Experimental Study on Reservoir Damage Mechanisms of Depleted Gas Reservoirs Considering Variable Pressure Depletion Rates During Multi-Cycle Injection and Production

The long-term operational reliability of underground gas storage (UGS) facilities in depleted reservoirs is significantly challenged by reservoir damage during multi-cycle injection and production (I&P). While the impact of cycle numbers has been extensively studied, the influence of variable pressure depletion rates remains insufficiently quantified. This study investigates the reservoir damage mechanisms of sandstone cores from the Sichuan Basin under different depletion rates (0.5 and 2.5 MPa/min) over 20 I&P cycles. Experimental results indicate that the pressure depletion rate is a decisive factor in permeability impairment. For the sample subjected to a fast depletion rate (2.5 MPa/min), the total permeability loss reached 18.2%, which is 2.16 times higher than that of the slow-rate sample (8.4% at 0.5 MPa/min). Notably, the high-rate sample sustained nearly 60% of its total damage within the initial three cycles, highlighting a critical window of vulnerability during early UGS operations. Theoretical hydrodynamic analysis suggests that at 2.5 MPa/min, the instantaneous shear force (6.42 nN) exceeds the representative adhesion force of clay minerals (~5.0 nN), which may increase the likelihood of clay mobilization under the present experimental conditions. Combined with the XRD-identified clay content and the observed permeability evolution, the damage is interpreted as being likely associated with fines migration and pore-throat plugging. Based on these findings, a “Slow-Start” operational protocol—maintaining depletion rates below 1.0 MPa/min during the initial cycles—is preliminarily recommended under the present experimental conditions to help preserve reservoir conductivity and extend facility longevity.