Evolution of injection-induced rock damage along fractures under thermo-hydro-mechanical coupling

Hot dry rock (HDR) reservoirs heat is extracted by injecting cold water into natural or hydraulic fractures. During injection, the rock around the fractures experiences repeated cooling and pressurization. This process causes rock damage and changes the reservoir behavior. However, most existing studies mainly focus on the hydraulic fracturing stage, whereas research on damage evolution within connected fracture networks during the production stage remains limited. In this study, a thermo-hydro-mechanical-damage (THM-D) model is developed to track the evolution of temperature, pore pressure, fracture aperture, and rock damage during production. The results show that curved or irregular fractures create stronger stress concentration at bends and tips compared with straight fractures. These locations develop larger damage zones. During injection, thermal stress and pore pressure buildup cause continuous damage accumulation around the fracture. Local porosity and permeability increase, and the fracture aperture becomes larger. Damage improves heat extraction in the short term, but it also leads to earlier thermal breakthrough and reduces the long term stability of the reservoir. Sensitivity analysis shows that injection rate, temperature difference, and thermal expansion coefficient are the dominant factors controlling damage evolution. In contrast, Young’s modulus and permeability have limited influence. This indicates that reservoir behavior is mainly controlled by fractures. The findings clarify how cyclic injection induces fracture damage and provide guidance for optimizing injection strategies and assessing long term production risks in HDR reservoirs.