Gases migration behavior of adsorption processes in coal particles: Density gradient model and its experimental validation

[Display omitted] The injection of CO2 or N2 to displace CH4 in coal seams can improve coalbed methane (CBM) recovery. However, the migration pattern of these gases in the coal matrix is disputed as the Fick diffusion model is not fully applicable. In this work, the isothermal adsorption experiments of CO2, CH4 and N2 were first performed on four kinds of coal samples. A mathematical model of gas diffusive adsorption was then established based on a free gas density gradient (FGDG) drive, which was solved numerically by finite difference method (FDM). The microchannel diffusion coefficient (Dfg) that reflected the transport capacity of free gas in the coal matrix was determined through matching the simulation curves of cumulative gas adsorption with experimental data. Furthermore, the effects of gas type, coal rank and adsorption pressure on the microchannel diffusion coefficient were quantitatively investigated. The results show that (i) the simulation results of the gases are well consistent with the experimental data, indicating that the migration behavior of CO2, CH4 and N2 in coal particles conforms to the FGDG-driven model; (ii) the Dfg of CO2 is much higher than that of CH4 and N2, and the Dfg of a single gas shows an asymmetric U-shaped trend with the coal rank; (iii) compared with Fick's diffusion coefficient and Darcy's permeability coefficient, the Dfg is only related to the pore structure of the coal and gas properties, independent of the state parameters such as pressure and time. Therefore, the FGDG-driven model better explains the essence of gases migration behavior in the coal particles, and this model can be considered as an important aspect of CBM production calculation and prediction.