In deep geothermal extraction, cyclic water injection frequently induces shear slip along pre-existing fractures, risking induced seismicity. Understanding the temperature-dependent cyclic shear behavior of reservoir fractures is critical for mitigating seismic hazards. In this study, cyclic shear tests were conducted on thermally treated sandstone fractures under constant normal stiffness (CNS) conditions. The effects of preheating temperature and shear cycles on the shear strength, shear stiffness, and frictional and dilatancy properties were comprehensively investigated. The test results reveal that the cyclic shear behavior of sandstone fractures exhibits a high temperature dependence. Under CNS conditions, the forward shear phase exhibits significant hardening. Key mechanical parameters evolve in two stages with temperature, showing a critical temperature of 400 °C. In contrast, the reverse shear phase exhibits reduced temperature sensitivity with a broader critical temperature range of 300 °C–600 °C. The shear strength and dilatancy parameters of sandstone fractures exhibit significant cyclic degradation, which is markedly accelerated by thermal treatment, especially when temperatures exceed 400 °C. With increasing shear cycles, the shear mechanism of sandstone fractures shifts from cohesive- to friction-dominated, characterized by a cyclically strengthening friction coefficient and degrading apparent cohesion. Acoustic emission (AE) tests reveal pronounced nonlinearity and temperature dependence in the damage evolution of sandstone fractures during cyclic shearing. Elevated preheating temperatures greatly reduce the decay rate of damage increment per cycle and promote the development of dominant damage across multiple cycles. This research provides an in-depth insight into understanding the thermo-mechanical coupling-dominated shear behavior of rock fractures in deep geothermal reservoirs.
Zhang et al. (Sun,) studied this question.