Apparent and True Diffusion Coefficients of Methane in Coal and Their Relationships with Methane Desorption Capacity

The diffusion coefficient of methane in coal is a key parameter for the prediction of coalbed methane production. The apparent diffusion coefficient is different from the true diffusion coefficient, which would result in the deviation of methane production. In this study, the particle method using the unipore model and the counterdiffusion method is adopted to measure the methane diffusion coefficients. The results indicated that the true diffusion coefficient obtained by the counterdiffusion experiment decreases first and then increases with increasing methane pressure. The apparent diffusion coefficients obtained by the particle method with two different grain sizes are lower than the true diffusion coefficient. The relationship between the apparent diffusion coefficient and the true diffusion coefficient is analyzed, and the desorption capacity factor (DCF) is proposed to reflect the gap between them. The apparent diffusion coefficient is closer to the true diffusion coefficient when the DCF is small. When using the particle method to estimate the methane diffusion coefficient, experiments with large coal particles and a small methane concentration gradient should be adopted.