A comprehensive model for dynamic prediction of formation pressure during alternate injection of gas storage

In the underground gas storage (UGS) facilities, formation pressure is a critical factor influencing both injection-production capacity and sand production prevention. Current prediction methods predominantly rely on numerical simulations and theoretical derivations. However, the existing theoretical framework for predicting formation pressure during the injection process in UGS remains limited. To address this gap, this study aims to develop a novel theoretical methodology for predicting formation pressure. The methodology begins with establishing the fundamental material balance equation for UGS, followed by deriving formation volume variation through comparative analysis of pore volume changes before and after gas injection. A mathematical model is developed to describe the correlation between natural gas inventory and formation pressure, which is validated against the classical material balance equation for the production process. Finally, application of this model to a representative UGS at X Oilfield in the Bohai Bay Basin demonstrates that formation pressure accelerates with increasing injection rates. During the 200-day alternate injection period, the formation pressure progressively rises 8% increment, from 19.75 to 21.33 MPa. The pressure differential in the alternate injection period between the bottomhole pressure and the formation pressure ranges from 0.07 to 4.05 MPa with an average differential of approximately 2.49 MPa. Model verification reveals over 91% accuracy in matching actual formation pressure. This study establishes that the injection-period material balance model provides an effective theoretical framework for UGS pressure prediction.