Study on Operation Strategy of Aquifer Underground Gas Storage Using CO₂ as Cushion Gas Available to Purchase

The cushion gas is indispensable to maintain the reservoir pressure, prevent the water intrusion and ensure the stability of underground gas storage (UGS). Considering the good thermodynamic properties, economic benefit and carbon storage, CO₂ is the ideal option for the cushion gas. However, it is inevitable that the cushion will mix with the working gas during the operation of gas storage, which reduces the thermal enthalpy and even changes the pressure dynamics of the reservoir. It is important to research how to control the injection of CO₂ to operate the UGS efficiently. This paper demonstrates the feasibility of CO₂ as the cushion gas and develops an operation strategy for aquifer UGS. The numerical simulation method with consideration of the effect of the dissolution of CO₂ in water and the diffusion between the gases is adopted to model the injection and production process of UGS with CO₂ as cushion gas. Firstly, the feasibility of CO₂ is demonstrated through the analyses of physical properties and mixing characteristics of CO₂ and CH₄. Then, the dynamics of UGS is analyzed and the influential factors are discussed. Eventually, the optimum operation strategy is recommended. This work can provides a better understanding of the dynamics of UGS and a scientific basis for the efficient operation of gas storage and production. This paper demonstrates the feasibility of CO₂ as the cushion gas, and develops an operation strategy for aquifer UGS, which provides a scientific basis for the efficient operation of gas storage.