Coupled THM Processes Drive Spatiotemporal Slip Evolution in Fracture Networks during Geothermal Heat Production

Le Zhang, Chuanyin Jiang, Qinghua Lei, Alexandros Daniilidis, Anne-Catherine Dieudonné, Longjun Dong, Thomas Hermans 

Understanding induced fracture slip in geothermal reservoirs requires clarifying the relative roles of rapid pore pressure propagation and slower cooling-related stress redistribution. We investigate this problem using coupled thermo-hydro-mechanical simulations with explicitly represented discrete fracture networks embedded in a poroelastic rock matrix. The study considers three fracture density levels (low, medium, and high), multiple realizations, and contrasting injection-temperature and pressure-gradient conditions. An isothermal reference configuration is used to help isolate the relative contributions of pressure and cooling. The results show a clear transition in the main control on slip. Fracture slip initiates mainly in response to pressure propagation, whereas long-term development of the cumulative seismic moment is sustained by cooling-related stress redistribution. Pressure perturbations spread rapidly through connected fracture clusters and define a broad early activation region, while cooling remains more localized within preferential flow corridors and becomes increasingly important for late-time moment accumulation. Higher fracture density increases both cumulative heat extraction and cumulative moment relative to low-density networks, but the two responses do not scale identically within the medium- and high-density networks. A small fraction of fractures carries a large share of the total moment, and the fractures with the largest moment contributions shift toward higher shear-to-normal stress states as cooling develops. Together, these findings explain why improved thermal performance does not correspond to a proportional increase in cumulative moment across fracture network realizations.