Study on Operation Strategy of Aquifer Underground Gas Storage Using CO2 As Cushion Gas

The cushion gas is indispensable to maintain the reservoir pressure, prevent the water intrusion and ensure the stability of underground gas storage (UGS). 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 dynamics of the reservoir. It is important to research how to control the injection of CO₂ to operate the UGS efficiently.

Based on the seepage theory of two-phase, this paper establishes a two-phase three-dimensional seepage model for UGS considering the effect of the dissolution of CO₂ in water and the diffusion between the gases. The optimal operation strategy for aquifer UGS is determined through the numerical simulation of the factors that have an impact on the process of injection and production.

It can be discovered that CO₂ and the mixture with a large amount of CO₂ have much higher density and viscosity than CH₄, since CO₂ is in a supercritical state within the operation pressure and temperature of the UGS. This is inclined to restrict the mixing of the cushion gas and the working gas. The CO₂ volume suffers a great loss due to the dissolution in the aquifer, which is more likely to weaken the protection of the cushion gas. It can be seen from the variations of the width of the mixed zone that the increase of pressure, permeability the proportion of the cushion gas will widen the mixing zone. Meanwhile, the content of CO₂ in the production gas increased observably, leading to a reduction in thermal enthalpy of the production gas. Comparing the two common injection mode of the cushion gas, the result shows that the cushion gas injected through peripheral injection well has less effect.