RESERVOIR STUDIES AT TATAPANI GEOTHERMAL FIELD, SURGUJA DISTRICT, INDIA

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1 PROCEEDINGS, Thirty-irst Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 30-ebruary 1, 2006 SGP-TR-179 RESERVOIR STUDIES AT TATAPANI GEOTHERMAL IELD, SURGUJA DISTRICT, INDIA P.B. Sarolkar, A. K. Das Geological Survey of India, Nagpur, India ABSTRACT Tatapani geothermal prospect is a promising geothermal resource in Surguja district, Chhattisgarh State. Total 26 boreholes were drilled in the area, out of which five boreholes produce hot water. The five boreholes, drilled up to the depth of 350m, initially produced 1800 lpm of thermal water with maximum temperature of C. At present, four boreholes have free flow discharge of 1159 to 1224 lpm. Thermal logging of the boreholes has indicated presence of thermal anomaly along ENE- WSW direction. The thermal gradient data and well testing data suggest that the reservoir may continue below 1000 m depth. The isotherms at 300m and 1500 m depths indicate that the aerial extent of the reservoir increases with depth. The thermal logging data suggest that the reservoir may have temperature of >150 C, at the depth of 1500m. The ENE extension of the reservoir and its depth persistence needs to be verified by deeper exploration to augment the presently established resource potential of 300 Kwe at the depth of 350m. 1. INTRODUCTION Tatapani Geothermal field is a promising hot water reservoir along the Son- Narmada lineament. Thermal manifestations in Tatapani consists of hot springs (50 C -90 C) in marshy ground and hydrothermally altered clay zones, covering an area of about 0.1 sq km (Ravishanker 1987). The thermal anomaly zone at Tatapani extends along the E-W direction and shows continuity to northeast. The Tatapani geothermal anomaly is located along a regional fracture extending for several kilometers in either direction (Guha 1987). Detailed investigations for assessment of geothermal resource were carried out by GSI since 1980s. Total 26 boreholes were drilled at Tatapani covering an area of nearly 20 sq km, of which five bore wells, drilled at the spacing of around 50 m, GW/Tat/6 and GW/Tat/ 23 to 26, produce hot water (Pitale et al 1995). The cumulative discharge of five borewells was measured to be 1800 lpm, producing water of 110 C at the surface. ig.1, Location map of Tatapani Geothermal area. Geological Survey of India in association with Oil and Natural Gas Corporation, carried out down hole testing of the production wells for measuring temperature and pressure profiles in the borewells (Sarolkar 1999). The available sub surface data and temperature - pressure profiles are utilized to prepare a preliminary reservoir model of Tatapani Geothermal area. 2. GEOLOGY The Tatapani geothermal field is located at the southern margin of the Tatapani- Ramkola coalfield at the contact with Achaean rocks. Tatapani fault, trending ENE-WSW, separates Archaean rocks exposed to the south of fault from Lower Gondwana sandstone exposed to the north west of the Tatapani fault (ig. 2). Lower Gondwana sandstone is encountered in the boreholes down to a depth of 120m, below which pink granitic gneiss with coarse feldspar is reported. Most of the hot springs are located along the Tatapani fault.

2 Geological map of Tatapani area, Surguja district Scale m Tat/04 INDEX Tat/26 Tat/06 Tat/24 Tat/23 Tat/25 Gondwana Sandstone Granites Tatapni fault To Ambikapur Borehole No. Hot spring - ault ig.2, Geological map of Tatapani area. 3. DISCHARGE Out of the 26 boreholes drilled at Tatapani, five bore wells GW/Tat/6, 23, 24,25 & 26 produce free flowing hot water. Monitoring of the wells at different times, for a period of up to two weeks has shown that on continuous flow there is slight variation in the discharge. Variation in discharge of boreholes since 1992 is presented in ig GEOTHERMOMETERS The chemical analysis of thermal water indicated moderate sodium, chloride, silica and sulphate content; low calcium, potassium and arsenic. The reservoir temperature indicated by various geothermometers varies as below: silica 127 C to 157 C, Na/K from 178 C to 232 C, and K/ Mg from 66 C to 97 C. Diagram showing variation in Discharge Discharge lpm Observation no. Tat/6 Tat/23 Tat/24 Tat/25 Tat/26 ig.3, Diagram showing variation in discharge of boreholes

3 The K/ Mg ratio re-equilibrates to the changing reservoir conditions and dilution by ground water adds more Mg to the water, affecting the K/Mg ratio significantly, which is reflected in lesser indicated reservoir temperatures. Dilution has affected the silica saturation also, resulting in lower indicated reservoir temperature. 5. TEMPERATURE O HOMOGENIZATION (Th) The temperature of homogenisation (Th) measured on 33 fluid inclusions from quartz and zeolites crystals, varies from 137 C to 258 C. The majority of fluid inclusions measured Th of >200 C and a few of the inclusions measured Th of <150 C. The rest of the fluid inclusions fall in the range of 150 C to 200 C, which is comparable to the temperatures indicated by the aqueous geothermometers. The frequency distribution diagram of Th is shown in ig.4. Cold-water injection test of the production wells reveals that the thermal profile has gentle slope up to 175m depth and temperature shoots up to 97 C at 200m and 110 C at 320m depths, due to convective circulation (fig.5). Depth m Temperature C Static P Static P Static T Static T lowing P low P low T lowing T 38% 15% 47% >200 C C <150 C ig.4, Diagram showing frequency of fluid inclusions at Tatapani. Thus, the fluid inclusion data suggest that the geothermal system at Tatapani has was initiated at around 250 C, which continued for considerable period at around 180 C. The Th of 140 C -150 C may be attributed to the effect of local mixing. The temperature of final melting TM measured in the fluid inclusions varies from -0.3 C to 21.5 C, indicating salinities ranging from 0.5%o to 23.3%o NaCl equivalent. The low salinity of most of the fluid inclusions suggests a meteoric origin for the geothermal waters. 4. THERMAL PROILE Temperature and pressure logging of the five borewells was carried out in association with Oil & Natural Gas Corporation. Static well temperature profile recorded temperature of 97 C at wellhead rising to 100 C at 50m and 110 C at 110m depths. Maximum temperature of C was recorded at 275m depth, below which temperature inversion is noticed. The flowing well temperature fluctuates within narrow range of 110 C at wellhead to C at 275m further reducing to 110 C at the bottom i.e. 350m depth (Pitale et al 1995) Pressure kg/cm2 ig.5, P and T profile of bore well Tat/ 23 The main thermal water feeder zones are at m, 275m and m depths. The permeable zone below 320m causes cold-water inflow resulting into the temperature inversion. 6. PERMEABILITY. 14 A 17 9 Tank Tetardih 8 Tatapani 7 3 ig.6, Location map of boreholes at Tatapani A'

4 RACTURE PATTERN ALONG SECTION AA' Depth in m Distance in m A A' O O fracture ig.6, Vertical section showing sub-surface fracture abundance in Tatapani area. The Gondwana sandstone is porous and permeable while the Talchir shale and siltstone are fine grained rocks, creating an impermeable horizon acting as a cap rock. The reservoir rocks are mostly granites and gneisses whose permeability is mainly controlled by fracture pattern. The borehole cores show profuse fracturing at 30, 40, 60 and 70 (Pitale et al 1995) to the horizontal. Besides fractures, development of cavities due to leaching is observed in the boreholes GW/Tat/25 and 26, which helps in improving the permeability. The fractures are mostly thin and are occasionally filled in with secondary silica, zeolites or platy calcite. Though the thin fractures in sub-surface rocks show secondary deposition, the major fractures are comparatively open and act as conduit for free flows of water. The fracture zones are few meters wide, but mostly inter connected, facilitating free flow of hot water within the bore wells. All the boreholes show inversion in temperature at the depth of around 300m indicating rather high permeability and good recharge zone at this depth. Joga Rao (1987) postulated low resistivity zones at the depths of about 300m to 600m, based on the Schlumberger soundings. The abundance of fractures was studied in the borehole cores. The relative abundance of the fractures with respect to the maximum fractures noted in the borehole cores was measured at different depths. The relative abundance of the fractures in selected boreholes is plotted along the section A A (ig 6). subsurface granite near boreholes Tat/23, Tat/7 & 8, show moderate to high fracture frequency suggesting better permeability. Most of the boreholes yielding hot water are located on the area showing good fracture abundance, thus, suggesting that the geothermal reservoir is confined to the zone of moderate to high fracture abundance Isotherms at depth of 350m Temperature C ig.7, Distribution of isotherms at 350m depth The sub-surface fracture abundance observed in the boreholes was plotted to get relative abundance. The fracture pattern near borehole Tat/10 clearly indicates that the Gondwana rocks west of Tatapani, and the granitic gneiss to the east of Tatapani near borehole Tat/15 show less abundance of fractures, hence, less permeability. The

5 Depth in meters indicating that the fractured granite / fault zone may provide conduit for the upward movement of hot water ig.8, Depth wise distribution of 100 C isotherm at Tatapani. The Thermal gradient in the boreholes varies from 26 C/km in borehole Tat/1 to 266 C/km in the borehole Tat/4A. The high thermal gradient zone is confined to the boreholes nearer to the Tatapani fault i.e.tat/23, 12 and 7A. The thermal gradient data as well as the sub-surface isotherms distribution, on correlation suggest that the thermal anomaly is at very shallow level near the production wells (i.e. boreholes Tat/23 & 25) with a subsidiary peak near the borehole Tat/7. The data suggest that the thermal anomaly zone has rather steep slope towards north and gentle slope towards south. Thus, the thermal reservoir may extend to south of the boreholes Tat/25, 26 i.e. south of Tatapani fault also (fig. 7). The boreholes Tat/9 & 10, drilled in Gondwana sandstone show less thermal gradient, while the boreholes drilled in granitic gneiss viz. Tat/ 23, 4A show higher thermal gradients. The maximum thermal gradient is observed in the boreholes located near Tatapani fault and fracture zones The thermal profile data shows positive correlation with the fracture pattern. The zones of high fracture abundance (ig.6) are also areas of high thermal anomaly, confirming the sub-surface limits of the geothermal reservoir. The observation also confirms that the fracture permeability controls the discharge of hot water at Tatapani. The thermal profile data were utilized to project configuration of isotherms at the depth of 350 m and 1500 m. The maximum temperature at the depth of 350 m is reported to be C and extrapolated temperature of 195 C at the depth of 1500 m. The area covered 100 C at 350m depth is 0.29 sq km and >7.2 sq kmat the depth of 1500m. The depth wise configuration of 100 C isotherm is shown in ig, 8. The average temperature of the reservoir may be assumed to be 130 C to 150 C, considering that at surface the temperature of the reservoir is less than the indicated reservoir temperatures and the possibility of mixing with cold ground water at shallow levels may reduce the temperature of hot water discharge. The sub-surface isothermal configuration section A-A, along the selected boreholes is presented in ig.9. The possible sub-surface temperatures in the area of study along selected boreholes suggest that the geothermal reservoir shows dome shaped or conical pattern with crest near boreholes Tat/23 & 25 and a secondary peak near borehole Tat/7, respectively. The sub-surface isotherms indicate persistence of reservoir to a depth of 1500m or more. The vertical section of isotherms distribution also indicates that the reservoir extends towards NE of Tatapani (borehole Tat/15), as supported by the bit map of isotherms at Tatapani geothermal area. Depyh in m 0 Subsurface temperature section along Tr. AA' A Distance in m Scale A' ig.9, Vertical section showing sub-surface distribution of isotherms at Tatapani o C

6 7. CONCLUSION The Tatapani Geothermal area is an active geothermal system of medium enthalpy. The geothermal reservoir at Tatapani is confined to granite and gneisses. The thermal water discharge is controlled by fracture permeability. The fracture abundance pattern and thermal anomaly zones show positive correlation suggesting that the reservoir is confined to the areas of fracture abundance and fault zones. The thermal reservoir displays conical shape with two separate sprouts, observed near boreholes Tat/23 and Tat/7, respectively. The thermal configuration at the depth of 350 m and 1500 m indicates ENE-WSW extent of the reservoir, mostly along Tatapani fault, and extension of reservoir towards NE of Tatapani village. The geochemical indicators, fluid inclusion studies, and thermal profiles data indicate that the reservoir temperature at Tatapani ranges from 150 C to 190 C. The study suggests that the Tatapani Geothermal prospect might have depth persistence beyond 1500 m, which needs to be confirmed by deep drilling. Acknowledgements The authors are thankful to Dr. K.S. Mishra, Dy. Director General, GSI, Nagpur for granting permission to publish this paper. acilities provided for work by Shri. S.C. Jain, Director, GSI, Nagpur are gratefully acknowledged. Sincere thanks are extended to Dr. R.S. Shukla for valuable suggestions. References GSI- ONGC (1993): Tattapani geothermal field, a development alternative (unpublished), Project document by GSI-ONGC at Dehradun. Guha S.K. 1986: Status of exploration for geothermal resources in India, Geothermics vol. 15 pp Joga Rao M.V, Rao A.P., Midha R.K., Keshavmani M (1987)- Results of geophysical surveys in Tatapani hot spring area, Surguja district, M.P., Rec. Geo. Sur. India, vol. 115, pt.5, pp Pitale U.L., Padhi R.N. Sarolkar P.B. (1995)- Pilot geothermal power plant and scope for commercial utilisation of Tatapani geothermal field, Surguja district, M. P., India, Proc, World geothernmal Congress, Italy, pp Pitale U.L., Sarolkar P.B., Rawat H.S., Shukla S.N. (1996): Geothermal reservoir at Tatapani geothermal field, surguja district, M.P., India, Proceedings Stanford geothermal workshop, 1996, pp Ravishanker, Thussu J.L., Prasad J.M. (1987): Geothermal studies at Tatapani hot spring area, Surguja dist, Central India, Geothermics, Vol. 16, pp Sarolkar P.B., Shukla S.N, Mukhopadhyay D.K. (1999): Shallow level sub surface characters of Tatapani Geothermal field, India, Proceedings Stanford geothermal workshop, 1999, pp Thussu J.L, Prasad J.M., Saxena R.K., Gyan Prakash, Muthuraman K. (1987): Geothermal energy resources potential of Tatapani hot spring belt, district Surguja, M.P., Geo, Surv. Ind, rec. Vol.115,pt 6,pp

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