Experimental Insights into Storage Capacity Enhancement and Seepage Mechanisms during Multicycle Injection–Production in Oil-Reservoir-Converted Underground Gas Storage

High-water-cut oil reservoirs are considered promising candidates for underground gas storage (UGS); however, the multiphase flow mechanisms during UGS construction in such reservoirs are inadequately understood. To address this gap, an experimental method is developed to simulate the multicycle injection-production process of UGS under high temperature and pressure conditions, facilitating the conversion of high-water-cut oil reservoirs into UGS. Additionally, a gas flooding experiment under high temperature and pressure is conducted to demonstrate the advantages of reservoir conversion in enhancing recovery rates, with a comparative analysis of oil displacement efficiency between two experimental groups. To quantify the process of displacing fluids and creating pore space for gas storage, the concept of liquid displacement efficiency is introduced. The results show that oil displacement efficiency increases progressively with cycles, peaking at Cycle 3, suggesting that initial cycles effectively mobilize residual oil and clear pore space, after which efficiency declines due to reduced movable oil. Continuous gas injection displaces significant liquid volumes with the volume of liquid displaced during gas injection phases exceeding the produced liquid volume by 26.3%. Gas relative permeability increased most significantly in Cycle 6, indicating a progressive improvement in gas flow capability. Compared with single-cycle gas flooding, six injection-production cycles enhance oil and liquid displacement efficiencies by 14.7% and 16.2%, respectively. These findings confirm that multicycle injection-production strategies not only enhance hydrocarbon recovery but also demonstrate the engineering feasibility of converting high-water-cut reservoirs into large-scale UGS.