Fluid Mobilization Mechanisms in Water-Invaded Gas-Reservoir Underground Gas Storage under Cyclic Injection-Withdrawal

Water-invaded underground gas-storage (UGS) reservoirs face challenges, including complex fluid mobilization and capillary trapping under cyclic injection-withdrawal. Here, we propose a microscopic two-phase pore-scale simulation method to elucidate these mechanisms. Cyclic gas injection and withdrawal induce increasingly complex gas–liquid flow and spatial distribution, complicating accurate evaluation of storage capacity and posing challenges for accurately determining storage capacity parameters. The study employs pore-scale simulations to investigate the dynamic mobilization mechanisms of gas and water in two key regions: the gas-drive expansion zone and the gas–water transition zone. In the gas-drive expansion zone, rapid cyclic gas injection and withdrawal mobilize residual liquids by expanding gas interfaces, displacing water into the main flow channels. Trapped water droplets in fine pores gradually coalesce and migrate along the pore walls, increasing accessible gas-storage space. In the gas–water transition zone, high-shear interactions between gas and water induce the formation of gas–water interlocking foams within narrow throats. This increases the flow resistance and inhibits complete gas recovery. Based on these insights, a theoretical model describing the spatial mobilization of fluids in water-injected UGS was developed. This model provides a reliable foundation for evaluating key storage performance indicators and optimizing operational strategies. Applied to a representative UGS field, the model yields a calculated maximum mobile inventory of 33.2 × 108 m3, closely matching the measured value of 35.1 × 108 m3 (94.6% accuracy). These results demonstrate the strong applicability of the model and provide guidance for the precision design and efficient operation of similar gas-storage systems in China.