraphic series, Quaternary aged talus and alluvium are found. Gonen area is greatly influenced by the North Anatolian fault (NAF) and NE-SW and EW oriented faults that form southern branch of NAF dominate the area. Active Yenice-Gonen strike-slip fault is one of those fractures that intersect the Gonen geothermal field in the NE-SW direction (Fig.1). A major earthquake occurred on this fault in 1953 causing a horizontal displacement of 3.5 m. The Hot Springs of Gonen probably ascends through this fault and flow into the alluvium and other deeper horizons.
them might act as the cap rock. The young Neogene aged volcanism created a high heat flow in the region. The active Yenice-Gonen fault and the other fractures developed in that connection served as an important conduit to transfer the heat from depth to the surface. Gonen geothermal system can be described as a low temperature fracture system (Hochstein, 1990). GEOPHYSICAL SURVEYING Tezcan, (1963) carried out gravity and resistivity surveys in the area. An area of 500 km2 was covered by gravity survey that defined the structure of Neogene basin, and distribution and magnitude of surrounding magmatic and pre-Tertiary formations. Furthermore, it provided the relative thickness of Neogene in the basin. Resistivity survey conducted on an area of 40 km2 also provided the thickness of Neogene and Alluvium, and topography of Neogene bottom. Moreover, apparent resistivity maps indicated anomalies created by geothermal fluids at different depths. Recently, a deep resistivity survey (Ozen, 1995) carried out covering an area of 20 km2 on the low resistivity anomaly discovered in the previous resistivity survey. This survey, while confirming SWNE oriented distribution of geothermal fluids along the Yenice-Gonen fault unearthed one shallow and a deep low resistivity zones (Fig. 2). On the other hand, the topography of Paleozoic aged basement and the resistive boundaries of the field were defined.
Figure. 1. Geological map of Gonen region (after Erisen et al.,1996). Gonen geothermal waters (Hot Springs and well fluids) have approximately 1750 mg/l of TDS and they are bicarbonate (soda) type waters as most of the geothermal fluids in Turkey. Classification of Gonen geothermal waters according to the cation and anion distribution are given as Na>Ca>Mg and SO4>Cl>HCO3, respectively. As for the resource temperatures, while silica geothermometers indicate the temperatures slightly higher than reservoir temperatures encountered (80-90oC vs. 110oC), cation geothermometers point out the temperatures around 150oC, which could be interpreted as deep temperatures. Isotopic studies indicate that the origin of Gonen thermal waters is of old meteoric, that is, paleowater (Yalcin, 1997). The geochemical studies showed that the possible recharge area of the field lies in the southern mountains. In the area, while limestone formations belonging to different ages and marbles in the basement probably serve as reservoir rocks and tuffs interlaying between
Figure. 2. N-S oriented structural geoelectric crosssection (after Ozen 1995). WELL TESTING In 1985 first flow tests were conducted on wells G-1 and G-2 (see Fig. 8 for the well locations), which initially had artesian flow of 14.7 l/s and 16.4 l/s, respectively and their flowing temperature was around 83oC. Both wells had wellhead pressures at that time, and Fig. 3 compares pumping test results of wells G-2 and G-16 conducted in 1985 and 2003, respectively. Prior to 1976 three caisson wells were used for health spa facilities. After the wells G-1 and G-2 were drilled, these two wells were used for the
same purpose until 1985. No changes in flowing parameters of those wells were observed during that period of 9 years. This fact seems mainly to have encouraged the municipality authorities to establish a small scale (for 600 households) district heating system. But in this context, an important interference test was overlooked. Although the full test records were not available for a complete interpretation, records (Orme, 1985) indicated that high flow rate (187 t/h) in G-2 resulted in decline of 29 t/h in flow rate of G-1. The static wellhead pressures of the wells G-1 and G-2 were observed as 1.1 kg/cm2g and 0.3 kg/cm2g. Reinjection possibilities were investigated, and during 68 hours disposal water at 20oC was injected in G-2 at a rate ranging from 160 to 210 t/h. On the other hand, with the help of 1m water head, the disposal water was injected into caisson wells at rates about 54–72 t/h during 200 hours.
On the other hand, recorded reservoir temperatures are a little bit higher, and they are illustrated in Fig. 4. Temperature profiles show that while some wells (G9) penetrated in the reservoir deeply and some others partially, one well (G-6) remained within the cap rock. Unfortunately, the temperature surveys were not run in all wells. Therefore, the completion problems observed in wells with profiles could not be diagnosed for some other wells without profiles, but, with apparent production problems. Table 1. Original Producing Temperatures of Some Wells (after Serpen, 2002). Wells Original Temp., oC Temp. in Aug.1998 Temp. in Nov.2003
G-2 82
G-3 79
G-4 75
G-7 77
G-8 76
81
68
74
70
72
*
-
*
64
62
* Wells caved in, - well not used. Producing temperatures of wells observed in 1997 and 1998 are illustrated in Fig. 5. Comparing with the original temperatures of some wells in Table 1, by the August 1998 those temperatures G-3, G-7 and G-8 declined 11oC, 7oC and 4oC, respectively. As seen in Fig. 6, the producing temperatures are oscillated in winter and summer seasons. While well G-7 seems to be the most affected one, G-3 and G-4 are influenced to a lesser degree. These changes can be attributed to reinjection conducted in between production wells. Figure. 3. First and last pumping test results. 85 80
Temperature, C 0
20
40
60
80
100
75
0
70 65
Jul 1998
Jun 1998
Apr 1998
May 1998
Mar 1998
Jan 1998
Feb 1998
Dec 1997
Oct 1997
Nov 1997
Sep 1997
Jul 1997
Aug 1997
Jun 1997
Apr 1997
May 1997
Feb 1997
Depth, m
Mar 1997
60
200
Date 400
Wells G-3 G-4 G-6
600
G-9
G-2
G-3
G-4A
G-7
G-8
Figure.5. Producing well temperatures of Gonen geothermal field in 1997and 1998.
G-10 G-11
800
Figure. 4. Temperature profiles of some wells in Gonen geothermal field (after Serpen, 2002). Producing temperatures encountered in the wells ranging 133 m and 816 m depth change between 75oC and 93oC and initial ones are given in Table 1.
Although there are not much recorded data available, during 1999-2000 heating season production rate and piezometric levels of some wells are recorded; they are illustrated in Fig. 6. As seen in total flow rate and static levels vs. time plot, water table as a whole in the reservoir responds to the production. Specifically, the well G-7 is the most affected one by the total production rate, and sequentially G-3, G-4 and G-10 follows it. Behavior of these wells is similar to 19971998 seasons. That is, the greater the pressure drop, the higher the temperature decline, in the sense that
490
60
420
55
40 210
35
140
Static Level,m
45
280
30
24-Feb-00
04-Feb-00
15-Jan-00
26-Dec-99
06-Dec-99
20
16-Nov-99
0 27-Oct-99
25 07-Oct-99
70
(average 43 m) in the midst of heating season. On the other hand, the data of pumping test (Fig. 3) illustrates that while static level of G-2 in 1985 is around -13.2 m it is around +47.5 m of a recently drilled well (G-16) in Dec. 2003; that is, total water level drop is more than 60 m after 16 years of exploitation. 4441500
50
350
17-Sep-99
Total Production, l/s
reinjection water front of 40oC moves in easily through fractures and due to pressure differential, and cools down the volume where the pressure declines. Dynamic levels of pumping wells are also available and they are following the same trend (Fig. 7).
G12
4441400
4441300
G6
4441200 G9
4441100 G8
4441000
G10
G7
Total Production G-4 G-8
G16
G2G11
G15
4440900
Date
G3
G-3 G-7 G-10
4440800
Figure. 6. Total production vs. static water levels of some Gonen geothermal wells in 1999 and 2000 heating season.
4440700
G5
G14
4440600
G13
4440500 G17
490
4440400 555000 555100 555200 555300 555400 555500 555600 555700
65 60
420
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Total Production, l/s
55 350
30 70
25
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04-Feb-00
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06-Dec-99
16-Nov-99
27-Oct-99
07-Oct-99
20 17-Sep-99
0
Date
Total Production G-4 G-8
G-3 G-7 G-10
Figure. 7. Total production vs. dynamic water levels of some Gonen geothermal wells in 1999 and 2000 heating season. A static water level survey was run in October 2003, and the results are illustrated in Fig. 7. The water level distribution in Fig. 8 is given with respect to sea level, and 31 m (sea level of Gonen) should be added for the measured ones in the field. As seen in Fig. 8 2 m contours shape the upflow region of the Gonen geothermal system. Considering the original wellhead pressures of wells and the current water table level, despite the reinjection, the pressure drop after 16 years of total exploitation due to stepping up production rate in stages is between 35m and 44 m. This decline is given at the end of the summer, that is, at the end of recovery season. As seen in Fig 7, static levels for 1999-2000 winter are at lower depths
Figure. 8. Water levels adjusted to sea level in Oct.2003 (after Aksoy, 2003). REINJECTION EXPERIENCE G-1 was utilized as a reinjection well at the initial stage; later, while G-1 was the main reinjection well G-3 and G-4 were added to reinjection scheme from time to time. After the well G-1 was caved in at the last stage, G-5 and G-8 were interchangeably included into reinjection line. Finally, G-15 was drilled to substitute G-1 close to G-1 together with G12, G-13, G-14, and G-15 in year 2002. This year (2003) G-15 alone has been utilized as reinjection well. Disposal water is reinjected at 40oC. Reinjection rates utilized in the past are not clearly known, but, it is roughly assumed that 25-40% of produced fluid has been used in health spa facility and discharged into the creek afterwards, and the rest has been reinjected into the geothermal reservoir. Actually, the same proportionality is maintained, and the facility uses 72-144 t/h of produced water, and 144-275 t/h of disposal water are reinjected. Injectivity of wells within the low resistivity anomaly seemed to be relatively high; however, since reservoir volume seems to be very limited in resistivity survey, and wells are very close to each other, injection of relatively cold disposal water into the center of a low
system were observed in 2001 and 2002. On the other hand, increasing flow rate has meant higher electric consumption in pumping and consequently, higher costs.
temperature reservoir has caused progressive cooling of the produced fluids. Even at second stage, after the enlargement of district heating system to 1200 households, the temperatures of produced fluids declined in 1998, as seen in Table 1. Fig. 9 and Fig. 10 illustrate the initial and the recent temperature distributions, respectively. As seen from those figures, particularly, the southwestern part of the geothermal reservoir has been considerably cooled down due to substantial reinjection in wells G-3, G-5, G-8, G-1 (G-15) and G-13 (recent). The east of the geothermal field also has been cooled down due to the reinjection into wells G-2 and G-4. The reinjection into G-4 was quit last year after the well was caved in.
If the reservoir pressure response to total production is taken into account, observed water levels have continuously been declining in the past. Flow rate of 350t/h with partial reinjection of 60-75% is the maximum one with gradual decline of reservoir pressure and temperature. Reinjection among the producing wells complicates the issue. Therefore, reinjection site for this field should be separate one. Fig. 11 indicates a proposed reinjection site that is approx. 400 m away from the main for this
4441500
G12 4441400
N
4441500
G12 4441400
G6
G6
4441300
4441300
4441200
G9
4441200
4441100
G4
G8 4441000
G10
G7
G9
4441100
4441000
G3
4440800
4440700
G10
G7
G15 4440900
G4
G8 G16 G2
G16 G2
G15 4440900
G3
4440800
G5
G14
4440700
4440600
G5
G14
4440600
G13 4440500
4440400 555000 555100 555200 555300 555400 555500 555600 555700 555800 555900
Figure. 9. Original producing well temperature distribution of Gonen geothermal field (after Aksoy, 2003). DISCUSSION OF THE RESULTS Gonen geothermal field is used to provide the necessary amount of heat for the installed district heating system of 2500 households. Available field records point out that inlet water temperature of heat exchangers of 66oC in September 2001 declined to 60oC in March 2002. To substitute the energy lost due to 6oC of cooling in the primary cycle would correspond 30% of increase in total production rate. This situation has both depleted and cooled the reservoir faster, creating a vicious cycle. Water levels in the same wells have dropped more than 60 m, and water temperatures declined approx. 15oC approaching to a critical level. This is not a sustainable way of exploitation for a geothermal field. Therefore, shot downs of the district heating
G13 4440500
4440400 555000 555100 555200 555300 555400 555500 555600 555700 555800 555900
Figure. 10. Recent (Oct.2003) producing well temperature distribution of Gonen geothermal field (after Aksoy, 2003). geothermal system. This site has already 3 wells to be used for reinjection purpose (G-13 has already been utilized), and should be tested for an entire heating season. If negative results are observed farther away wells could be drilled in that direction. Reinjection site is chosen on the basis of temperature distribution, information about the permeability and the geochemical data. Geochemistry indicates that the intake area of the field is on that direction in Kazdag Mountains. It is believed that reinjected water would be heated on its way to the main production area, provided that sufficient distance and depth is given. Wells in the northern section such as G-6 and G-12 have very poor permeability. The above mentioned data indicate that even if a successful reinjection scheme was established the reservoir pressure will gradually proceed to decline
due to its limited dimensions, if an efficient heating is achieved. That leaves two alternatives to provide the demanded heating requirements: (1) to improve district heating system by revising the system and installing heat exchangers at every building and saving through energy conservation and (2) to develop nearby other outflowing regions of a deep geothermal system. Anomalies observed in geophysical surveys and related hot springs may provide the lacking heat, and there is even a possibility of enlargement of the existing district heating system. In our opinion both alternatives merit the consideration.
•
Reservoir engineering studies for geothermal system must be conducted.
the
The following recommendations are given after this study: • Monitoring of necessary parameters such as pressures, temperatures and repeated well testing should be conducted in the field to be able to predict the future performance of the field. • With the collected data a reservoir engineering study should be immediately commenced. • Hydraulic and heat balance of district heating system should be revised and heat exchangers for buildings should be installed to operate an efficient heating system and conserve energy. • Geological and geophysical surveys should be started on nearby low resistivity anomalies.
REFERENCES
Figure. 11. Proposed production and erinjection sites for Gonen geothermal field (after Aksoy, 2003). The district heating system has been developed in two stages, doubling the capacity at every turn. No comprehensive reservoir study and no systematic data collection were conducted in this considerably large size facility. The heating system has continuously been enlarged by increasing the number of wells in a limited area indicated by geoscientific data, and finally, the system has run aground. CONCLUSIONS AND RECOMMENDATIONS In the light of above mentioned the following conclusions are reached: •
•
•
At current flowing and reinjection rates it will not be possible to maintain the reservoir pressure and temperature in Gonen field, and the geothermal system turns out to be unsustainable. Current exploitation of Gonen geothermal field will lead to depletion of geothermal system and the heat demand the subscribed people will not be met in the long run. Sustainability of a geothermal system can not be provided by increasing the number of wells for this field.
Aksoy, N., (2003), “Gonen District Heating System, Observed problems and Recommended Solutions,” unpublished report to Gonen Kaplicalari Isletmesi AO., Gonen. Erisen, B., Akkus, I., Uygur, N., Kocak, A., (1996), “Geothermal Inventory of Turkey”, MTA, Ankara. Hochstein, M.P., Zongke, Y., Ehara, S., (1990), “The Fuzhou Geothermal System (Peoples Republic of China): Modeling Study of a Low Temperature Fracture-Zone System,” Geothermics, Vol. 19, No. 1, 43-60. Kartal, T., (1973), “Hydrogeologic Report of Gonen Hot Springs (Balikesir),” MTA Report No. 50792690, Ankara. Okay, A., Siyako, M., Burkan, K.A., (1990), “Geology of Biga Peninsula and Tectonic Evolution,” Bulletin of TPJD, C. 2/1, 63-122. Orme Jeotermal, (1985), Feasibility Report of District Heating System of Balikesir-Gonen to Gonen Kaplicalari Isletmesi AO., Gonen. Ozen, N., (1995), “Resistivity Survey of BalikesirGonen Area,” MTA Report, Ankara. Serpen, U., (2002), “Considerations on the Latest Situation of Gonen Geothermal Field”, unpublished report to Gonen Kaplicalari Isletmesi AO., Gonen.
Serruya, S., (1962), “Les Thermes de Gonen,” MTA Report No. 3236, Ankara. Tezcan, K. (1963), “Gravity and Resistivity Surveys in Balikesir-Gonen,” MTA Report No. 3511, Ankara. Urgun, S., (1963), Thermomineral Study of GonenEksidere-Ilicaoba Region, MTA Report, No. 3636. Yalcin, T., (1997), “Hydrogeological Investigation of Gonen and Eksidere Thermal Waters”. In: Schindler, C., Pfister, M., (Eds.), Active Tectonics of Northwestern Anatolia-The Marmara Poly-Project. Vdf Hochschulverlag AG an der ETH, Zurich, 301320.
Yalcin, T.H., (1990), “Hydrogeology of Gonen and Eksidere Thermal Springs Area,” Proc. of X Congress of World Hydrothermal Organization, Istanbul, 359-372.
