Review of Using CO2 as Cushion Gas in Underground Natural Gas Storage (UGS)

Underground natural gas storage (UGS) and CO2 sequestration are both critical research topics in our country. There is always a part of natural gas be treated as cushion gas to maintain the reservoir pressure and thus, become “dead capital”. Therefore, using CO2 to replace natural gas as cushion gas could: (1) Maintaining the reservoir pressure; (2) Storing CO2 in underground; (3) Reducing UGS operation cost. This study, based on recently published journal and conference papers comprehensively organized the mechanism of using CO2 as the cushion gas, concluded how academia using physical experiments and numerical simulation deep dives into this area, and gave this area’s possibly research trend in the future. The following result could be obtained: (1) The mixing between CO2 and natural gas is inevitable. The diffusion effect is relevant with pressure and temperature. Since CO2 has higher density and viscosity than natural gas, gas mix zone will become smaller during injection/production cycles. Moreover, CO2 could also help reservoir to maintain pressure due to high compressibility; (2) Formations’ properties will change after the coupling reaction between CO2 gas–water–rock, but the law of properties alters still needs further research; (3) UGS reconstructed from aquifers and salt caverns were also studied in this paper. Cushion gas work efficiency and produced natural gas purity could be effectively increased by clarifying creep speed and temperature and pressure condition during inject/produce cycle in salt caverns. As for reservoir aquifers, solubility and reservoir’s micro fractures should be considered, inject pressure and flow rate should be controlled in order to prevent water breakthrough. This study suggests adding CO2/CH4 mixing gas–water–rock coupling reaction in future research to know the dispersion effect, solubility mechanism and formation properties alteration trend better. Starting CO2/CH4, CO2/CH4-water mix fluids’ phase behavior research during multi-injection/production cycles to analyze CO2/CH4-water system phase balance mechanism. Adding water-cut reservoir, salt cavern into experiments and numerical simulations to evaluate feasibility of rebuild them into UGS. By combining previous achievements and these research topics, injection/production plan could be further optimized, UGS could be developed safely and effectively and moreover, assist our country to realize “dual carbon” goal.