The effect of pore structure and gas pressure upon the transport properties of coal: a laboratory and modeling study. 2. Adsorption rate modeling

Abstract

The effect of coal composition, pore structure, and gas pressure upon methane and carbon dioxide gas transport in Cretaceous Gates Formation coal is investigated. Coal matrix gas transport models, which assume a homogeneous unimodal pore structure and linear adsorption isotherms, are not appropriate for modeling methane or carbon dioxide adsorption rates in all coal lithotypes. A new numerical model for matrix gas diffusion/adsorption is developed and applied to methane and carbon dioxide volumetric adsorption rate data. The model accounts for nonlinear adsorption in microporosity, a bimodal pore volume distribution, and time-varying gas pressure external to coal particles. Methane and carbon dioxide adsorption rate behaviour of bituminous coals with a multimodal pore volume distribution, such as dull or banded coals, are accurately captured with the current numerical model and an analytical solution which assumes a bimodal pore structure. Single parameter (diffusivity) models may be adequate for some bright coals. Careful consideration of coal pore structure is therefore required for accurate modeling of gas transport through the coal matrix. Carbon dioxide numerical and analytical model diffusivities are larger than methane diffusivities obtained for dry coal. In addition, methane diffusivities obtained using the models for wet coal are smaller than the model diffusivites obtained from dry coal. The numerical model diffusivities, which are corrected for the effects of nonlinear adsorption, are larger than diffusivities obtained for analytical models for pore diffusion. Methane and carbon dioxide gas analytical and numerical model effective diffusivities are sensitive to the starting pressure in an adsorption step. The pressure-dependence of the analytical solution diffusivities is likely due to the nonlinearity of the adsorption isotherm. The effect of gas pressure upon diffusivities, obtained from the numerical model, indicate that the mechanism of gaseous diffusion is bulk diffusion. Results of the current study have important implications for coalbed methane reservoir characterization, the determination of gas contents for gas resource calculations, gas production simulations, and the prediction of outbursting in coal seams.