The fracture network of a deep geothermal reservoir forms the place for heat exchange between injected fluid and rock mass with high temperature. The fracture resistance ability of reservoir rocks will affect the formation of fracture-network structure, heat exchange and transmission characteristics, and reservoir mechanical stability. However, there are few reports on the fracture toughness and trajectory prediction of geothermal reservoirs with different depths. In this paper, the modified maximum tangential stress criterion (MMTS) is analyzed. The results show that the experimental data are significantly different from the theoretical estimate of MMTS under the influence of different occurrence depths. It is found that the fracture process zone (FPZ) seriously affects the accuracy of predicting fracture initiation angle and mixed-mode (I+II) fracture toughness by MMTS. The FPZ value, considering the influence of different occurrence depths, is modified, and the accuracy of MMTS in predicting the fracture mechanical characteristics of granite is improved. In addition, the mechanical test results show that the Brazilian splitting strength (σ_t) of granite fluctuates increase with the increase in temperature. With the increase in deviatoric stress, the Brazilian splitting strength and the Brazilian splitting modulus of rock show a trend of first increasing, then decreasing, and then increasing.

In the geothermal development of hot dry rock (HDR), both the drilling of the wellbore and the heat exchange of the heat reservoir involve the effects of different cold and hot conditions on the high-temperature rock mass. The testing machine for rock mechanics was used to conduct a uniaxial compression test and carry out micro testing on the treated samples; furthermore, with the help of scanning electron microscopy the fracture mechanism of granite subjected to different temperatures and cooling methods was studied. The results show: (1) With the gradual increase in temperature, the compressive strength of granite under the two cooling methods gradually decreases. (2) The failure modes of the samples under the two cooling methods are mainly shear failure of the "Y" type. The degree of damage of the sample under water cooling is significantly greater than that under natural cooling. Electron micrographs could confirm these results. (3)It can be obtained by testing the mineral composition and element changes of granite at different temperatures. When the temperature reaches 600℃, its change is more pronounced. The results of this study can provide a theoretical reference for the failure of the wellbore and the degree of fracture of the thermal reservoir rock mass during geothermal development.

In this paper, a self-developed multi-functional high-temperature rock triaxial servo control testing machine was used to carry out uniaxial compression tests on the granite after the cooling and heating cycles under real-time temperature. The physical and mechanical properties of two types of granite damaged by hot and cold cycling under real-time temperature were discussed, and the changes in apparent color, longitudinal wave velocity, elastic modulus, uniaxial compressive strength, and damage characteristics of the specimen were revealed. The research results show the following: (1) With the increase in temperature or the increase in number of cycles, the uniaxial compressive strength, longitudinal wave velocity, and elastic modulus of the samples under the two cooling methods all show a decreasing trend, but the decrease in the range is different. The change range of the sample with temperature is greater than that with the number of cycles. (2) Under the dual action of real-time temperature and cold heat cycle damage, the failure form of granite is very random, but it is mainly shear failure, longitudinal splitting failure, and conical failure, and it is accompanied by a high temperature with the increase in the number of cycles, and the degree of crushing of the test piece gradually increases. For example, the sample under 600 °C water cooling for 25 cycles is crushed and destroyed. (3) As the temperature and the number of cycles increase, the surface of the water-cooled sample becomes rougher with the increase in the temperature and the number of cycles and the higher temperature, along with more cracks and debris; the increase in the temperature cycle, no obvious cracks appeared on the surface. The test results in this paper can provide relevant theoretical guidance for the stability and safety of rock in geothermal mining.