The underground gas storage is one of the major storage and peak-shaving means in the world. It needs the cushion gas to keep the reservoir pressure

prevent the water intrusion and ensure the stability of the gas storage. The cushion gas volume generally accounts for 30 % to 70 % of the total volume of the gas storage. If we just took the natural gas as the cushion gas

a considerable amount of natural gas would not be mined when the storage was abandoned

and a large amount of natural gas would be wasted. The study of thermodynamic properties and the mixed experiments of CO

2

and natural gas show that taking CO

2

as the cushion gas for gas storage rebuilt upon the depleted gas reservoirs is feasible

becau

se CO

2

is in the critical state under the operating pressure and temperature. Its physical properties of “high density” and “high viscosity” make itself separate from the natural gas

rather than mix.