Heat Extraction Evaluation of CO₂ and Water Flow through Different Fracture Networks for Enhanced Geothermal Systems

Enhanced geothermal system (EGS) technologies have been developed to improve geothermal energy production from hot dry rock (HDR). In this study, discrete fracture network models for geometric topological networks that consider different parameters (the fracture density and the fracture length index) were built on the basis of fractal geometry theory. The heat extraction processes of CO2 and water as the working fluid through different discrete fracture networks were simulated with the application of the thermal-hydraulic-mechanical (THM) coupled method. A series of sensitivity analyses were carried out to reveal the influences of fracture parameters on heat transfer processes. Based on the simulation results, heat extraction efficiencies and temperature distributions in the reservoir of CO2 and water as the working fluid were compared, which showed that CO2 as the working fluid can bring a faster thermal breakthrough. It was found that the fracture length index a = 2.5 and the fracture density I = 5.0 can provide the highest heat extraction rate compared with other cases. This study provides a detailed analysis of fracture parameters and working fluids, which will contribute to the optimized management of geothermal energy production.