Geothermal Characteristics and Productivity Potential of a Super-Thick Shallow Granite-Type Enhanced Geothermal System: A Case Study in Wendeng Geothermal Field, China

Abstract

Super-thick shallow granites without a cap layer are widely distributed in the Wendeng geothermal field. To evaluate the field’s productivity potential for an enhanced geothermal system (EGS), we carried out field tests, laboratory tests and numerical simulations in succession. The geothermal characteristics and deep rock mechanical properties were identified based on real geological and core data from the borehole ZK1 in Wendeng geothermal field. Then, a numerical model of reservoir hydraulic fracturing based on a discrete fracture network was established. Thermal extraction simulations were then conducted to assess the long-term productivity of an EGS project based on the fracturing results. Possible well layout patterns and operational parameters were considered. Results indicated that, for naturally fractured formations, large well spacings should be used and reservoirs with overdeveloped natural fractures should not be selected. For the same reservoir, created by stimulation, the production performances of five-spot and triplet-well modes were different. The pressure indicator was more sensitive to the choice of well layout mode than the temperature indicator. The power generation of the five-spot well mode was slightly improved above that of the triplet-well mode. When selecting the target reservoir, the formations with high temperatures, moderate natural fractures, and high in-situ stress shielding are preferable. On this basis, a large volume of fracturing fluids should be injected to stimulate the reservoir, making the reservoir length and width as large as possible. If the desired large-scale reservoir is created, the five-point well mode should be selected.