Abstract: The mechanism of supercritical CO₂ 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 CO₂ 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 CO₂-based flooding and storage in gas reservoirs.A supercritical CO₂-natural gas diffusion test offered opportunities for observing behaviors of mixed gas front during flooding process.The result indicates that CO₂ does not diffuse much in na-tural gas and can form only a narrow mixing phase belt,which is good for an effective CO₂ displacement.Based on an analysis of CO₂ and natural gas mixing behaviors,we carried out a CO₂ displacement experiment on a long core from tight gas reservoirs.The results show that gas recovery rate can be increased by 12% after supercritical CO₂ 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 CO₂ displacing natural gas.It suggests that a higher gas recovery is possible because of a partial mixing of natural gas with CO₂ at the flooding fronts helping maintain gas reservoir pressure and of a supercritical CO₂ "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 CO₂ flooding target in tight gas reservoirs.

Key words: numerical simulation, natural gas, CO₂ flooding, low-permeability tight gas reservoir, EOR