The mechanism of supercritical CO

2

flooding in gas reservoirs was studied through both laboratory and numerical simulations to explore ways to increase recovery of low-permeability tight gas reservoirs.Behaviors of mixing supercri-tical CO

2

with natural gas were observed first to reveal the mixing patterns of the two phases.A laboratory quantitative measurement was carried out for an equilibrium phase behaviors to define the in-situ physical properties of both carbon dioxide and natural gas.The results show that the differences between the properties of the two gases are favorable for CO

2

-based flooding and storage in gas reservoirs.A supercritical CO

2

-natural gas diffusion test offered opportunities for observing behaviors of mixed gas front during flooding process.The result indicates that CO

2

does not diffuse much in na-tural gas and can form only a narrow mixing phase belt

which is good for an effective CO

2

displacement.Based on an analysis of CO

2

and natural gas mixing behaviors

we carried out a CO

2

displacement experiment on a long core from tight gas reservoirs.The results show that gas recovery rate can be increased by 12% after supercritical CO

2

flooding.Finally

based on the experiments

we used numerical simulation method to establish long core model

single injector-single producer dip mechanism model as well as anticline model.These models were then used to systematically verify the mechanism of supercritical CO

2

displacing natural gas.It suggests that a higher gas recovery is possible because of a partial mixing of natural gas with CO

2

at the flooding fronts helping maintain gas reservoir pressure and of a supercritical CO

2

"pad" formed at the lower part of gas traps favorable for gas recovery.The displacement mechanisms expounded through both laboratory works and numerical simulations provide a basis for selecting potential CO

2

flooding target in tight gas reservoirs.