Evaluation of Working Gas Volume and Influencing Factors in Gas Storage Reservoirs Utilizing CO₂ as Cushion Gas

The transformation of depleted gas reservoirs into gas storage facilities serves as a vital strategy for balancing natural gas supply and demand. Utilizing CO₂ as cushion gas provides dual advantages of economic benefits and environmental protection. Enhancements in the efficiency and accuracy of working gas volume evaluation for this type of storage are achieved by analyzing the construction model of gas storage facilities converted from depleted gas reservoirs with CO₂ as the cushion gas. Grounded on the principle of material balance in reservoirs, this study incorporates the solubility of CO₂ and CH₄ in formation water and the effects of water body to develop a working gas vo-lume evaluation model. A sensitivity analysis of factors such as water body multiples, bound water saturation, and formation temperature on the working gas volume of the storage facility was conducted. Results indicate that water body multiples, original formation pressure, bound water saturation, and formation temperature positively correlate with the working gas volume, whereas abandoned reservoir pressure, water salinity, and cushion gas injection vo-lume exhibit negative correlations. Among these, the injection volume of cushion gas significantly influences the working gas volume with a correlation coefficient of -0.777 3; water body multiples have a secondary impact with a coefficient of 0.459 6, while formation temperature has the least effect, yielding a coefficient of only 0.002 7. The findings offer theoretical guidance for the design and assessment of working gas volume in further transformations of depleted gas reservoirs into gas storage facilities.