Molecular Dynamics Simulation of CO2-ECBM Under Different Moisture Contents

The interactions among water molecules, coal beds, and gases during the process of coal bed methane mining are highly complex. The water and methane (CH4)/carbon dioxide (CO2) molecules compete for adsorption and undergo a series of reactions that affect gas diffusion. In this study, Monte Carlo and molecular dynamics methods were used to investigate the microscopic mechanism of CH4/CO2 competitive adsorption and diffusion during CO2-enhanced coal bed methane mining (ECBM) under different moisture contents, and the geological storage potential of CO2 was predicted. The results showed that when the CO2 and water binding sites were independent of each other, the water molecules changed the electrostatic potential around the coal molecules, resulting in enhanced CO2 adsorption performance, as verified by the surface electrostatic potential. When the water molecules formed a water molecule layer, the adsorption capacity of the secondary adsorption sites provided was larger than that of the surface of the coal molecules, so the CO2 molecules were preferentially adsorbed on the secondary adsorption sites. However, the number of secondary adsorption sites available was not as large as that on the surface of the coal molecules. The interaction energies revealed that when the displacement effect of CH4 in the process of CO2-ECBM and the sequestration effect of CO2 were considered comprehensively, the best CO2 sequestration effect and a good CH4 displacement effect were obtained at a 3% moisture content. The worst CO2 sequestration effect was found at a 5% moisture content. After CO2 injection, the main adsorption layer of CH4 shifted from X = 5 and X = 9 to X = 8.7 and X = 12.5, respectively, and obvious detachment and diffusion occurred. The distribution of the molecular motion and diffusion coefficient revealed the considerable displacement and dispersion of the gas molecules. The distribution of the gas molecular velocity and diffusion coefficient indicated that a 3% moisture content was the ideal condition for CO2 displacement of CH4, and the CO2 sequestration effect was good.