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CO2 is used as a cushion gas for underground natural gas storage reservoirs, and during the operation of the gas storage, unreasonable injection and production parameters and other factors lead to the problems of transitional mixing between CO2 and natural gas and a low natural gas recovery rate. The comparison of physical properties of CO2 and CH4, and the visible phase reactor were carried out to demonstrate the feasibility of CO2 as cushion gas. A two dimensional model of the gas storage reservoir was established to simulate the process of CO2 as cushion gas injection and production, and the formation parameters and operating parameters affecting the mixing degree of CO2-CH4 were analyzed. The results show that the proportion of CO2 cushion gas is not more than 20%, the molar content of CO2 in produced gas is not more than 3%, the mixing degree of CO2-CH4 and the average reservoir pressure drop are small. The higher the permeability, the more CO2 cushion gas carried by working gas recovery, the smaller the proportion of actual CO2 cushion gas, and the insufficient CO2 cushion gas to provide “power” for working gas recovery. Porosity has little effect on the mixing degree of CO2-CH4. Reasonable control of gas production rate can effectively inhibit the development of mixed zones. In other words, the gas production rate should be increased at the peak of gas consumption, and the gas production rate should be reduced at the trough to ensure the minimum gas consumption standard. The research results provide theoretical guidance for the engineering practice of using CO2 as cushion gas in underground natural gas storage. [ABSTRACT FROM AUTHOR]

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 70×106 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 7.236 5%. 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 70×106 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. [ABSTRACT FROM AUTHOR]

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The feasibility of CO2 as cushion gas is summarized from the comparison of thermodynamic properties of CO2 and CH4, and related gas mixing experiments, aiming at the relevant research on cushion gas of underground natural gas storage. From the aspects of static and dynamic factors, the influencing factors of CO2 as cushion gas mixing with working gas are systematically discussed，including reservoir porosity ，permeability，temperature，pressure，CO2 cushion gas ratio，etc. The influence of gas-brine-sandstone interaction on reservoir structure and stability is discussed. This study can provide a theoretical reference for the analysis of injection and production performance of CO2 as cushion gas in underground matural gas storage and the improvement of gas recovery in gas storage. [ABSTRACT FROM AUTHOR]

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Copyright of Journal of Petrochemical Universities / Shiyou Huagong Gaodeng Xuexiao Xuebao is the property of Journal Editorial Department Of Liaoning Shihua University and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Petroleum Geology & Recovery Efficiency is the property of Petroleum Geology & Recovery Efficiency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)