Numerical study on micro-cracking behavior and damage model for granite during thermal-cooling cycles

Deep energy exploitation involves heat exchange between fluids and rocks, where temperature variations can induce rock damage and lead to engineering challenges. In this study, a numerical rock core was established using a discrete element method (DEM)-based particle flow model to analyze the effects of mineral composition, heterogeneity, and particle size on microcrack development and mechanical behavior under heating–cooling thermal cycles. When the thermal heating temperature rises, thermal cracks evolve from intergranular tensile cracks to intragranular shear cracks within quartz grains. During cooling, intergranular tensile cracks are predominantly generated. Compared to heterogeneity and particle size, quartz content was the more important factor to affect the damage in mechanical property of rock after thermal cycles. Rocks with 70 % quartz content exhibit reductions in compressive strength and elastic modulus by 97.3 % and 98.2 %, respectively, after two cycles with cyclic temperature of 600 °C without confining, while when the confining pressure increased to be 120  MPa, rock compressive strength and elastic modulus reduce by 34.1 % and 30.9 %, respectively. Confining pressure can suppress crack numbers while the ratio of shear to tensile cracks increases. In this study, the Weibull function was verified to effectively characterize the evolution of rock thermal damage with respect to the maximum volumetric thermal strain. Confining pressure has a more significant influence on the damage model parameters than the microstructural factors. Our study results can provide theoretical support for rock mechanical property prediction after irregular thermal cycles damage, which is the basis for the safety analysis for engineering applications such as deep well drilling and geothermal energy exploitation.