Utilization of CO2 and N2 as cushion gas in underground gas storage process: A review

Due to the significance of gas storage in recent years, the importance of its most crucial operational and economic aspect, the cushion gas, has come under great scrutiny. The cushion gas is injected into the reservoir to raise the pressure to a specific range, ensuring optimum rate and sufficient deliverability during the production phase. 20 to 70 % of the total gas in an underground gas storage (UGS) operation can be comprised of cushion gas, and it is widely known that it's the most expensive part of the procedure. If the natural gas itself is used as the cushion gas, it cannot be produced, reducing the efficiency of resource delivery to the consumer market. Consequently, nitrogen and carbon dioxide have been considered to replace a portion of the cushion gas. Although this application can prove beneficial, there are still gray areas that have not been addressed conclusively. This paper aims to categorize the advantages and disadvantages of the application of nitrogen and carbon dioxide as cushion gas from previous studies for the first time and provide future researchers with challenges that are yet to be resolved. Initially, a brief history of prior operations and studies is given. Secondly, the properties of nitrogen and carbon dioxide and their potential effect on the operation and mixing between cushion gas and working gas are discussed. After that, the operational parameters affecting the mixing phenomenon concerning each gas are presented, and future challenges and opportunities regarding the utilization of nitrogen and carbon dioxide as the cushion gas are introduced. It was concluded that the abnormal changes in CO2's properties, specifically density, and viscosity, can heavily impact the cushion gas selection process. Furthermore, an increase in production and injection rates can increase the mixing percentage. Simultaneous injection of cushion gas and working gas can potentially limit the mixing phenomenon and decrease the duration of the UGS process before it is ready for the initial production phase.