A new pressure-based gas diffusion coefficient model: Modeling, verification and analysis

Gas diffusion plays a critical role in the gas migration process in dual-porosity coal, such as coal and gas outburst, gas extraction, CO₂ geological sequestration etc., due to the adsorption of a quantity of gas in coal matrix. Existing gas diffusion models can not accurately describe the dynamic evolution of gas diffusion coefficient (GDC) on temporal scale and spatial scale. In this paper, a new pressure-based dynamic gas diffusion model was constructed, and verified with experimental data. The results showed that on the temporal scale, GDC declines with the advance of gas diffusion process, and on the spatial scale, GDC surrounding the center of the coal particle is larger than it surrounding the boundary of the coal particle. With the decrease of gas pressure, the resistance of gas molecular directional motion decreases, resulting in the increase of Fick diffusion coefficient. On the contrary, with the decrease of gas pressure, the gas diffusion channel sizes decrease, causing the decrease of Knudsen diffusion coefficient. The pressure-based GDC D_p is the results of competition of Fick diffusion and Knudsen diffusion. The lower limit of Fick diffusion weighting factor c_Fmin is 0, and the upper limit of Knudsen diffusion weighting factor c_Kmax is 1. During gas diffusion process, the lower limit of Knudsen diffusion weighting factor c_Kmin increases from 0 to 1 gradually, indicating that the pore structures in coal matrix dominate the gas diffusion performance, especially the later stage of gas diffusion process. The research of this paper provides a new insight into the gas diffusion process in coal and has certain guiding significance for the gas related engineering practices.