This study explores the innovative use of post-mining subsurface voids by proposing a coal mine goaf-based underground reservoir energy storage system. By fully utilizing the geothermal potential and insulation properties of the mined-out coal seam, we established a finite element model considering the thermal-fluid coupling process to describe the seasonal energy storage process of the underground coal mine reservoir and analyzed the feasibility of achieving seasonal hot water storage through the underground reservoir, based on the model calculation results. By studying the temperature state of the hot water after the end of the heat storage process and the hot water output effect, the thermal energy storage and utilization potential of the underground reservoir were quantitatively analyzed using the factors of heat storage and heat production. The research results have clarified the feasibility of using coal mine underground reservoirs for cross-seasonal heat storage applications. The results of this study can provide new ideas for the utilization of coal mine goaf areas and underground goaf residual spaces and serve as a reference for the development and design of new energy storage facilities.

Hot dry rock (HDR) is a promising renewable energy resource whose vast reserves and wide distribution have attracted extensive attention in recent years. However, exploiting HDR resources requires hydraulic stimulation, which is typically accompanied by substantial microseismic activity, posing significant risks to project safety and public acceptance. Current understanding of microseismic mechanisms, particularly the role of fracture geometry under varying injection schemes, remains inadequate. This study employs a three-dimensional block-based discrete element method to construct a fluid–mechanics coupled model founded on a discrete fracture network, aimed at investigating the mechanical behavior of fractures and the spatial distribution of microseismicity during hydraulic stimulation. Our results quantitatively demonstrate that fractures oriented at 45° to the maximum principal stress are most susceptible to shear reactivation and microseismic clustering, with event magnitudes strongly correlated to both fracture orientation and intra-fracture fluid pressure. Consequently, preventing critically high fluid pressures in natural fractures near the injection well, particularly those at approximately 45° to the maximum principal stress direction, is essential for risk mitigation. Cyclic injection can shear more fractures and slightly reduce magnitudes via staged pressure relaxation, but its effectiveness in controlling microseismic magnitude is limited. Therefore, it is recommended to implement measures to control the entry of fracturing fluid into these high-risk fissures, such as segmented fracturing or temporary plugging techniques. This strategy is expected to enhance seismic risk mitigation, thereby contributing to the safe and efficient exploitation of deep geothermal resources.