Mechanical properties and failure characteristics of granite treated with a combined water–air cooling cycle

Granite is the main material used to enhance geothermal reservoirs, and its mechanical properties play a key role in the safe and efficient development of geothermal materials. High temperature and cold shock issues are often encountered during the process of geothermal development. For this reason, numerous studies have been conducted on the mechanical properties of granite treated at different temperatures and with various cooling methods. In addition, granite is often affected by combined water-air cooling, which affects the development direction and stability of reservoir heat transfer channels. However, few studies on the mechanical properties of granite treated with combined water-air cooling have been conducted thus far. In this study, uniaxial compressive tests were conducted on granite to study the effect of localized water cooling at different temperatures and numbers of heating-cooling cycles. The results show that the thermal cracks in granite subjected to combined cooling treatments exhibited regional distribution characteristics with temperature and cycle times; there were no apparent thermal cracks at 200 °C, thermal cracks were mainly distributed in the water-cooling zone between 300 and 400 °C, and thermal cracks were distributed throughout the granite specimens above 500 °C as the number of cycles increased. Within 300 °C, the peak axial stress and Young's modulus of the granite exhibited slow linear decreasing trends with temperature and cycle times, but the peak axial stress and Young's modulus exhibited nonlinear decreasing characteristics when the temperature exceeded 400 °C. The failure mode gradually changed from brittleness to ductility. Above 500 °C, thermal cracks near the interface of the water-cooling and air-cooling zones became denser than those in the other two areas (the water-cooling and air-cooling zones) as the number of cycles increased, and the splitting failure during loading failure shifted from the water-cooling zone to the interface of water cooling and air cooling. The temperature condition for fracture and peeling between the air-cooling and water-cooling zones of the granite was 600 °C.