Molecular simulation of CO2/CH4 self- and transport diffusion coefficients in coal

CO2 enhanced coal-bed methane recovery (CO2-ECBM) project has become the most promising method for carbon sequestration and natural gas recovery. The diffusion properties of carbon dioxide (CO2) and methane (CH4) in coal lie at the heart for the project. Quantitatively understanding the diffusion, especially the transport diffusion in microporous coal, remains challenging. In this paper, based on the Wiser bituminous coal model, molecular simulation was performed to obtain the transport diffusion coefficient from self-diffusion coefficient via thermodynamic factor, and first used to study the properties of CO2 and CH4 diffusion in coal micropores. Comparisons were made between CO2 and CH4, between different coal models, also between simulations and experiments. The effects of temperature and pressure were further analyzed. It turns out that: (1) The calculated transport diffusion coefficients are in good agreement with the experimental data. It is validated that the diffusion in micropores is predominant in coal for CO2 and CH4. (2) The self-, corrected, and transport diffusion coefficients of CO2 are all larger than CH4, while the calculated diffusion activation energy of CO2 is smaller than CH4. It is indicated that CO2 diffusion in coal micropores is faster than CH4. (3) All three distinct diffusion coefficients increase with rising temperature. The transport diffusion coefficients first increase to reach a peak and then decrease with the pressure rising. At relatively low pressure, the coal matrix swells seem to be ignorable and the dominant transport mechanism is the surface diffusion. With pressure rising, the coal swells quickly and the diffusion activation energies enhance, so the configurational diffusion gradually becomes dominant. The peak pressure of CO2, at which the peak of diffusion coefficient occurs, is lower than CH4. The work is expected to reveal the mechanism of gas diffusion in coal micropores and provide some fundamental data for CO2-ECBM projects.