Injecting CO

2

into depleted gas reservoirs can simultaneously achieve enhanced oil recovery and CO

2

geological storage

offering broad application prospects under the "dual carbon" background. Currently

CO

2

-EGR（CO

2

enhanced gas recovery） is still largely in the theoretical research phase. To address the unclear mechanisms

this review summarizes the mechanisms of CO

2

-EGR in different gas reservoirs. For conventional gas reservoirs

the mechanisms include pressure restoration and displacement effects

gravity segregati

on

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 CO

2

can effectively inhibit water invasion. The contributions of each enhanced recovery mechanism to different types of gas reservoirs vary. CO

2

-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 CO

2

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.