Mechanism and development direction of CO2-EGR

Injecting CO2 into depleted gas reservoirs can simultaneously achieve enhanced oil recovery and CO2 geological storage, offering broad application prospects under the "dual carbon" background. Currently, CO2-EGR（CO2 enhanced gas recovery） is still largely in the theoretical research phase. To address the unclear mechanisms, this review summarizes the mechanisms of CO2-EGR in different gas reservoirs. For conventional gas reservoirs, the mechanisms include pressure restoration and displacement effects, gravity segregation, viscosity-difference-assisted displacement, and dissolution-enhanced reservoir modification. For condensate gas reservoirs, pressure-maintained production is possible, along with viscosity reduction, dissolution expansion, and extraction-deblocking effects. For shale gas, coalbed methane, and tight gas reservoirs, the adsorption displacement effect is more significant. In water-bearing gas reservoirs, injecting CO2 can effectively inhibit water invasion. The contributions of each enhanced recovery mechanism to different types of gas reservoirs vary. CO2-EGR has been proven feasible at the theoretical level, but to achieve field application, further breakthroughs are needed in several areas, including gas phase characteristics of mixed gases, diffusion and gas mixing mechanisms, enhanced recovery potential evaluation, and characterization of enhanced recovery mechanisms. Research shows that injecting CO2 into depleted gas reservoirs can restore formation pressure and replenish formation energy. Due to physical property differences, a stable displacement process is formed, achieving enhanced recovery under the combined action of multiple mechanisms. It is a highly promising method for increasing production.