A Novel Method for Calculating Average Formation Pressure of Gas‐Reservoir‐Type Underground Natural Gas Storage

Natural gas is a low-carbon and environment-friendly clean energy. It plays an important role in the process of carbon neutralization. Gas-reservoir-type underground natural gas storage (UGS) is a significant part of the natural gas industry, and it is considered as one of the most efficient ways to store energy. The safe operation of UGS requires accurate calculation of the average formation pressure of the reservoir. The periodic large-scale gas injection and production lead to uneven pressure distribution in the UGS, resulting in serious well interference. The pressure of a single well cannot represent the average formation pressure of UGS. This paper proposed a novel method for calculating the average formation pressure under multi-well production conditions in UGS. The point source function, line source function, and Laplace transform methods were used to solve the radial diffusion equation. The Stehfest numerical inversion method was used to obtain the pressure solutions in the real domain of the vertical well and horizontal well models of the UGS, and the balanced pressure value of a single well was obtained by extending the shut-in testing time. The pressure superposition principle and weighting method of gas injection and production rate were used to obtain the average formation pressure. To validate its reliability, an example simulation was performed by a commercial numerical simulation software and the proposed model in this paper separately. The method was applied to the Xiangguosi UGS in China. The Xiangguosi UGS has been operated for nine injection–production cycles. The total gas injection volume and total gas production volume are 146.0 × 10 8 m 3 and 105.6 × 10 8 m 3 , respectively, and the current inventory volume is 42.4 × 10 8 m 3 . The results show that with the multi-cycle operation of Xiangguosi UGS, the average formation pressure increased from 3.41 MPa before the construction of the UGS to 27.89 MPa, and it can be divided into the period of rapid pressure rise in the 1st–5th injection–production cycle, and the period of slow pressure increased in the 6th–9th injection–production cycle. The average formation pressure at the end of gas injection in the ninth cycle is 27.89 MPa, and it is close to the upper limit pressure for the UGS of 28 MPa. The average formation pressure at the end of gas production is 14.08 MPa. This study provides analytical methods and tools for the calculation of average formation pressure and safety operation and monitoring of the gas-reservoir-type UGS.