Objective In order to deal with the loss of cushion gas in underground gas storage,CO2,an economical gas,is used as cushion gas.There are various restrictions in applying CO2 as cushion gas,and few studies on its application have been conducted during the shut-in period.Therefore,it is necessary to study the influencing factors of CO2 as cushion gas during the shut-in period in the operation of natural gas storage.Methods The influence of dynamic parameters of the gas reservoir(gas injection pressure,gas injection rate and CO2 cushion gas ratio)on the gas mixing zone is studied when CO2 is used as cushion gas by finite element simulation.Results The results show that the injection pressure has little effect on the mixing zone during the shut-in period.The injection pressure is controlled at about 12 MPa,which is the most reasonable,and the proportion of gas mixing zone in the gas storage area is 23.731 5％.The proportion of the mixing zone decreases with the increase in gas injection rate,but the mixing zone near the gas injection port leads to a large amount of mixed gas when recovering natural gas.Therefore,it is reasonable to control the gas injection rate at 70X 106 m3/d,and the proportion of mixing zone is 18.324 6％.CO2 cushion gas ratio has an obvious effect on the mixing between natural gas and CO2.When the CO2 cushion gas ratio is 20％,the proportion of the gas mixing zone is 7.2365％.Conclusions According to the experimental results,the control measures for the gas mixing zone are designed.When the injection pressure is controlled to 12 MPa,the gas injection rate is 70 X 106 m3/d,and the CO2 cushion gas ratio is 20％,the operation of the gas storage can be more efficient,and the reference can be provided for the actual construction of the gas storage.
underground gas storageCO2shut-in periodgas injection pressuregas injection ratecushion gas ratioproportion of gas mixing zone
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