Molecular Dynamic Simulation on the Dynamic Process of CH₄ Displacement by CO₂ in Shale Pores

The technology of CO2-ESGR(CO2-enhanced shale gas recovery) is an effective means to practice energy saving and emission reduction and has drawn the attention of countries all over the world for its promising future. At present, the possibility and potential of the CO2-ESGR have been confirmed by some studies. However, the mechanism of shale gas occurrence is complex; existing studies have mainly focused on the static results of CH4 displacement by CO2. Therefore, the dynamic process of CH4 adsorbed in the shale being displaced by CO2 was simulated in this study. The results show that the process of CH4 displaced by CO2 is divided into the CH4 self-resolution phase, the displacement of residual CH4 by CO2, and the stabilization phase. Moreover, the diffusion and desorption of CH4 are less affected by pressure and pore size, and the effect of pressure and pore size on the diffusion and adsorption of CO2 is more significant. The high pressure enhances the adsorption capacity between the pore wall and CO2, which can promote the effect of CH4 displacement by CO2. In addition, another displacement phenomenon is derived in this study by examining the offset of the CO2 and CH4 density peak, in which CO2 only occupies part of the CH4 adsorption site and then both coexist for adsorption. This is different from the traditional theory that CO2 displaces CH4 by taking the place of it. It is conducive to revealing the dynamic mechanism of CO2 displacing CH4. An in-depth study of the dynamic repulsion mechanism of CH4 by CO2 in geological rock formations can promote the application of the CO2-ESGR technology.