Experimental studies on physical and mechanical behaviors of heated rocks with pre-fabricated hole exposed to different cooling rates

In enhanced geothermal systems, thermal shock occurs around the borehole when cold water is injected into the hot rock of a geothermal reservoir. Experiments were performed on granite and sandstone specimens to simulate and investigate the effects of temperature and cooling rate using five temperature gradients and two cooling methods. Scanning electron microscopy was applied to examine the microstructural changes after thermal treatment, and a series of uniaxial compress tests were conducted on rock specimens containing a simulated borehole. The digital image correlation (DIC) method was applied to analyze the crack evolution during loading. The test results show that the thermal damage induced by the propagation of both inter- and intra-granular cracks destroy the rock microstructural integrity, and that cold water breaks the bond between grains and further widens thermal-induced cracks. When the temperature increases from 20 to 800 °C, the physical and mechanical properties of the two investigated rock types are both weakened, including the density, P-wave velocity, and compression strength. Rapid water cooling more significantly affects the rock mechanical properties than slow cooling owing to the intense thermal shock. The presence of thermal-induced cracks at high temperature significantly influences the rock homogeneity and elastic modulus, and the quenched specimens showed less brittle characteristics at high temperature. The DIC analysis indicates that thermal-induced cracks alter the local stress concentration around the simulated borehole and increase the unstable crack propagation regions owing to stress redistribution. Article highlights The evolution of thermal-induced micro-cracks at high temperature weakens the physical-mechanical properties of rocks. Greater damage is caused to quenched rocks by thermal shock and water weakening than slow cooling. The DIC technique is applied to study the deformation behavior around the borehole after temperature treatment.