Study on Fracture Surface Damage and Fluid Flow Characteristics of Hot Dry Rock with Different Failure Modes under Repeated Thermal Shock

In the process of geothermal energy exploitation, the fracture surface of hot dry rocks (HDRs) is easily damaged by repeated injection of cold water. To reveal the damage and flow characteristics of granite fracture surfaces with shear and tension, Brazilian split tests, shear tests, three-dimensional laser scanning tests, repeated thermal shock tests, and fracture surface fluid flow tests are carried out in this study. The fracture damage of the samples is characterized by using the average roughness Ra, the root means square roughness Rrms, the fractal dimension of the fracture surface, the fractal dimension of the gap of the fracture surface, and the three-dimensional fracture surface pictures. The fluid flow characteristics of the fracture surface are characterized by the hydraulic aperture and Reynolds number. The results show that the repeated thermal shock test at 500 celcius has the greatest influence on the fracture morphology. The variation value of the roughness coefficient of the tensile fracture is 0.0772 and that of the shear fracture is 0.0485. However, the influence of heat treatment on fluid flow in tension fractures is greater than that in shear fractures. This shows that the fluid flow in the fracture is related not only to the heat treatment but also to the fluid flow channel. Under repeated thermal shock, the section damage of tensile fractures is larger than that of shear fractures.Using the fractal dimension of the section surface and the fractal dimension of the void to reveal the fluid flow characteristics of tensile fracture and shear fracture after thermal shock.After repeated thermal shock, the fluid flow characteristics of the fractures are more significant as the change value of void fractal dimension increases.