Mechanical properties and damage evolution of granite under high temperature thermal shock

Hot dry rock (HDR) is an underground rock with high temperatures (usually above 180 °C), low porosity, and low permeability. The extraction of geothermal energy from HDR generally requires the stimulation of man-made reservoirs. In the enhanced geothermal system (EGS) project, high-pressure water is usually injected into the deep HDR reservoir from the injection well, and the artificial fracture network is stimulated via fracking. The ultimate goal is to enhance fluid flow and heat exchange between injection and production wells. During this period, thermal shock induced by the injected cold water, also known as thermal stimulation, leads to thermal fracture of the HDR, which contributes to the formation of fractures near the injection well. However, this process results in a series of rock damage problems to the high-temperature rock mass, such as borehole collapse and microseismicity. To analyze the mechanical properties and damage evolution of high-temperature granite after thermal shock, the uniaxial compression test of granite specimens at different temperatures in the range of 25 °C–600 ℃ was conducted, and the stress–strain relationship of the specimens was obtained. Based on the theory of damage mechanics, a thermal–mechanical coupled damage constitutive model considering the combination of the initial thermal shock damage and the microelement fracture damage during loading was proposed, and the relevant parameters of the statistical damage constitutive model were theoretically solved. Furthermore, given the effect of pore structure deterioration caused by thermal shock, the constitutive relationship of thermal shock granite was modified by introducing a compaction coefficient. The statistical damage constitutive model was also verified by the experimental results. The influence of temperature on the damage evolution of thermal shock granite under uniaxial compression was discussed. Results showed that with the increase in thermal shock temperature, the initial thermal damage of the granite specimen increases continuously, resulting in a nonlinear compaction stage in the stress–strain curve. The statistical damage constitutive model modified by the compaction coefficient can accurately characterize the nonlinear compaction characteristics of thermal shock granite specimens in the initial loading stage. When the thermal shock temperature is low, the damage variable evolution curve rises steeply. However, with the increase in the thermal shock temperature, the increase rate of the curve gradually slows down and changes from nonlinear to linear. The research results not only help elucidate the deterioration process of the mechanical properties of thermal shock granite but also provide important theoretical guidance for the construction of accurate numerical calculation models and engineering scheme demonstrations.