Heat is energy and geothermal energy is the heat contained within the earth. Geological, geophysical and hydrological processes in favourable environments form a geothermal system. The underground circulating water in aquifers acquires heat from the earth's thermal gradient of around 30°C/km. At places, geothermal gradient is more than the normal due to some geological conditions. The heat is transferred from the interior of the earth to the circulating water by conduction and convection. Geothermal energy can be located at various depths but the economically recoverable energy is confined to depths of 3 to 4 km. The recoverable heat energy is possible only from circulating hydrothermal fluids. Geothermal resources account for only a small part of the world's present day energy consumption.
The geothermal fields are invariably tectonically controlled, and often found in areas of block faulting, grabens or rifting, collapsed caldera structures, with reservoir depth within 1-3 km. Typical settings for the occurrence are around active plate margins, such as subduction zones (e.g. Pacific Rim), spreading ridges (e.g. Mid-Atlantic), intra-continent rift zones (East Africa) and within orogenic belt (Mediterranean, Himalaya). High temperature geothermal systems are generally associated with volcanic belts. Hot spring or boiling spring typically discharge chloride water with a total dissolved solids (TDS) concentration of ~3000-5000 mg/lit. Silica sinter is often deposited around these springs. High temperature fields with non-volcanic or tectonic related systems are less common.
Low temperature or low-enthalpy systems can occur in a variety of geological setup, with slightly elevated or normal heat flow. Deep circulation of water through faults or folded permeability strata with thermal gradient or residual heat from intruded plutons at shallower level can also yield low temperature geothermal fields. They usually discharge water, with total dissolved solids concentrations of around 1000 mg/kg , and temperature in the range of 30° C to 65°C. Many such springs show mineral deposits such as calcite and gypsum with minor or no silica around the springs.
The Geothermal Resources can be classified into three categories, viz i) hydrothermal , ii) geopressure and iii) hot dry rocks. Hydrothermal resources are those that are associated with natural convection systems. Their thermal reservoirs consist of porous or fractured rocks, containing hot water or steam, which in the natural state, transported towards the surface by density-driven thermo-artesian flow. All the currently identified geothermal resources are hydrothermal in origin. Hydrothermal system may be further divided into liquid-dominated or vapour-dominated, depending on the ratio of water to steam in the reservoir. Liquid dominated hydrothermal systems may be of low, moderate or high-temperature type and are the most common kind of geothermal system being exploited commercially today. Vapour dominated reservoir is much less common but includes Larderello in Italy and Geyser in USA.
Geopressure Resources are an example of conduction dominated heat flow system. Their energy is confined in the porous spaces as hot water and sealed so that fluid convection is restricted and heat transfer is by conduction. Their main characteristic is that their pore pressure is considerably greater than the hydrostatic. Such reservoirs exist at depths >4 km in young sedimentary basins having only slightly greater than average thermal gradients. Tertiary basins along the Gulf Coast of Louisiana and Texas, USA are some examples.
Hot dry rock resources are another example of a sub-economic conduction dominated geothermal resources. They occur in areas of high heat flow but low permeability and heat is transferred by conduction. In these areas temperature exceeds 300°C at 2 to 3 km depth. To derive geothermal resources it is required to open a fracture system at depth using hydraulic fluid pressure by drilling and then penetrated by a secondary drill hole. Heat is extracted by circulating treated water from the surface down the well, through the hot fracture network, and recovered up in the second well. The technique is in the preliminary stage and is being applied in England, France and USA.
Depending upon the enthalpy of the geothermal fluid at the wellhead, geothermal resources can be placed in one of the following three categories of potential use.
Electrical conversion yield
Economically viable uses
Space heating, domestic hot water, agricultural and food industries
Chemical Industry, fresh water by distillation, evaporation in sugar refining, drying farm products, canning of food
Drying of timber, heavy water via hydrogen sulphide process, refrigeration by ammonia absorption, electricity production (binary cycle)
Geothermal Studies in India:
Schiagintweit documented ninety-nine well-known thermal springs in India in 1864. R. D. Oldham in the 19th century published the monumental work of his father, T. Oldham (1882) where an inventory of three hundred thermal springs covering the entire country. La Taiche published a list of mineral springs in 1918. Subsequently, studies on the hot springs were carried out by Hollam (1905), Heim and Ganssar (1938), Pranvananda (1949), Ghosh (1954), Seitz and Tewari (1959), Deb (1964), Chatterjee and Guha (1964). The Ministry of Power and Irrigation constituted a committee on 'Hot Springs' in the year 1963 to explore the commercial utilization potential of thermal springs in India. The committee inducted members from the GSI, NGRI and Jadavpur University, Kolkata. All the thermal springs of India were classified on the basis of their geo-tectonic setup and grouped into six Geothermal Provinces as follows:
I. Himalayan Province - Tertiary Orogenic belt with Tertiary magmatism
II. Areas of Faulted blocks - Aravalli belt, Naga-Lushi, West coast regions and Son-Narmada lineament.
III. Volcanic arc - Andaman and Nicobar arc.
IV. Deep sedimentary basin of Tertiary age such as Cambay basin in Gujarat.
V. Radioactive Province - Surajkund, Hazaribagh, Jharkhand.
VI. Cratonic province - Peninsular India
Geothermal Provinces of India
Geothermal Resources of India
There are some 340 hot springs spread allover India. Of this, 62 are distributed along the northwest Himalaya, in the States of Jammu and Kashmir, Himachal Pradesh and Uttarakhand. They are found concentrated along a 30-50-km wide thermal band mostly along the river valleys. Naga-Lusai and West Coast Provinces manifest a series of thermal springs.Andaman and Nicobar arc is the only place in India where volcanic activity has been reported. Some of the islands like Barren are still active. The area is in the continuation of the Indonesian geothermal fields and can be good potential sites for geothermal energy.
Cambay graben geothermal belt is 200 km long and 50 km wide with Tertiary sediments. Thermal springs have been reported from the belt although they are not of very high temperature and discharge. The area contains oil and gas at considerable depths. High subsurface temperature and thermal fluid have been reported in deep drill wells in depth ranges of 1.7 to 1.9 km. Steam blowout have also been reported in the drill holes in depth range of 1.5 to 3.4 km. The thermal springs in the peninsular region are more related to the faults, which allow down circulation of meteoric water to considerable depths. The circulating water acquires heat from the normal thermal gradient in the area, and depending upon local condition, emerges out at suitable localities. The area includes Aravalli range, Son-Narmada-Tapti lineament, Godavari and Mahanadi valleys and South Cratonic Belts. Some of the important geothermal fields of India are described as follows.
• Geothermal Fields of Puga-Chhumathang, Jammu & Kashmir
Puga and Chhumathang geothermal fields are located at altitudes of 4000 and 4400 m in Ladakh district, Jammu & Kashmir along Leh-Chusul road, about 180 km and 150 km, respectively from Leh town, Thermal manifestations have been observed in the form of geyser, hot springs and hot water pools along the Indus river in case of the latter and along Puga nala in case of the former. Present day thermal spring deposits/fossil deposits are found as carbonate/travertine material. The area lies in the vicinity of Indian and Asian Plate suture zone where basic-ultrabasic volcanism took place in Upper Cretaceous age. Later, several phases of acid igneous activity took place in the Upper Cretaceous to Tertiary times during the Himalayan Orogeny. These magmatic bodies are considered to be the sources of heat at shallow level. In Puga Geothermal belt, the thermal manifestations are in the temperature range of 22°C to 84°C spread over an area of 4 sq km2 and the rock types are gneiss, quartz-biotite schist and garnetiferous schist of Puga Formation (Proterozoic age). Several Tertiary granites, quartz and pegmatite have intruded the above rock units. The Puga valley is filled up with glacial deposits.
A total of 35 shallow holes have been drilled by GSI in the Puga Valley to depths of 28.5 to 200.0 m. The hydrochemistry of Puga has indicated that the thermal water from springs and drill holes are neutral to alkaline, pH range from 7.2 to 6.5 and TDS from 1912 mg/l to 2360 mg/l. The water is mainly bicarbonate and NaClHCO3 type. The Cl/B atomic ratio of springs and drill holes are constant 0.08 to 0.10, which indicates a common reservoir for springs and drill holes. There is hardly any chemical difference of thermal springs and drill hole water, which suggests there is no mixing of hot and groundwater in the area. The Na/K ratio is in the range of 10 to 16 which suggests a high temperature in the reservoir but this assumption is not supported by low Cl/HCO3 and SiO2. An evaluation of geothermal potential in Puga valley, based on the studies of GSI, was carried out by Drs. Tsvi Meidav and A. K. Truesdell, United Nations Experts on geothermal resource development, who visited Puga in August 1980. According to them the exploratory holes which erupted with existing pressure and flow rates were impressive for the shallow depths, but considered to be insufficient for long term, high efficient turbine performance.
Blow-out in drill hole, Puga Geothemal Field
Thermal spring, Puga Geothemal Field
Sulphur deposit, Puga Geothemal Field
At Chhumathang, thermal manifestations are spread over an area of 0.5 sq km along river Indus with the springs present on both the banks. The spring temperature ranges from 30°C to 97°C, the latter being the boiling temperature at an altitude of 4000 m. A total of 6 slim holes were bored at Chhumathang. The drill depth ranged from 21.5 m to 221m. The hydrochemistry of Chhumathang indicates that the thermal water contains TDS 977 to 1438 mg/l with pH of 7.1 to 8.4. It has been observed that there is hardly any difference in chemical composition of thermal springs and drill hole water. The maximum Cl is 108 mg/l, SO4 250 mg/l and HCO3 751 mg/l . The Cl/B ratio is almost constant around 0.06-0.09, which indicates thermal water of springs and drill holes originate from the same reservoir. The Na/K ratio is always >21 but Cl/ HCO3 <1 suggests a low temperature geothermal field. The base temperature comes out to 140±10°C based on empirical relationships.
A total of 35 shallow holes have been drilled in the Puga Valley to depths of 28.5 to 200.0 m. Two deep drill holes of 500 m each were proposed close to the southern ridge on western side of the valley, based on geophysical anomalies. However, these could not be drilled to greater than 385 m and 146.92 m. The maximum down-hole temperature has been assessed to be 140°C and shut in pressure in holes 2-3 Kg/cm2. A productive exploratory well of 10-12 cm diameter had discharge of 7-25 tonnes/hour of wet steam. Out of 26 holes in the eastern part of Puga valley, 17 had blowout condition Wellhead discharge measurement carried out on 8 holes indicated water-steam mixture of 190 tonnes/hour. The hydrochemistry of Puga has indicated that the thermal water from springs and drill holes are neutral to alkaline, pH range from 7.2 to 6.5 and TDS from 1912 mg/l to 2360 mg/l. The water is mainly bicarbonate and NaClHCO3 type. The Cl/B atomic ratio of springs and drill holes are constant 0.08 to 0.10, which indicates a common reservoir for springs and drill holes. There is hardly any chemical difference of thermal springs and drill hole water, which suggests there is no mixing of hot and groundwater in the area. The Na/K ratio is in the range of 10 to 16 which suggests a high temperature in the reservoir but this assumption is not supported by low Cl/HCO3 <1 and SiO2 moderately high up to 170 mg/l. An evaluation of geothermal potential in Puga valley, based on the studies of GSI, was carried out by Drs. Tsvi Meidav and A. K. Truesdell, United Nations Experts on geothermal resource development, who visited Puga in August 1980. According to them the exploratory holes which erupted with existing pressure and flow rates were impressive for the shallow depths, but considered to be insufficient for long term, high efficient turbine performance.”
• Geothermal Fields of Beas and Parbati Valleys, Himachal Pradesh
Beas and Parbati valleys are well known for their hot springs in Kulu district, Himachal Pradesh. The springs lie between altitude of 1300 m and 3000 m. The famous thermal springs in the Beas valley are Bashist, Kalath, Rampur and Kulu, whereas in the Parbati valley, the springs are at Jan, Kasol, Manikaran, Pulga and Khirganga. The rocks in the area belong to Proterozoic age and are classified in Vaikrita, Kulu and Rampur Groups. The temperature of springs ranges from 22°C to 59°C in Beas and 21°C to 96°C (96°C being the boiling point at the altitude) in Parbati valley, respectively.
Geothermal drill hole at Beas Valley, Himachal Pradesh
Geothermal manifestation (96°C) at Manikaran, Himachal Pradesh
In the Parbati valley, the thermal springs and drill holes contain low TDS water, which rarely exceeds 1000 mg/l in the Beas valley, except at Kulu where it goes up to 4094 mg/l. The pH varies from neutral to slightly alkaline. Classification of thermal water has been made on Shoeller and Giggenbach diagrams, which indicate water is bicarbonate type and has gone shallow subsurface circulation. The water has meteoric origin and is of peripheral nature. The shallow circulation in presence of high CO2 has converted water rich in HCO3. The Na/K ratio in springs and drill hole is <3 or very high >22, which does not favor high temperature reservoir. Springs and drill hole plot on Na-K-Mg diagram shows that samples fall in the immature field and thus no rock-water equilibrium has taken place with respect to rock minerals in the reservoir. This supports the concept of low temperature reservoir in Beas valley. Parbati valley shows that the thermal water from springs and drill holes are again of low TDS of around 1000 mg/l. The silica content hardly exceeds 110 mg/l.
• Geothermal Field of Tapoban, Uttarakhand
The geothermal field falls in Dhauliganga Valley, a tributary to Alaknanda river in Chamoli district, Uttarakhand. There are several thermal springs in this part of Dhaluliganga and Alaknanda Valleys in the temperature range of 25°C to 65°C. One of the most prominent springs is at Tapoban, located 15 km from Joshimath along Joshimath-Malari road. The exposed rocks belong to Crystalline and Garhwal Groups separated by the Main Central Thrust. High thermal water discharge of 950 lit/min with 65°C has been observed in the area, but the maximum temperature in free flow discharge is 92°C with thermal fluid discharge of 800 lit/min.
Drill hole discharging geothermal water at 65°C at Tapoban, Dhauliganga Valley, Uttarakhand
Drill hole with blowout preventer Tapoban, Dhauliganga Valley, Uttarakhand
The drill hole and spring water in the area is neutral to alkaline with TDS less than 1000 mg/l. There is high SO4 and HCO3 content but low Cl (<20 mg/l) and silica (<120 mg/l). The Na/K ratio is either very low (<5) or high (>20). All the above chemical characters indicate that the thermal reservoir is of low to medium temperature. Further, the Na-K-Mg diagram indicates immature water for Tapoban geothermal field. The chalcedony and quartz geothermometry suggests a base temperature of the geothermal reservoir to be not more than 100±10°C, thereby bringing it in low enthalpy.
• Geothermal Field of Sohana, Haryana
The Sohana Hot Spring is located close to Gurgaon, Haryana. The rocks of the area belong to Alwar and Ajabgarh Groups of Proterozoic age. Post intrusive in the form of pegmatite and quartz veins are of Post Delhi age. The Geological Survey of India drilled a total of 10 holes in the depth range of 90.1 to 547.2 m. Down hole measurement indicated a maximum temperature of around 56°C. Water from drill hole is neutral to alkaline with high TDS (up to 4818 mg/l) and Cl (1348 mg/l). Despite high Cl and TDS contents, very low silica (<49 mg/l) and high Mg (226 mg/l) do not support a high temperature environment for the geothermal field. The exacted base temperature in the area is 50°±10°C, based on log SiO2 vs log K2/Mg diagram of Giggenbach 1988.
• Geothermal Field of Tattapani, Chattisgarh
The incoming of primates including hominids and their evolution giving rise to modern man was the most interesting biotic event during the Cenozoic Era. Early ancestors of man were ape-like non-human primates. Among the earliest pri¬mates akin to hominids, was Ramapithecus, whose fossils have been found in 12 million year old rocks of Siwalik hills in India. These probably walked on their two feet for a while. However, the first definite member of human lineage known as Australopithecus appeared about 4 million years ago; these were ape-faced pre-humans who could stand erect, walk on two hind limbs and were probably the first stone tool users. Earliest human (Homo habilis) appeared about 2 million (20 lakh) years ago in Africa. In the initial stages probably, he did not look like a human. Gradually, he used tools, started living in caves, hunted animals, lived in settlements and finally became the man of the modern age.
Blowout in geothermal drill hole at Tattapani, Sarguja District, Chhattisgarh
In all, 26 holes were drilled at Tattapani area in the depth range of 99.8 to 643.25 m. The geothermal water of Tattapani area is of low TDS and is as good as fresh water. The TDS hardly exceeds 500 mg/l but the Cl content is high (up to 70 mg/l) in comparison to other constituents. There is very little difference in the chemistry between the cold and thermal water. The pH is neutral to alkaline and the water bicarbonate having characteristic of dilute HCO3-Cl type and of peripheral origin. The plot on Li-Rb-Cs diagram suggests rock dissolution rather than rock water interaction with the reservoir in the alluminosilicate rocks. The computed reservoir temperature by Chalcedony geothermometry comes out to around 110±10°C.
• Geothermal Field of West Coast, Maharashtra
The area has about 60 thermal springs all along the west coast of Maharashtra. The belt extends along the coast for a distance of about 350 km from Konkere in the north to Rajapur in the south with average width of 20 km. The eastern boundary is marked by NNW-SSE trending Sahyadri mountain range constituting part of the Western Ghats whose western limit is demarcated by the Arabian Sea coast line. Geologically, the area is marked by thick Deccan Basalt Flows overlying rocks of Precambrian age. The prominent thermal springs are Rajapur (42°C), Math (61°C), Sangameshwar (45°C), Rajwadi (61°C), Tural (62°C), Aravil (43°C), Khed (35°C), Unhavre (Khed-71°C), Unhave (Tamhane-54°C), Vadavli (35°C), Sov (42°C), Pali (43°C), Akloli (54°C), Ganeshpuri (52°C), Sativli (58°C), Haloli (43°C), Paduspada (42°C), and Koknere (54°C).
13 exploratory holes were drilled along the coast. The spring and drill hole water are neutral to alkaline and low in TDS except at Unhavre where it is up to 2391 mg/l. The Cl content in thermal discharge is high but silica is low in amount-maximum around 100 mg/l. Water has been classified as Na-Cl type on Cl-SO4-HCO3 diagram and the plots fall close to Cl apex but none is geothermally matured. The diagram also indicates mixing of thermal and seawater in the area. The Na/K ratio is very high (>70), which confirms a low temperature reservoir.
Source: 1. Geothermal Atlas of India, GSI Special Publication No 19.2. Geothermal Energy Resources of India, GSI Special Publication No 69.3. Compilation of Data on Chemical Analysis of Water and Gas Samples from North West Himalaya and Adjoing Ares, Bulletin Series-C, No. 5.
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