Competitive Adsorption and Selective Diffusion of CH₄ and the Intruding Gases in Coal Vitrinite

Abstract

Here, the adsorption and diffusion of CH4 and the intruding gases in coal were systematically simulated via Monte Carlo. The adsorption selectivity of carbon dioxide over methane (SCO2/CH4, >1) decreases significantly at P < 6 MPa and is kept stable when P > 6 MPa. However, SN2/CH4 (<1) monotonously increases with the increasing pressure, temperature, and bulk mole fraction (BMF) of N2. Both the cross exchange (Di,j) and diagonal diffusion (Di,i) coefficients in the nCO2 + mCH4 and nN2 + mCH4 systems gradually increase with the increasing temperature. Di,j is far higher than Di,i for these two systems, indicating the weaker coupling strength of gas–gas interactions than the gas–coal interactions. D111 (or D112) increases while D221 (or D222) decreases with the increase of CO2 (or N2) BMF. The swelling ratios of nCO2 + mCH4 and nN2 + mCH4 increase slightly at temperatures lower than 338 K and significantly at temperatures higher than 338 K, and both of them are positively related to BMFs of CO2 and CH4, respectively. Both SCO2/CH4d and SN2/CH4d increase with the increasing BMF of carbon dioxide and methane, respectively, indicating that the replacement effects of CO2 and N2 engineering are weightily related to the mole fractions of the invading gases. SCO2/CH4d first increases (≤398 K) and then decreases (398 < T < 438 K). Concerning the geological conditions, the optimization injection depths were 800–1100 m (7.94–10.88 MPa) and 600–900 m (5.98–8.92 MPa) for carbon dioxide and nitrogen, respectively.