Experimental Study on the Mechanical Damage and Permeability Evolution of Tight Sandstone Reservoir Under Triaxial Loading

This study systematically investigates the evolution of mechanical damage and the permeability response of tight sandstone under triaxial compression and alternating load conditions, with a focus on the safety and stability of deep underground tight sandstone gas storage reservoirs in China subjected to complex geological environments and alternating stress conditions. By integrating conventional triaxial testing, cyclic loading experiments, CT scanning, and fractal dimension analysis, this study elucidates the enhancement effects and transformation mechanisms of confining pressure on the strength behavior and failure patterns of sandstone. It identifies the influence mechanisms of fault roughness on permeability and its convergence behavior under high-stress conditions and comprehensively characterizes the three-stage evolution of sandstone damage at the microscale under cyclic loading. Experimental results showed that with increasing confining pressure, both the peak strength and elastic modulus of sandstone displayed an increasing trend. With confining pressure increasing from 10 MPa to 40 MPa, the peak deviatoric stress increased from 98.42 MPa to 171.00 MPa and the elastic modulus rose from 8.70 GPa to 12.65 GPa. The failure mode transitioned from brittle shear failure under low confining pressure to a ductile-plastic failure pattern under high confining pressure. Alternating loading resulted in a 17.23% reduction in sandstone strength (from 98.42 MPa to 81.46 MPa at 10 MPa confining pressure). At confining pressures > 25 MPa, the permeability differences among faults with different roughness converged to within 10%. These research findings offer a robust experimental foundation and theoretical framework for evaluating the long-term stability and predicting the sealing performance of deep underground gas storage reservoirs.