Simulation Study on Competitive Adsorption of CO2 and CH4 in Shale Organic Porous Media Based on the Lattice Boltzmann Method

Summary Carbon dioxide (CO2) injection into shale gas reservoirs facilitates methane (CH4) desorption from shale micropores, effectively enhancing individual wells’ estimated ultimate recovery while achieving CO2 sequestration. To clarify the CO2-CH4 competitive adsorption mechanism and improve shale gas recovery efficiency, we developed a two-component flow model accounting for this competitive adsorption. Within this model, the surface reaction kinetics equation converts the surface concentration relationship for competitive adsorption into concentration gradient variation at the solid boundary, providing an effective method for characterizing two-component competitive adsorption. The finite difference method was used to simplify the gradient term, and a corresponding evolution scheme for its distribution function was also developed. Furthermore, the presented model integrates pressure-driven velocity fields, diffusion processes dominated by concentration gradients, and adsorption/desorption boundary conditions to simulate gas adsorption and desorption behaviors in shale gas reservoirs. The model accuracy is validated by comparing results with the analytical solution of the Gaussian Hill convection-diffusion problem. In this study, we analyze the effects of convection-diffusion intensity and direction, as well as the impacts of gas ratio and composition, on adsorption and desorption capacities based on the physical model of shale pores. The underlying mechanism of competitive adsorption affecting shale gas production performance is further discussed. Simulation results show that (1) the recovery efficiency of single-well huff and puff is higher than that of gas displacement; (2) CO2 competitive adsorption demonstrates superior extraction efficiency compared with pressure reduction extraction under identical CH4 partial pressure conditions; (3) when reservoir pressure drops to 10 MPa, free gas recovery approaches 80%, while adsorbed gas recovery remains below 30%. The desorption of adsorbed gases mostly occurs below 10 MPa; (4) Finally, the mechanisms by which CO2 injection enhances shale recovery include reducing CH4 partial pressure, maintaining high reservoir pressure, and diminishing CH4 adsorption capacity through competitive adsorption.