Evaluation of Rock Mechanical Properties and Production Pressure Differential in Underground Gas Storage Under Multi-Cycle Injection/Production Conditions

Under the dual challenges of energy supply demand imbalance and the efficient operation of underground gas storage (UGS) facilities, this study investigated the mechanical behavior of reservoir rocks and optimal production pressure differential in a depleted gas reservoir in China under multi-cycle injection-production. For the first time, we reveal the mechanical degradation mechanism of hydration and cyclic fatigue for three typical lithologies in depleted sandstone reservoirs. Rock mechanics tests were conducted to analyze the effects of lithology, water saturation, and cyclic loading on mechanical properties, and appropriate failure criteria were evaluated. The main findings are as follows: (1) Under a confining pressure of 45 MPa, the peak strength of fine sandstone was the highest at 160.13 MPa, and the peak strength of argillaceous sandstone was the lowest at 114.92 MPa. The strength increased approximately linearly with confining pressure. (2) Increasing water saturation significantly weakened rock strength, particularly in argillaceous sandstone due to hydration effects. At 45% water saturation, its strength decreased by 37.38%. while Young’s modulus and Poisson’s ratio remained relatively unaffected. (3) Rock strength progressively degraded with the number of loading cycles. Siltstone showed the most significant degradation, with a strength reduction of 28.50% after 200 cycles. The damage induced by cyclic loading was less severe than that caused by hydration. (4) Among five failure criteria evaluated, the Mogi–Coulomb criterion demonstrated superior predictive capability by incorporating three-dimensional principal stress effects, showing closest agreement with the experimental data. We further established a depth-dependent production pressure differential profile and proposed a lithology-specific injection-production strategy. These findings provide theoretical foundations for optimizing injection-production strategies and sand control measures in depleted reservoir UGS systems.