Understanding gas transport mechanisms within the coal matrix is crucial for optimizing coalbed methane extraction and preventing gas-related accidents. However, current academic research often ignores the release characteristics and transport behaviors of gas stored within rocks. This study first determined the adsorption constants of coal and rock particles for different gases and conducted isothermal desorption experiments for both carbon dioxide and methane under constant-volume conditions. Based on the free gas density gradient diffusion theory, a mathematical model for gas transport was developed and solved numerically using the finite difference method. Furthermore, a novel method integrating dimensionless analysis criteria with an empirical desorption formula was proposed to determine the micropore diffusion coefficient. The results demonstrate that (1) for both rock and coal particles, and for both carbon dioxide and methane desorption, the simulation curves based on the density gradient diffusion model show fundamental consistency with the experimental data, verifying the model's correctness; (2) within the same coal/rock particle, the micropore diffusion coefficient of carbon dioxide is greater than that of methane. For anthracite under 2 MPa, the methane diffusion coefficient is 6.3 × 10−8 cm2/s, while the carbon dioxide diffusion coefficient is 4.9 × 10−7 cm2/s. For the same gas, the micropore diffusion coefficient is greater in coal particles than in rock particles; (3) although the total gas content stored in the rock matrix is less compared to coal particles, it is not negligible. Subsequent research should pay greater attention to the storage and transport behaviors of gas within rocks.
Wu et al. (Sun,) studied this question.