A model for gas diffusion in multiscale pores of coal coupling multiple diffusion mechanisms

Abstract

ABSTRACT Gas diffusion in coal is an important process for coalbed methane extraction. Multiple diffusion mechanisms and various gas states exist in the pores of coal, causing gas diffusion to be a complex process. In this study, a gas diffusion model for multiscale pores of coal coupling multiple diffusion mechanisms was established, and the effect of pore size distribution type on gas diffusion was studied. Verified through experimental results, we found that the tortuosity for coal is very high and can be expressed by a power function relationship between tortuosity and porosity with the index n set as 3. The calculations show that the average pore size and pore type can clearly affect gas desorption, including the desorption ratio, gas diffusion type and diffusion coefficient. For free gas in coal, bulk diffusion dominates in macropores, and transition diffusion dominates in mesopores and micropores. Free gas diffusion plays a leading role in macropores and is also the main form in mesopores, while in micropores, surface diffusion accounts for the main proportion and cannot be ignored. The effective diffusion coefficient for coal with macropores is of the order of magnitude of 10−11 – 10−10 m2 s−1, increasing in gas desorption; the effective diffusion coefficient for coal with mesopores is of the order of magnitude of 10−12 – 10−11 m2 s−1, having a limited increase as gas desorption; the effective diffusion coefficient for coal with micropores is on the order of magnitude of 10−12 m2 s−1, rarely affected by desorption; and the effective diffusion coefficient has a great difference in coal having bimodal distributions, showing the gas diffusion heterogeneity in coal.