Molecular Dynamics Simulation of CO 2 -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 (CH 4 )/carbon dioxide (CO 2 ) 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 CH 4 /CO 2 competitive adsorption and diffusion during CO 2 -enhanced coal bed methane mining (ECBM) under different moisture contents, and the geological storage potential of CO 2 was predicted. The results showed that when the CO 2 and water binding sites were independent of each other, the water molecules changed the electrostatic potential around the coal molecules, resulting in enhanced CO 2 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 CO 2 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 CH 4 in the process of CO 2 -ECBM and the sequestration effect of CO 2 were considered comprehensively, the best CO 2 sequestration effect and a good CH 4 displacement effect were obtained at a 3% moisture content. The worst CO 2 sequestration effect was found at a 5% moisture content. After CO 2 injection, the main adsorption layer of CH 4 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 CO 2 displacement of CH 4 , and the CO 2 sequestration effect was good.