Active Pressure Management for Seasonal Aquifer Gas Storage: From Artificial Gas-Cap Establishment to Cyclic Injection–Production

Seasonal balancing of energy supply and demand in China is constrained by the limited capacity of existing salt-cavern and depleted-reservoir storage facilities. To evaluate aquifer storage as a supplementary option, this work investigates the Baiju coastal aquifer using a CMG-GEM compositional model and examines seasonal multi-cycle operation under active pressure management. Sensitivity analyses were carried out for the artificial gas-cap construction period, brine-well bottomhole pressure, daily gas discharge rate, vertical-to-horizontal permeability ratio, and cushion-gas composition (N2, CH4, and CO2), with emphasis on injection–discharge performance, pressure response, and discharged-gas purity. The simulations show that extending cushion-gas injection moderates pressure oscillation during subsequent cycling, and the working-gas mole fraction in discharged gas rises above 97% by the fifth cycle, while values varied across the tested operating and cushion-gas scenarios. Within the fixed eight-well configuration, lowering the bottomhole pressure of brine-discharge wells was associated with higher injectivity, although the incremental benefit became limited once the pressure dropped below the initial reservoir pressure. Within the tested range, higher daily discharge rates were associated with lower capacity-expansion efficiency; the magnitude depends on the metric normalization. A lower vertical permeability mainly constrains gas injection, while its influence on gas discharge is comparatively minor. Among the candidate cushion gases, CH4 provides the largest gas-cap volume and CO2 provides the strongest damping of pressure disturbance; considering both economics and storage performance, N2 provides the best overall compromise because its working-gas injection and discharge volumes are close to those obtained with CH4.