Detailed study on self- and multicomponent diffusion of CO2-CH4 gas mixture in coal by molecular simulation

Abstract

Gas diffusion plays a key role in CO2-enhanced recovery of coal bed methane (ECBM), where more than one types of gases coexist and multicomponent gas diffusion occurs. Such process is now usually described by non-coupled two-component gas diffusion equations which exclude the interactions between gases. Self-diffusion and mutual diffusion of CO2–CH4 mixture are investigated through molecular simulation for the first time. The self-diffusion coefficients of CO2andCH4 decrease with gas concentration but increase with temperature. The mutual diffusion coefficients of binary gas mixture of CO2–CH4incoal are computed through Maxwell–Stefan diffusion theory. A 2D diffusivity matrix |D| (with diagonal element Di and non-diagonal element Dij) is obtained to depict the mutual diffusion of the gas mixture. It is found that CO2 (CH4) diffusion is coupled with CH4 (CO2). The diffusion coupling strength of CO2andCH4 decreases with increasing gas concentration. Temperature positively affects Dij but minimally influences Di, resulting in large ratios of Dij/Di at high temperatures. It means that CO2–CH4 diffusion correlation interactions, which are not present in non-coupled pure gas diffusion equations, are necessary to analyze gas mixture diffusion in coal. In this case, non-coupled pure gas diffusion equations are inadequate for description of CO2–CH4 mixture diffusion. The coupling between the gases can be ignored only at very high temperatures (T>400K), which means a large depth of coal bed in ECBM engineering.