Molecular dynamics analysis of influencing factors of CH4 diffusion in coal

In order to analyze the influence factors of methane diffusion in coal from the microscopic point of view.Taking three kinds of coal samples from Sunjiawan, Datong and Shuangyashan as the research objects.Based on the XRD results, three kinds of coal macromolecular structure models were constructed.The effects of pressure, temperature, CO2 and H2O on the diffusion of CH4 molecules in coal were studied by molecular dynamics simulation.The change law of CH4 diffusion coefficient in coal under different influence factors is given.The results show that with the increase of pressure, the diffusion coefficient of CH4 molecules in the three coals decreases first and then tends to be stable. When the pressure increases to a certain value, the diffusion coefficient of CH4 molecules in the three coals from Sunjiawan, Datong and Shuangyashan will be stable at 1.084×10 -8, 0.770×10 -8 and 1.137×10 -8 m 2/s, respectively. Under the same pressure condition, the order of CH4 molecular diffusion rate from large to small is Shuangyashan coal,Sunjiawan coal,Datong coal. With the increase of temperature, the mean orientation shift of CH4 in the three kinds of coal increases, which is conducive to the diffusion of CH4 molecules, but not conducive to its adsorption. The effect of temperature change on the diffusion rate of CH4 molecules in three kinds of coal is from the largest to the least as Sunjiawan coal, Shuangyashan coal,Datong coal. Within a certain range, the increase of H2O content has a hindrance to the diffusion of CH4 molecules. The increase of H2O content has a greater effect on the diffusion rate of CH4 molecules in Sunjiawan and Datong coals, but a smaller effect on the diffusion rate of CH4 molecules in Shuangyashan coals. H2O gas injection method is more effective in Sunjiawan and Datong coals. The molecular diffusion coefficient of CH4 decreases with the increase of CO2 content. The inhibition effect of CO2 on the diffusion of CH4 molecules in coal is stronger to weaker than that in Datong coal,Sunjiawan coal,Suangyashan coal. Compared with H2O, CO2 has a stronger inhibition effect on the diffusion of CH4 molecules in coal seam. From the perspective of molecular dynamics diffusion coefficient, the coal seam CO2 injection gas recovery method is more effective.