Mechanical behaviors of carbonate rocks in ultra-deep reservoirs under real-time high-temperature and high-stress conditions

In 2024, the "Deep Earth Tarco-1" well exceeded 10,000 m, thereby establishing a new benchmark for drilling depth in Asian. However, the mechanical behavior of rock formations at such depths remains poorly understood. In this study, triaxial compression tests were conducted on carbonate rock specimens from the Tarim Oilfield in China under ultra-high temperature (200 °C) and ultra-high confining pressure (180 MPa) (UTUP) conditions, using a self-developed multi-field coupling experimental system. The findings revealed variations in strength, deformation, and fracture characteristics of carbonate rock specimens under such extreme conditions, which offer valuable data for the exploitation of ultra-deep oil and gas reservoirs. Additionally, computed tomography (CT) was employed to analyze the internal fracture network distribution patterns of rock specimens under different temperature and confining pressure conditions. The experimental results under the coupling effects of high temperature and high stress showed that the strength of the rock increases nonlinearly with an increase in confining pressure but decreases with rising temperature. Furthermore, the deformation of the rock exhibited a distinct brittle-ductile transition, and the fracture morphology of the rock evolved from single fractures to complex fracture networks. A power-law Mohr–Coulomb strength criterion considering temperature dependence was proposed to describe the nonlinear strength evolution of rocks under UTUP conditions. It shows that the strength envelope expands nonlinearly with increasing stress but shrinks with increasing temperature. The criterion was validated experimentally to demonstrate that it effectively captures the failure strength characteristics of rocks in ultra-deep reservoirs, providing a crucial tool for the design and safety assessment of ultra-deep oil and gas reservoir development.