Mechanical Behaviors of Reservoir Sandstone Under Coupled High Temperature and True Triaxial Stress Conditions: A Discrete Element Modeling Study

In geothermal development, deep reservoir rocks are usually subjected to thermal shock induced by cold‐fluid injection and true triaxial stress state ( σ 1 > σ 2 > σ 3 ), leading to complex strength, crack‐propagation, and brittleness evolution mechanisms. However, a unified three‐dimensional analytical framework for this coupled response remains insufficient. In this study, a three‐dimensional thermo‐mechanically coupled grain‐based model (3D TM‐GBM) was constructed based on the mineral composition of sandstone, considering its mineral heterogeneity. After conventional triaxial calibration, the model was extended to true triaxial compression simulations. The results show that during slow heating, thermal cracks are dominated by intergranular cracking, whereas water cooling shifts the dominant damage mode to intragranular cracking. Under true triaxial compression, increasing σ 3 significantly increases the rock strength, whereas σ 2 exhibits a nonlinear effect characterized by initial strengthening followed by weakening and promotes stress‐induced anisotropic failure. Moreover, increasing treatment temperature enhances the relative dominance of tensile cracks, while increases in σ 2 and σ 3 promote grain‐boundary shear slip, resulting in the transition from tensile‐dominated to mixed tensile‐shear failure. Additionally, the brittleness evolution of sandstone is jointly controlled by temperature and stress conditions. The rock brittleness decreases with increasing σ 3 , increases before stabilizing with increasing σ 2 , and generally decreases with treatment temperature under true triaxial conditions. These findings inform laboratory‐scale assessments of intact, dry sandstone strength and brittleness, with potential applications to geothermal reservoir engineering.