Geothermal Exploration and Exploitation:

Abstract

Earth is on the verge of climate change and there are unprecedented dangers before mankind. The need of the hour is to replace carbon-based fuels with renewable alternatives so as to reduce the emission of carbon dioxide. The bold decision to move from carbon-based sources to carbon-neutral sources like geothermal, solar, tidal, wind etc. require worldwide unity and common goal setting. The shift in energy source should be gradual as it will require new infrastructure, new laws, and new policies for harnessing the renewable or carbon neutral source and new technological advancements as the current method for exploitation of renewable sources is costlier than the carbon-based sources. The recent increase in conventional fuel prices has resulted in an increase in the interest to natural sources of energy such as geothermal energy. Geothermal energy is the vast reservoir of heat energy in the earth’s interior, whose surface manifestation are volcanoes, fumaroles, geysers, streaming grounds, and hot springs. Utilization of geothermal resources is an alternative, sustainable energy, which needs to be harnessed in India. Exploration of the resource is the first step to understand potential leads and prospects in the subsurface. The exploration and exploitation of geothermal resources in India are in the nascent stage and that motivated to look into the aspect in great detail. Although some work has been done in Puga, Tatapani, Manikaran, Bakreshwar, Chhumatang, Tapoban etc., no major work has been done in Gujarat. The focus of the present research is an exploration of geothermal resources in Gujarat and demonstrations of the efficacy of geoscientific techniques for prospect generation. Two sites in Gujarat namely Dholera and Unai and were chosen, where geoscientific studies were carried out to understand the subsurface water flow model and relate the same to the surface manifestations in terms of hot springs. Remote sensing data, gravity data and magnetotelluric data utilized to prepare subsurface anomaly maps in terms of cross sections and tomograms. The maps and cross-sections were further utilized to understand the resource potential in Dholera using resource estimation techniques. The same could not be carried out in Unai because of lack of well data in the vicinity of the study area. The study also brought out the different techniques used for resource estimation and also categorized them in the context of reliable applicability in geothermal resource estimation. The energy potential of prospects identified in Dholera and Unai through the above-mentioned studies may help industries in creating geothermal power plant and utilization of energy in space heating & district heating/cooling. Dholera is located 30 km away in southwest direction from Dhandhuka village of Ahmedabad district and 60 km away in the north direction from the city of Bhavnagar. Hot springs exist over gravity high, which is the surface manifestation of deep and shallow water sources. Present research focuses on vegetation index for the study area and land surface temperature; alteration of water chemistry in association with subsurface rocks and understanding of subsurface models using gravity and MT surveys. Studies from existing wells and surveys have suggested the presence of a sizable low enthalpy geothermal resource in the field. In this study, commonly accepted concepts of Ionic and Silica geothermometry have been applied to understand conditions in the reservoir. Interpretation of the data has given us some input on the reservoir characteristics like reservoir temperature, mixing process and multiple fluid origins. The current research additionally imagines the significance of graphical portrayals like Piper chart, Scholler and so forth individually to decide variety in hydro-chemical facies and comprehend the advancement of hydro-chemical forms in the Dholera geothermal field. These studies give a broader idea about the nature of the geothermal reservoir. 2D geoelectric maps and gravity contours have helped in understanding the aquifer geometry and dimensions. 2D resistivity sections depict the aquifer connectivity across Dholera at depths of 3 and 4 km. MT survey suggests that the volcanic rocks are exposed at places. However, it also depicts locations where sediments exist and are sandwiched between basaltic lavas. MT model is also constrained using residual gravity data and corroborates well with the MT data. The present study will help in harnessing the geothermal energy entrapped within the geobodies and also to understand connectivity between the geobodies. These studies were carried out one after the other, to delineate geothermal reservoir and increase the probability of success. The best location for further exploration and exploitation by drilling has been identified by integration of all these methods. The physio-chemical properties of water play an important role in the efficiency of a process, selection of materials and the lifetime of conduits used to carry the fluid used either in industries or for irrigation. A study was undertaken to assess the quality of groundwaters in Dholera geothermal field. Samples were collected from nine geothermal wells and tested for pH, Total Dissolved Salts(TDS), Na+ , K+ , Ca2+, Mg2+ , F - , HCO3 - , Cl- and SO4 2- and their distribution was observed using spatial distribution contours. Piper plot, ternary plot and various other diagrams were used to analyze the water for manifold direct and indirect uses. Unai geothermal field, located in the Narmada-Son lineament, has been found to be a potential zone based on the preliminary studies carried out such as remote sensing and geochemical analysis of the water from the hot springs located in the field. With the aim of developing Unai as a potential geothermal field and exploiting it in the future, geochemical and geothermometric study was undertaken. The samples were collected from various Unai geothermal locations and analysis of the chemical composition of water obtained from different wells was done. The concentration of Silica, Carbonate and ions like Na and Cl have been analyzed to delineate the path of water movement in the subsurface and classify the reservoir based on the enthalpy. The ratio of the concentration of ions like Na+ and K+ and the relative proportions of various sets of ions were also used to characterize the geothermal reservoir and the reservoir fluid by the use of ternary diagrams. Na-K-Ca, Chalcedony, Quartz, and Silica geo-thermometers have been studied. The present study also envisages the importance of graphical representations like Piper diagram, Scholler etc. to determine variation in hydro-chemical facies and to understand the evolution of hydro-chemical processes in the Unai geothermal field respectively. Researchers have carried out data acquisition, processing, and interpretation of 2D MT survey at Unai. Data were acquired on 66 stations in Unai in uniformly spaced profiles along the ENE-WSE direction (geological strike direction) with the station spacing of 2 km. Deep and shallow 1D and 2D models of electrical resistivity distribution were prepared. The one dimensional Occam models and two-dimensional geoelectric models confirm the presence of a low resistivity anomaly near the surface ranging from the depth of 100- 700 m. This suggests the presence of a shallow geothermal zone. Apart from the shallow geothermal resource, this study also delineates a deep conductive body which may be attributed to a matured geothermal reservoir. Based on the interpretation and results of 2D MT; 3D MT survey was carried out for further identification of the geothermal resource and better mapping of subsurface anomalies. 3D MT survey was varied out in a square grid fashion with the grid spacing of 1 km. The results of 2D and 3D MT survey were matched to identify the best possible location for subsurface exploration of geothermal energy. Nowadays, a spotlight on the direct manipulation of water from the geothermal fields is laid for manifold applications. In order to identify the suitability of the water for the various direct and indirect uses, various hydro-chemical parameters were evaluated. The hydro-chemistry of the samples collected from hot spring (depth 100-150 ft.) was studied and samples were examined by calculating different parameters. The complete study was done individually for both industrial and irrigational uses of geothermal water. The key Water Quality Indices (WQI) such as Langelier Saturation Index (LSI), Ryznar Satbility Index (RSI), Puckorious Scaling Index (PSI) and Larson Skold Index (LS) were examined for industrial utilization and the key indices like Sodium Absorption Ratio (SAR), Sodium Percentage (SP), Kelly Ratio (KR) Residual Sodium Carbonate (RSC) and Permeability Index (PI) were examined for irrigational utilization of geothermal water. The results of this study would help the end users to identify the necessary water-treatments before utilizing the water for industrial and irrigation purposes in the study area. After effective investigation on various exploration activities, from a geothermal prospect, stakeholders are constantly anxious to know of its potential. A study was undertaken to calculate energy potential of the Dholera geothermal field. Using various parameters from the geoelectrical model, the resource potential beneath the subsurface was calculated by applying Monte Carlo simulation. Using various parameters from the geoelectrical model and applying Monte Carlo simulation, the resource potential beneath the subsurface was calculated. It was calculated considering all uncertain parameters (random values) within the span of the minimum, the most likely and the maximum triangular distribution. The result shows the frequency distribution of energy values. Energy estimated at 3 km depth in Dholera is 3.73 × 1010 J (P50 Case). Energy estimated for P90 case is 2.90 × 1010 J and for P10 case is 3.73 × 1010 J. To access the geothermal reservoir, shallow wells are drilled. SP and resistivity logging are also conducted for borehole investigation. Surface and subsurface exploration data are also integrated for better understanding and confidence. Effective exploitation strategy was prepared based on the exploration results. One of the major challenges in exploiting the shallow geothermal reservoir at Dholera was the temperature limitation. The problem is solved by introducing a heat pump to raise the temperature to a sufficient level to generate electricity using Organic Rankine Cycle (ORC). Heat pump is designed in dual mode, which can be also used for air conditioning of temple auditorium. The design studies and working fluid selection for the heat pump are discussed in detail in the Project. The study is aimed at a complete study of exploring, drilling and developing low enthalpy geothermal reservoirs. The study also discusses the future expansion possibilities of the field based on the exploration activities carried out. The current research is all about careful, proper study, understanding and implementation of the above concept at Dholera geothermal site for feasibility and might convert it into a successful small scale geothermal power plant which can provide clean green and cheap power energy. This thesis also covers the careful study of ORC for low enthalpy geothermal reserves. A simulation program has been developed in EES software to simplify the calculations and compare the results of different fluids. A systematic approach is followed to compare the fluids for least volume flow rate and VFR, maximum network output and efficiency, least total irreversibility and overall heat exchanger area, by analyzing first and second laws of thermodynamics. The research concludes with the outcome of exploration efforts and also systematically developing schemes for harnessing the energy for domestic and commercial uses. This research can be a starting point for further research work who would like to work on the exploitation strategies of geothermal resources and its utilization.