Shear mechanical properties and surface damage characteristics of granite fractures treated by real-time high temperature

In recent years, granite fractures have been considered an effective research object to improve the permeability of enhanced geothermal system (EGS). Granite fractures are always in a high-temperature environment, and their shear behavior treated by real-time high temperature is the key to promoting EGS permeability. Hence, the granite fractures shear tests under real-time high temperature were conducted with MTS 816 rock testing machine with a high-temperature sealed shear box. The results indicate that fracture peak shear strength and shear stiffness show a continuous deterioration trend as the temperature increases, which can be classified into a slowly declining stage (30–200 °C) and a rapidly declining stage (200–400 °C). High temperature deteriorates the asperity shear failure on granite fracture surface, which leads to the reduction of the dilatancy deformation of fracture surface with increasing temperature, and its decreasing amplitude becomes larger with increasing normal stress. Coupling synergy between temperature and normal stress can exacerbate shear damage on granite fracture surfaces, and the percentage of shear damage area increases by 21.41% on granite fracture surface as temperature and normal stress increases from 10 MPa and 30 °C to 30 MPa and 400 °C. High temperature aggravates the damage of asperities, which is the principal cause of the degradation of fracture surface roughness with increasing temperature. The research results can provide a theoretical guide for enhancing reservoir permeability and connectivity in geothermal energy development engineering.