Investigation of Heat Extraction in an Enhanced Geothermal System Embedded with Fracture Networks Using the Thermal-Hydraulic-Mechanical Coupling Model

This paper presents a numerical study on thermal energy mining from hot dry rock (HDR) using an enhanced geothermal system (EGS). In these simulations, the thermal-hydraulic-mechanical (THM) coupling model is employed on the basis of the embedded discrete fracture model. The evolution of physical fields of the fractured reservoir, including temperature field, pressure field, and stress field is studied over time, and the effects of different controllable factors, such as fracture morphology, fluid injection rate, and the distances between the injection well and producing well on the heat recovery capacity are investigated. The results show that the fracture morphology significantly influences heat extraction performance. The working fluid mainly flows along with the fracture networks, which causes locally low temperatures and low mean effective stress near fractures. The porosity and permeability increase due to the decrease in mean effective stress. For reservoir models with inclined fractures, there will be a significant decrease in the extraction temperature. In the 30th year, the decline in the heat recovery rate is 46.6%, which is much higher than the model without inclined fractures. Moreover, the increasing injection temperature barely influences the production temperature, while it significantly decreases the heat recovery of the EGS. When the injection and production well spacing is small, increasing the well spacing is an effective way to improve the thermal extraction performance of the EGS. In the model in the paper, the heat production increases up to 13.7% when the injection-production well spacing is increased from 150 m to 450 m. The results of this work could provide guidance for the optimization and operation of EGS.