Molecular simulation of self-diffusion of CH4, H2O and CO2 in coal macromolecule

In this paper, the self-diffusion coefficients (SDC) of CH4/CO2/H2O in coal molecules under different ensemble conditions and the effects of temperature and pressure are calculated by grand canonical Monte Carlo and molecular dynamics (MD) methods. Results indicated that the self-diffusion coefficients of CH4 and CO2 gradually increase with the increasing temperature under the NPT ensemble. The SDC of H2O increases undulatory and it is higher than that of CH4 and CO2 at each temperature, where the diffusion activation energy followed the order of \begin{document}$E({{\mathrm{H}}_2{\mathrm{O}}})<E({{\mathrm{CO}}_2})< E({{\mathrm{CH}}_4}) $\end{document}, indicating that the CH4 diffusion was the most difficult to occur, and the diffusion of CO2 is more easy to occur. With the increase of pressure, the swelling rate of various gas molecules gradually increases. At high pressure, the molecular swelling effect is significant, indicating that the adsorption of H2O and CO2 in the enhanced coalbed methane recovery process can cause significant coal matrix swelling, resulting in narrow gas transmission channels, which is not conducive to the production of coalbed gas. The diffusion activation energies of CH4, H2O and CO2 all increase gradually with the increase of pressure, and the increase trend is increasing, indicating that the self-diffusion of CH4, H2O and CO2 needs to overcome the higher molecular structure energy barrier when the pressure is high. CO2 should be injected into coal body at a faster rate, so as to improve the recovery rate of coalbed methane.