Multi-Field Coupling Mechanisms and Heat Extraction Characteristics of CO2-Based Enhanced Geothermal Systems in Hot Dry Rock Reservoirs

Abstract Hot dry rock (HDR) geothermal energy, as a clean and renewable resource, plays a significant role in transforming the energy structure and promoting green, low-carbon development. Supercritical carbon dioxide (SC-CO2), with its low viscosity and high mobility, exhibits strong application potential in HDR geothermal development, enabling efficient heat extraction while facilitating carbon sequestration. This study begins from a theoretical perspective and proposes several modifications to the traditional governing equations, considering the heat and mass transfer behavior of SC-CO₂. A thermo–hydro–mechanical–chemical (THMC) coupling model is developed specifically for CO2-EGS applications. Based on this framework, a discrete-fracture numerical model is established to analyze the effects of well pattern configuration, operational parameters, and fracture network characteristics on thermal performance. The results show that conventional horizontal well configurations are sufficient for efficient CO2-EGS development. In contrast, vertical well patterns tend to experience early thermal breakthrough due to pressure gradient distribution and gravity-driven flow, thereby reducing the thermal service life of the system. Sensitivity analysis reveals that higher matrix porosity and permeability can lead to reduced heat extraction efficiency due to CO2 sequestration and flow diversion effects. While injection temperature has limited impact, increasing injection rate significantly enhances the system's thermal performance. It is therefore recommended to lower injection temperature and moderately increase injection rate to improve energy recovery. Further analysis indicates that a uniformly distributed fracture network improves heat extraction efficiency. Although secondary fractures enhance CO2 recovery, their contribution to overall thermal output is limited. By combining theoretical modeling with numerical simulation, this study systematically investigates the multi-field coupling mechanisms and key influencing factors in CO2-EGS development. The results help to refine the theoretical framework of CO2-EGS systems and expand their application potential, offering theoretical and technical support for achieving carbon peaking and neutrality goals.