Variations in Stress Thresholds for Heated Granite Subjected to Rapid Cooling under Different Confining Pressures

Liquid nitrogen has been proposed to promote fracture networks generated by hydraulic fracturing in geothermal reservoirs. To compare the deterioration in mechanical properties due to water and liquid nitrogen cooling under conditions of high temperatures and confining pressures, three stress thresholds (crack initiation stress, crack damage stress, and peak stress) of granite were studied by performing triaxial compression tests (under 0, 20, 40, 50, and 60 MPa) after heating and rapid cooling. The relationships between the three stress thresholds and the confining pressures were analyzed. The effects of different cooling treatments on the stress thresholds were analyzed under different confining pressures. The thermal damage mechanism and confining pressure effect were revealed. For both rapid cooling treatments, the three stress thresholds showed convex nonlinear relationships with the confining pressure. The temperature sensitivity on the stress thresholds can be weakened by confining pressure. Under low confining pressures (< 40 MPa), the three stress thresholds of samples cooled by liquid nitrogen were higher than those by water, and the differences were suppressed at higher confining pressures. The higher mechanical properties observed under liquid nitrogen were due to the heat transfer restriction by the Leidenfrost effect and the crack closure induced by volumetric contraction. A higher thermal expansion heterogeneity contributed to the complexity of cracks induced by liquid nitrogen. Under high confining pressures (> 40 MPa), there was a trend that the stress thresholds of samples cooled by liquid nitrogen were lower than those by water, which was related to cohesion.