Experimental Study on Triaxial Mechanical Properties of Deep Carbonate Rocks Under Thermo-Hydro-Mechanical Coupling

Global oil and gas exploration and development are gradually expanding into deep and ultra-deep formations. Deep limestone exists in a long-term multi-field coupled environment featuring high temperature, high in situ stress and high pore pressure, which brings great challenges to reservoir stimulation and wellbore stability. To investigate the effects of confining pressure and pore pressure on limestone under high temperatures, triaxial compression tests were conducted on limestone at various temperatures (25~150 °C) using the GCTS RTR-2000 rock mechanics testing system. This paper investigates the evolution laws of strength and deformation parameters of limestone under varied temperature, confining pressure and pore pressure. The results indicate that: (1) Within the 25~150 °C range, the peak strength and elastic modulus of limestone exhibit a “decrease-then-increase” trend, with a strength rebound occurring at 150 °C driven by the “thermal expansion and compaction” effect. (2) Under a pore pressure of 50 MPa, temperature and confining pressure exert a significant coupled control effect on the mechanical properties of the rock, characterized by a critical confining pressure threshold of approximately 100–110 MPa. Below this threshold, high temperature acts as a weakening factor, whereas above it, high temperature acts as a strengthening factor and induces intense brittle failure under high pressure. (3) In the pore pressure coupling tests, the rock undergoes ductile failure as confining pressure increases at normal/room temperature, while a temperature of 100 °C strengthens the rock under high confining pressure. (4) Energy evolution analysis reveals that within the 75~125 °C range, the thermal pressurization of pore water and local thermal stresses induce massive microcracks, causing the dissipated energy to surge sharply to nearly 80%. The research findings provide a theoretical basis for wellbore stability analysis and fracturing parameter optimization in deep carbonate reservoirs.