Experimental study on the physico-mechanical properties and temperature field evolution of granite subjected to different heating–cooling treatments

Rock cracking caused by thermal shocking occurs widely in geothermal exploitation processes, such as wellbore drilling, reservoir stimulation, and long-term geothermal extraction. In this study, the permeability, P-wave velocity, uniaxial compressive strength, and tensile strength of granite subjected to heating (100 ~ 300℃) and cooling in water with different temperatures (0 ~ 60℃) were experimentally investigated. Moreover, the evolution of temperature field, cooling rate, and temperature gradient inside the samples during rapid cooling were tested. The results show that a decrease in the temperature of the cooling water aggravates the deterioration of the physical and mechanical properties of the granite after rapid cooling; this deterioration is specifically manifested in the increase in permeability and the decrease in P-wave velocity and mechanical strength of the granite samples. The heat transfer mechanism of the high-temperature granite under rapid cooling is characterized by unsteady heat conduction. The evolution of the internal temperature, cooling rate, and temperature gradient of the granite samples with time can be divided into three stages: the first 200s is the rapid cooling stage; 200s ~ 400s is the slow cooling stage; 400s ~ 600s is the temperature basically constant stage. The evolution of the cooling rate and temperature gradient formed in the high-temperature granite specimens under rapid cooling is consistent, and they all show a trend of rapid increase first followed by a slow decrease. The maximum values of the cooling rate and temperature gradient always appear in the area near the outer surface, which is closest to the solid–liquid heat exchange interface. This study has potential guiding significance for the application of rapid cooling in the development of enhanced geothermal system.