Carbon Dioxide as Cushion Gas for Natural Gas Storage

Natural gas storage is used to smooth the natural gas supply to meet high peak demand. In natural gas storage, the working gas (methane) is injected and produced seasonally while a cushion gas that is not extracted is used to provide pressure support. In the case of depleted gas reservoirs being used for gas storage, the cushion gas is commonly leftover native gas (methane). Another approach is to produce most of the methane from the reservoir since it can be sold for profit and inject a cheap inert gas for use as the cushion gas. Carbon dioxide injection during carbon sequestration with enhanced gas recovery can be carried out to produce the methane while simultaneously filling the reservoir with carbon dioxide. Carbon dioxide undergoes a large change in density near its critical pressure, an advantageous feature if used as a cushion gas. Furthermore, the injection of carbon dioxide into the ground may in the future be economically favorable through carbon credits or tax advantages offerred to encourage carbon sequestration. Reservoir simulations of methane injection into a model gas storage reservoir with carbon dioxide as cushion gas demonstrate that 30% more methane can be stored relative to a native gas cushion. Along with economic considerations of carbon dioxide and natural gas prices, the critical issue for the use of carbon dioxide as a cushion gas is limiting the rate of mixing between methane and carbon dioxide through careful reservoir selection and operations.