Abstract: During hydraulic fracturing in hot dry rock, elevated temperatures weaken the fracture surface strength of the thermal reservoir rock, resulting in fracture slip behavior. This weakening phenomenon is closely related to thermal variations in cohesion and internal friction angle. Therefore, understanding the mechanical properties of granite at high temperatures is crucial for the stability assessment of thermal reservoir rocks during hot dry rock development. Variable angle shear tests (45°, 55°, 65°) were conducted on coarse and fine-grained granite under different high-temperature conditions. The study analyzed the impact of elevated temperatures on the cohesion and internal friction angle of granite with different particle sizes. In conjunction with scanning electron microscopy and low-field nuclear magnetic resonance tests, a further analysis was conducted on the influence of thermo-mechanical coupling on the evolution of microcracks and pore structures in granite. The results indicate that the cohesion of granite initially increases and then decreases with the increase of temperature. The variation in the internal friction angle is minimally influenced by temperature. The threshold temperature for the transformation of granite cohesion is identified as 300 ℃. Prior to the threshold temperature, the cohesion increase in coarse-grained granite is approximately 5.73 times greater than in fine-grained granite, and the effect of temperature strengthening is more significant on coarse-grained granite. As the temperature increases, the form of microcracks in granite transitions from intragranular cracks to intergranular cracks. The nuclear magnetic resonance (NMR) T₂ spectrum exhibits multiple discontinuous peaks. With increasing temperature, compaction affects the large pores in coarse-grained granite, leading to a reduction in relaxation effects. Conversely, thermo-mechanical coupling has a limited effect on the development of large pores in fine-grained granite. These findings are expected to provide valuable insights for assessing the stability of reservoir rocks in hot dry rock geothermal energy development.

Key words: real-time high temperature, granite with different grain sizes, variable angle shear test, scanning electron microscopy, nuclear magnetic resonance