Experimental study on physico-mechanical responses and energy characteristics of granite under high temperature and hydro-mechanical coupling

Thermal and hydro-mechanical coupling effects on granite's physico-mechanical responses and energy characteristics can influence its carrying capacity, hydraulic and heat transfer performance. The evolution of these properties is crucial for geothermal reservoir stability assessment and the optimization of deep geothermal energy development techniques. Hence, a variety of physical tests and rock mechanics experiments were conducted. Key findings include: (1) As temperature rises, granite undergoes thermal expansion and structural integrity degradation. The peak strength σ_p, elastic modulus E, total energy density U, and elastic energy density U_e initially rise at 25 °C–150 °C and subsequently decrease above 150 °C, while the proportion of dissipated energy η_d is opposite. Granite also initially experiences hardening, then turns to brittle-ductile transition. (2) The crystallinities of quartz, albite, and orthoclase demonstrate substantial deterioration beyond 450 °C. (3) 150 °C and 450 °C are regarded as the temperature thresholds for mechanical properties. (4) As confining pressure rises, granite experiences hardening, with σ_p, E, U, and U_e increasing, and η_d decreasing. (5) Pore water pressure increases η_d and decreases σ_p, E, U, and U_e, and its effect on the mechanical responses is pronounced when it reaches 80 % of confining pressure or exceeds 450 °C.