Simulation study on dynamic characteristics of gas diffusion in coal under nitrogen injection

Abstract

To reveal the mechanism of desorption of methane in coal seams by inert gas N2, the desorption behavior of CH4 after N2 injection was studied by using Giant Canonical ensemble Monte Carlo (GCMC) and Molecular Dynamics (MD) methods with wiser bituminous coal as the research object. The results show that the adsorption isotherms of CH4 and N2 in the molecular structure model of bituminous coal are in good agreement with the Langmuir adsorption isotherm model. The adsorption capacity of the two gases in the bituminous coal structure model is CH4 > N2. The higher the N2 injection pressure, the higher the temperature, and the more methane desorption. N2 can replace some adsorbed CH4 through competitive adsorption with CH4. Compared with injecting high-temperature nitrogen to desorb methane in coal seams, in high-pressure nitrogen, the diffusion effect of CH4 flowing in coal is more significant. The higher the nitrogen injection pressure, the better the effect of N2 promoting CH4 desorption. The relative concentration of CH4 in the vacuum layer gradually increases with the increase of water content. This indicates that the water in coal promotes the desorption of CH4. The mechanism of N2 injection and CH4 desorption in coal seams mainly includes gas displacement and gas dilution and diffusion. This study provides theoretical support for methane extraction technology in goaf.