Mechanical properties of sandstone under in-situ high-temperature and confinement conditions

Low to medium maturity oil shale resources boast substantial reserves, offering promising prospects for in-situ conversion in China. It is of great significance to investigate the evolution of the mechanical properties of reservoir and caprock under in-situ high-temperature and confinement conditions. Compared to conventional mechanical experiments on rock samples after high-temperature treatment, in-situ high-temperature experiments can more accurately characterize the behavior of rocks in practical engineering, thereby providing a more realistic reflection of their mechanical properties. Therefore, an in-situ high temperature triaxial compression testing machine is developed for conducting in-situ compression tests on sandstone under different temperatures (25, 200, 400, 500, and 650℃) and different confining pressures (0, 10, and 20 MPa). Based on the experimental results, the evolution characteristics of compressive strength, peak strain, elastic modulus, Poisson’s ratio, cohesion, and internal friction angle with temperature are deeply analyzed and discussed. It is observed that the mass of sandstone gradually decreases with increasing temperature. The thermal conductivity and thermal diffusivity of sandstone exhibit a linear relationship with temperature. Peak stress decreases with rising temperature and increases with higher confining pressure. Notably, the higher the temperature, the worse the effect of the confining pressure on the peak stress. Additionally, the Poisson’s ratio of sandstone decreases as temperature rises. The internal friction angle also decreases with increasing temperature, with 400°C serving as the threshold temperature. Interestingly, under uniaxial conditions, the damage stress of sandstone is less affected by temperature. The damage stress decreases with increasing temperature when the confining pressure is 10 and 20 MPa. This study enhances our understanding of how the mechanical properties of sandstone strata change under in-situ high-temperature and confinement conditions. It provides valuable references and experimental data that support the development of low to medium maturity oil shale resources.