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GRC Transactions, Vol. 34, 2010

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KeywordsGeothermal exploration, hot-water dominated systems, Turkey

ABSTRACT

This paper outlines the exploration and development effort to discover a geothermal reservoir that is suitable for power genera-tion in the Gümüşköy Area, Aydın, Turkey. The area, located on the western end of the Büyük Menderes Graben (BMG), adjacent to the Germencik Geothermal Site (232ºC), was originally discarded by Mineral Research & Exploration General Directorate (MTA) owing to very limited surface manifestations and failed shallow exploration attempts. Exploration efforts were taken over by BM in early 2004, as Turkey’s first private sector medium/high enthalpy geothermal exploration initiative. The usual geothermal explora-tion tasks were this time supported by deep exploration methods, and especially a comprehensive 3D Magnetotellurics and TDEM study conducted in a total of 3 stages and 393 measurement sta-tions. Joint analysis of all results using a GIS database revealed that there exists a deep geothermal system at greater depths (from 1000 to 1500 meters). Three gradient wells were drilled in the area, with the latter having an unexpected blowout at 120m depth. Wildcat exploration well GK-1 was completed on 28 May 2009 at a total depth of 2100 meters, where flow tests confirmed the presence of a geothermal resource in the Gümüşköy formation, yielding a flow rate of 230 tons/hour and a maximum reservoir temperature of 178°C. Reservoir modeling, volumetric assessment and feasibility studies were initiated immediately thereafter for development of the 12 – 20 MW Gümüşköy Geothermal Power Plant Project, with expected commissioning in mid 2012. Explo-ration drilling was likewise continued with ORT-4 at 2330m and GK-3a at 2000m depth, albeit with varied degrees of conformity with the MT measurements.

Introduction1.

In general, Western Turkey is tectonically active and has com-plex strike-slip and normal faults similar to the Basin and Range

Region in western USA. In a transtensional geological setting, higher heat flow and magmatic geothermal activity are widespread in both regions (Faulds, 2008). Systematic geothermal exploration studies in the Büyük Menderes Graben (BMG) started in the early 1960’s. Previous studies were mostly limited to the shallow parts of the geothermal systems, mainly comprising surface manifesta-tions, geochemistry, gravimetry and 2D resistivity studies.

The geothermal areas in the BMG show surface manifestations along the more active fault zones along the northern boundary, with hot springs having surface temperatures of up to 99°C (e.g. Şimşek, 2003). Shallow reservoirs are present in the Neogene sediments, where the deeper reservoirs consist primarily of karstic marbles and fractured gneiss, quartz and calc-schists of the Menderes Massif (MM). Numerous reservoirs were discovered prior to 2000, includ-ing Turkey’s hottest geothermal sites Kızıldere (242°C) in 1968 and Germencik (232°C) in 1982. As determined through succeeding studies, the MM shows a high surface heat flow (100-300 mW/m²) and Curie depth points between 6 to 12,4 km. (İlkışık, 1995; Aydın et al., 2005; Dolmaz, 2005; Akın et al., 2007).

The Gümüşköy / Ortaklar geothermal area, which is located at the western end of the BMG adjacent to Germencik, reveals limited surface manifestations (hot springs up to 30°C). It is known that MTA (Turkey’s national mineral and geothermal ex-ploration company) conducted studies in the Ortaklar Geothermal Area in early 2000’s. Past studies by MTA include geochemi-cal prospecting surveys covering a larger area extending to the Bozköy Geothermal Reserve in the immediate north / northeast of Gümüşköy – Ortaklar. Initial measurements showed higher shallow temperatures in the Bozköy – Çamur area, reaching up to 62°C. Gümüşköy – Ortaklar area was studied through airborne magnetics, gravimetry and 2D resistivity measurements as early as the mid 1980’s. Ultimately, the AOG-I well was drilled to a total well depth(TWD) of 600 meters with 39.3 °C bottom hole temperature inside quartz. Based on the above studies, the vicinity of AOG-I and an existing hot spring in the area was defined as a blocked area and the remaining portion of the Gümüşköy – Or-taklar exploration program was abandoned.

Exploration efforts were taken over by BM in early 2004. BM’s primary interest in this area stemmed from structural analyses

Exploration and Discovery of the Gümüsköy Geothermal Reservoir in Aydın, Turkey

Özgür Çaglan Kuyumcu, Umut Destegül Solaroglu, and Ali Ünal Akman

BM Engineering and Construction Ltd.

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BMG change direction (from E-W to NE-SW) and boundary fault patterns (normal to oblique-slip). It is known that similar structural positions in the Basin and Range System in the United States have also kept several concealed geothermal reservoirs (Faulds et al., 2009). Overall, The Gümüşköy / Ortaklar geothermal area can be identified as a hot-water dominated convective hydrothermal resource (White and Williams 1975). This system results from deep circulation of water along fractures settled in the BMG.

Geology2.

The Western Anatolia Region of Turkey is considered among the most active tectonic and seismic areas in the world. The Men-deres Massif is one of the largest core complexes in the world and into horsts and grabens by high-angle normal faults. The Büyük Menderes Graben (BMG) contains the hottest geothermal reser-voirs within The Menderes Massif that rapid exhumation began with extensional tectonics in Late Oligocene (?) - Lower Miocene. Precambrian and Eocene metamorphism and deformations have been occurred (e.g. Şengör et al, 1984; Bozkurt and Oberhansli, 2001). The Menderes Massif includes a variety of metamorphic rocks that crop out around the central part in the northern and southern parts including the Dilek Nappe ( continuation of the Cycladic core complex), Lycian Nappes, Afyon and Tavşanlı zones (e.g. Okay and Satır, 2000; Okay, 2001; Ring et al., 2001; Bozkurt and Satır, 2000).

The Menderes Massif mainly consists of a Precambrian core of gneiss and metagranite. The core is overlain by a cover of Permo-Carboniferous marble, quartzite and phyllite and a thick sequence of Mesozoic marbles. This sequence is overlain by red pelagic recrystallized limestones, followed by a weakly metamorphosed flysch sequence with serpentinite blocks (e.g. Okay, 2008). The Ortaklar area is located between the Dilek Nappe and the Central Menderes Metamorphics (CMM).

There are three Neogene series in the region. The first Neo-gene series on the CMM consists of terrestrial, lake and volcanic sequences of the Early-Middle Miocene age. Exhumation of the CMM began with low-angle detachment faulting in the Middle Miocene by a divergent dome along the northern boundary of the BMG and the southern boundary of the SAG. The second series consist of the terrestrial, lake and volcanic sequences accompa-nied by this tectonic event. The last series consist of terrestrial sediments in the high-angle fault-bounded basins formed during the Neotectonic stage in the last 5 ma. The BMG, KMG and SAG are cut by normal faults.

There are discussions on the cooling pattern of the CMM that has been controlled by the cooling history from Pliocene times till present (e.g. Ring et al, 2003). The highest heat flow and the shallowest curie-depth points are also around the CMM (e.g., Dolmaz, 2004). The area is tectonically active based on the GPS, seismic and field data. The CMM contains the hottest geothermal reservoirs within western Anatolia; these are probably related to deep circulation of fluids. The BMG and SAG have the several geothermal reservoirs. BMG’s active geothermal sites, from east to west along the northern boundary fault of the BMG, are Kızıldere (242°C), Buharkent -Tekkehamamı, Pamukören (188°C), Nazilli – Gedik, Sultanhisar (146°C), Salavatlı (171°C), Yılmazköy (142°C), Umurlu – Çiftlik, İncirliova – Sandıklı,

Germencik (232°C) and Bozköy - Çamur (142°C). 2D MT re-sults indicate low-resistivity zones between the BMG and KMG, where additional concealed geothermal reservoirs may be present (Akman et al. 2009).

Exploration Activities3. 3.1 Exploration Methodology

The exploration program followed for this particular region was shaped by the complexity of the area as well as the following legislative and market-specific limitations:

3.1. a. Legislative limitations: Exploration concessions in Turkey are granted for 3 years, and can be extended by one more. This stipulation puts a significant amount of pressure on explo-ration companies. A second limitation is introduced by customs regulations, which require a high number of permits in order to allow temporary import of exploration tools and equipment. This renders sourcing geophysics and drilling equipment from abroad extremely difficult in most cases and impossible for daily-rate based drilling operations.

3.1. b. Lack of staff and professional consultancy services: Geothermal exploration concessions were mostly government controlled until recent years, most activities since 1960 were performed by MTA. Private sector interests and services in explo-ration remained almost exclusive to the oil and gas industry until 2007. This resulted in a potentially strong geothermal market with inadequate availability of staff and consultancy, mainly limited to a number of retired specialists from MTA, geothermal depart-ments at a small number of universities and small-scale service providers that mainly specialized in testing of existing wells. In all cases, they had not developed into full scale service compa-nies that were able to properly schedule their work and adhere to predetermined deadlines.

Most services had applications sourced from the oil industry following quick training programs in geothermal. In the meantime, the lack of availability necessitated bypassing numerous essential exploration steps and continuing with riskier operations such as wildcat drilling in order to adhere to concession deadlines.

3.1. c. Quality of locally available geophysics services: The earlier industry offered two groups of geophysics services. First, there were the methods used by the oil and gas industry, mainly seismic and gravimetric measurements. These were readily available and provided high quality data. Second group comprised shallow conductivity based geophysics services such as the resistivity, VES and SP offered by small companies. While these methods were suitable to shallower geothermal exploration, they did not offer deep penetration and were performed without any industry standards for acquisition, filtering and processing techniques. This necessitated the import of deeper geophysical measurement methods such as TDEM and MT from foreign com-panies, introducing limitations owing to (a) increased costs owing to mobilizing and demobilizing and foreign working conditions and (b) time limitations due to mobilization times, busy schedules and resulting lag times between succeeding acquisition stages.

3.1. d. Lack of slim-hole drilling equipment or services: MTA’s common practice did not include slim-hole drilling for exploration work. Contractors were available for slim-hole drill-

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ing contracts, but they lacked geothermal equipment such as mud cooling systems, BOPs (blow-out preventer), drilling spools and raised working platforms to support proper wellhead equipment. Sourcing these items from abroad on a rental and purchase basis was not practical because of compatibility problem with different contractors’ equipment dimensions. The option was therefore to skip drilling slim-hole wells until a full rig setup including pumps, mud systems, drilling and fishing tools etc. could be purchased by the exploration company.

3.1. e. Lack of drilling rigs and equipment: Similarly, conventional size (i.e. 26" start to 8-1/2" completion) drilling con-tractors were also limited. They included small companies working with 50 – 80 ton Russian truck-mounted drilling rigs with shallow drilling depth capacities, which also lacked the required BOPs, cooling units, etc. Second group comprised oil and gas companies were restricted from utilizing their drilling equipment imported under Petroleum Law’s customs exemption for the geothermal industry. The only viable option was to hire The Turkish Petroleum International Contracting Co., Turkey’s gov-ernment oil drilling company as the drilling contractor. However, TPIC’s drilling program was already overbooked with oil wells and it required at least one year in advance to undertake any serious drilling job. Deciding on a second well based on the outcome of the first was practically impossible.

Looking at the situation as of 2010, it is pleasing to see a quick response in the service and rental sector that mirrors the booming private sector investment interest. Explora-tion activities (including different types of geophysical surveying such as MT, CSAMT, active seismic survey) and field develop-ment activities including drilling and testing are ongoing in close to ten areas (Serpent et al., 2010), three other companies are about to start development stage on geothermal areas purchased from MTA and three addi-tional geothermal energy sites were recently tendered. In a quick transition, good quality and standardized services and equipment are becoming more and more commonly available.

3.1. f. Resulting Exploration Program: Tasks including structural mapping, lineament analysis and geochemistry studies were completed where during the early stages of exploration. These were supported by a comprehensive 3D Magnetotellurics and TDEM study conducted in 3 stages using 393 measurement stations. The overall exploration program included.

Literature surveys• Surface geology studies• Tectonics, geodynamics and lineament analysis from re-• mote sensing Detailed structural studies • (Figure 1)Geochemistry studies• Acquisition of existing gravity and resistivity data• Gradient wells (GS-1 and others)•

Gravity, MT, TDEM (Stage-1)• Gradient wells (ORT-1, G-GK-1, G-GK-2)• SZL-1 wildcat exploration well• MT (Stage-2)• 2D resistivity and micro gravity surveys• MT (Stage-3) • Exploration well (GK-1), completion and flow testing• Exploration well (ORT-4), completion and flow testing• Volumetric Assessment study (Monte Carlo Analysis)•

Results from all studies were compiled into a GIS system analysis. Results showed a geothermal system (at a depth of 1000 to 1500m) within the basement beneath a low permeability cap rock and a second geothermal system in the form of an isolated (inferred) aquifer.

3.2 Geochemistry Studies

Geochemistry studies in the region were initiated on exist-ing irrigation wells (<250m) and springs in the area. Initial geothermometer calculations yielded expected temperatures of 130 – 174°C (Quartz), 136 - 160°C (Na/K) and 115 - 197°C (Na-K-Ca) and a clear total dissolved solids (TDS) concentration map that was in general agreement with the surface thermometry (Yıldırım, N. 2009).

Detailed geochemistry characteristics of the reservoir were identified upon completion of GK-1 well testing. The reservoir fluid was found to be sodium chloride-bicarbonate water, consistent with a moderate to high temperature liquid-dominated artesian geothermal reservoir within carbonate-bearing reservoir host rocks, with similar chemistry with other geothermal fields within the Menderes Graben. The most distinctive chemical feature of the resource is the predominantly high concentration of carbon dioxide

Figure 1. Structural features and well locations in license no.2006/1.

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(CO2) dissolved in the reservoir fluid (%99.8 by volume of total gas), based on analysis of dry gas sampled in stainless steel bottles. The ratio of NCGs relative to water by weight in the reservoir averages 0.021 in GK-1, within the range of the Non-Condensable Gases (NCG) concentrations observed in the Menderes Graben of 0.01 to 0.003 mg NCG: mg brine. The total dissolved solids of the reservoir fluid from GK-1 averages approximately 8000 mg/kg or 0.8% dissolved solids by weight which is relatively common for geothermal fluids world-wide. TDS of shallow geothermal fluids sampled from surface manifestations and shallow wells average 2400 mg/kg, indicating that these fluids are mixtures of ground-water and the deep fluids of GK-1(Haizlip, J. 2010).

3.3 Geophysics StudiesGeophysics studies were initiated with the acquisition of exist-

ing magnetic and gravity data from TPAO and MTA. As the second step forward, 3D Magnetotelluric and TDEM surveys comprising a total of 330 stations was conducted by Geosystems S.R.L. of Italy on December 2006. The stations were placed 2km apart for the purpose of identifying deep (>1500m) reservoirs and upflow zones present within the system. The surveys were performed during December 2006 to March 2007 and the final report was received on December 2007. The 3D inver-sion maps in the final report suggested the presence of two reservoirs, a larger reservoir located underneath Gümüşköy and another smaller one underneath Ortaklar regions, pos-sibly connecting below 3500 meters. A second gradient well, ORT-1 (located in the SE corner of the license area) was tested to measure the temperature in this area. The results appeared to be compatible with the previous geochemistry findings for Electrical Conductivity (EC) and deterministic

mineral concentration areas. The second MT study was conducted in October 2008, and the final report received in February 2009. The resulting MT inversion maps showed a stronger separation between the two reservoirs in Gümüşköy and Ortaklar, where the Ortaklar anomaly zone was now much more apparent than in the previous inversion. A better overall conformity with previous geochemistry and heat flow studies were also noted. The new maps showed the first gradient well GS-1 was coincident with the suggested a pos-sible upflow zone, where ORT-1 gradient well was now shown to be slightly outside the reservoir center. The resistivity cross sections indicated a geothermal resource out (>1500m; Figure 2).

3.4. Wells and TestingThree gradient wells were drilled in order to measure thermal

gradient and heat flow potential zones. ORT-1 was drilled to a depth of 750m at the up flow zone from Stage I MT profiles. Two more gradient wells were drilled in the area. The third well was terminated at 120 m following a blowout. (Gradient and explora-tion well locations are shown in Figure 1)

The presence of higher gradients at shallow depths of the Gümüşköy area (Southwestern section of the 2006/1 prospect license in Figure 1) and the geochemistry and geophys-ics studies led to the decision to drill the first wildcat exploration well, GK-1 to the inferred up flow zone. GK-1 was completed on 28th May 2009, at TWD of 2100 meters. GK-1 from bottom to top encountered Paleozoic gneiss, quartz-calc-mica schist, marble interactions uncon-formably overlain by Mesozoic schist and recrystallized limestone formations. The well, which was drilled, confirmed presence of a geothermal resource in the marble and alternating of quartz-schist and calcareous-schist of the Menderes Massif Metamorphics. The well flowed at a rate of 230 tons/hour and reached a temperature of 178 °C. Initial well tests yielded the follow-ing (Figure 3):

Figure 2. Magnetotelluric Survey Profile for Ortaklar, NW-SE resistivity cross section from 3D inversion.

Table 1. Gümüsköy and Ortaklar gradient wells.

Well Name ORT-1 G-GK-1 G-GK-2

Purpose / Target

Test medium depth gradient at

SE bound ary of prospect

Test shallow gradient at MT upflow zone

in Gümüşköy

Test shallow gradient at NE boundary of

prospect

Date of Completion 10.02.2008 25.11.2007 15.09.2007Total Well Depth 750m Aborted at 120m 85mAvg. Gradient 8,9°C/100m 65°C/100m 60°C/100mMax. Gradient 10,9°C/100m 65°C/100m 60°C/100mMax. Temperature 70°C at 750m depth 98°C at 120m depth 71°C at 85m depth

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GK–1 Exploration WellGeothermal gradient: 9,5 – 25,5°C / 100m • Measured flow rate: 61 lt/s• Measured wellhead pressure: 7-8 bars (105 - 120 psi)• Max. reservoir temperature: 178°C• Shallowest production zone depth: 1100m •

Following the discovery of the Gümüşköy Reservoir, reservoir development studies were initiated starting with the MC Volumet-ric Assessment method and exploration studies were continued for assessing the second inferred reservoir underneath the Ortaklar area. Two exploration wells were considered at the time. These were ORT-2, located in a structurally complex zone and ORT-4 located within the MT low resistivity anomaly. Out of those two ORT-4 has been selected to be drilled.

ORT-4 was completed on 25/07/2009, at TWD of 2350 me-ters. ORT-4 encountered from bottom to top Paleozoic gneiss, quartz-calcareous-mica schist, and marble (partial) unconform-ably overlain by Tertiary sandstone, siltstone, pebble stone and Quaternary alluvium. Initial findings from the well logs and mud temperatures while drilling, suggested a possible production zone located in the marble and quartz-schist-calcareous-schist of the Menderes Massif metamorphic, at around 1100 – 1300 meters depth. During the initial production tests well had a flow rate fluctuating between 1 to 15 lt/sec, which produced 4 lt/sec overall. Down hole temperatures reached 115 °C in the possible produc-tion zone and 130 °C at TD. Based on geochemistry studies it was decided to acidize ORT-4. The acidizing operation was conducted on 06/02/2010. Short term flow testing after the acidizing opera-tion showed a significantly improved artesian flow of 35 lt/sec and temperatures remained unchanged.

ORT-4 Exploration Well (Post-Acidizing)Measured flow rate: 35 lt/s artesian• Measured wellhead pressure: 18-22 bars (270 - 330 psi) • static, 3 bar (45 psi) dynamicMax. down hole temperature: 130°C•

Shallowest production zone depth: • 1100m (now extending down to 1750 meters, from previous 1300 meters)

4. Discussions and Conclusions

The following differences between the production from GK-1 and ORT-4 were noted as follows:

The elevation difference between 1. GK-1 and ORT-4 is approximately 80 meters, GK-1 being located at a higher elevation, which is equivalent to 8 bar (120 psi) of hydrostatic pressure dif-ference. The water table level in GK-1 was measured at -80 meters elevation from the well head. The static well head pressures measured in ORT-4 is 18 bars, which is equivalent to a static pressure difference of 18 bars between the geothermal systems present in the two wells.

The compositions of fluid samples from the two wells are 2. different. Cl/B ratios yield values between 30-40 for GK-1 and between 7-10 for ORT-4. This difference suggests that two fluids do not have the same origin. The isotope analysis of GK-1 well, with high Cl and low B the high TDS values indicate deep circulation for the fluid. The Mg content of the GK-1 fluid sample is lower in comparison to ORT-4, where owing to this difference and the low TDS values in ORT-4, it can be interpreted that ORT-4 and GK-1 wells could be tectonically bounded by the faults in the area.

These differences suggest that The Ortaklar Geothermal Reservoir may be a separate reservoir from Gümüşköy reservoir. The conclusion is also suggested by the MT inversion maps and static temperature measurements from the two wells. ORT-4 well static temperature logs show a lower production zone tempera-ture (102 – 115°C from a depth of 950 – 1800 meters) and a steady geothermal gradient of 3.9°C/100m (slightly above the natural geothermal gradient) below this production zone. There is no temperature reversal at greater depth. By comparison, GK-1 supports much higher production temperatures (level 1 between 975 – 1400m producing 155-158°C and level 2 between 1600 – 2050m producing 175-178°C) and geothermal gradients of 10°C/100m in transition zones. The above also supports the belief that the Ortaklar Geothermal Reservoir was formed by fluid convection up to 950m, at which it was capped under low-permeability schists and gneisses. In comparison, GK-1 is located within the upflow zone of a separate Geothermal Reservoir.

Volumetric reservoir assessment studies based on the Monte Carlo method were performed for the overall geothermal project area, which includes both reservoirs. The volumetric model predicted with 90% probability that at least 29 MWe can be pro-duced on cut-off temperature of 110°C, which rises to 33 MWe for 100°C and 37 MWe for 90°C cut-off temperatures, all for a period of 25 years. The following activities were initiated in order

Figure 3. GK-1 dynamic PTS (Pressure Temperature Spinner) log (21/07/2009).

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to reduce the uncertainties of the MC analysis and understand the reservoir better:

Surface heat flow, alteration and gradient studies to assess • reservoir boundariesHydrogeological modeling studies based on isotope and • mineral analyses from wells, existing shallow wells and springs Drilling and logging of a second production well in the • Gümüşköy reserve for additional data and interference testingLong term production and interference tests from produc-• tion wellsRevised structural modeling based on new data• Reprocessing of 3D MT inversion model with constraints • to be determined based on new well dataRevised volumetric assessment study with constraints to be • determined based on new data, in order to assess production volumes from both reservoirs more accuratelyFurther geological studies to determine possible re-injection • areas

In line with the above decisions, the second production well in the Gümüşköy Reserve, GK-3a, was initiated on 30/03/2010. A third shallower well for temporary reinjection during long-term interference testing, R-GK-1, was also initiated on the same day. Assuming successful completion of the two wells, long term interference testing of GK-1 and GK-3a production wells of ap-proximately 6 weeks is planned. ORT-4 long term well tests are in progress. If it can be established that the Ortaklar Reserve can effectively contribute to the Gümüşköy power generation system or otherwise feasibly support a separate power system, a new - larger diameter - exploration well to fully exploit the reserve from 950 meters. ORT-4 may be utilized as a future production or reinjection well. Meanwhile, cored wells up to 1500 meters depth are being considered to identify reservoir boundaries.

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