Characteristics of Methane Diffusion in Coal Matrix and Its Effect on Gas Production

Diffusion is one of the key steps of methane transport in coal seam, yet our understanding of it is still insufficient. Taking the high rank coal bed methane (CBM) reservoir in the southern Qinshui basin, China as the study area, the patterns and quantitative characterization of methane diffusion in coal seam were analyzed based on microflows and nanoflows mechanics theory; the influence of diffusion property on gas production in coal seams of different coal textures was studied by using a numerical simulation software (Simed) in this study. Results show that the diffusion of methane in coal is driven by a chemical potential gradient and the diffusion modes include bulk diffusion, Knudsen diffusion and configurational diffusion. Various diffusion modes coexist and vary during the extraction of coalbed methane; the diffusion coefficient is influenced by temperature, gas pressure, gas type, moisture and pore structure of coal matrix, and the size of micropore varies due to the adsorption of methane and thereby the diffusion path will be transformed; the dynamic adsorption of methane on the coal matrix determines that methane diffusion in coal is a non-equillibrium process and that the diffusion coefficient is a function of adsorbed gas concentration; the results of numerical simulation based on quasi-steady diffusion show that the diffusion property has a slight influence on the long-term cumulative gas production while it exerts a significant effect on the short-term gas rate; if the diffusion coefficient is low, that is, the sorption time constant is relatively high, the peak gas rate will be relatively low while the gas rate will be relatively high in a period of time after reaching the peak gas rate; the gas production is more sensitive to sorption time constant for the low-permeability tectonically deformed coal seam than for the high-permeability one.