Dynamics of CO ₂ -Brine Mass Transfer during Cyclic Injection and Production Operations in Underground Gas Storage Reservoirs

In underground gas storage systems utilizing CO2 as a cushion gas, pressure fluctuations arising from cyclic injection–production operations significantly influence the CO2 dissolution process. However, the impact of pressure fluctuations on CO2 convective dissolution remains underexplored. To address this gap, a numerical model integrating periodic pressure-driven solubility variations based on linear stability analysis was developed. Using the finite volume method, century-long simulations spanning 100 years with annual pressure cycles were conducted to investigate CO2 migration dynamics in saline aquifers. Based on the simulation results, fitting formulas were proposed to characterize the CO2 dissolution behavior under various pressure fluctuation scenarios. The findings demonstrate that compared to pressure maintained at the average pressure, the fluctuations significantly accelerate CO2 convective dissolution by enhancing peak concentrations at the gas–brine interface. This effect manifests as a reduction in the onset time of convection by up to 40%, an increase in the fingering evolution distance by approximately 45%, and an enhancement of the lateral diffusion rate by about 35%, all following a power-law relationship. The study further reveals that fingering velocity is predominantly governed by the mean concentration, which in turn dictates that the long-term dissolution flux is primarily controlled by the time-averaged concentration. Overall, periodic pressure fluctuations increase the CO2 convective dissolution rate by approximately 8–12%. These findings provide a critical theoretical foundation for optimizing the design of natural gas storage reservoirs utilizing CO2 as a cushion gas.