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

The effect of coal composition, pore structure, and gas pressure upon metha ne and carbon dioxide gas transport in Cretaceous Gates Formation coal is i nvestigated. Coal matrix gas transport models, which assume a homogeneous u nimodal pore structure and linear adsorption isotherms, are not appropriate for modeling methane or carbon dioxide adsorption rates in all coal lithot ypes, 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 vo lume 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, a re accurately captured with the current numerical model and an analytical s olution which assumes a bimodal pore structure. Single parameter (diffusivi ty) models may be adequate for some bright coals. Careful consideration of coal pore structure is therefore required for accurate modeling of gas tran sport through the coal matrix. Carbon dioxide numerical and analytical model diffusivities are larger than methane diffusivities obtained for dry coal. In addition, methane diffusiv ities obtained using the models for wet coal are smaller than the model dif fusivites obtained from dry coal. The numerical model diffusivities, which are corrected for the effects of nonlinear adsorption, are larger than diff usivities obtained for analytical models for pore diffusion. Methane and carbon dioxide gas analytical and numerical model effective dif fusivities are sensitive to the starting pressure in an adsorption step, Th e pressure-dependence of the analytical solution diffusivities is likely du e to the nonlinearity of the adsorption isotherm. The effect of gas pressur e 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 methan e reservoir characterization, the determination of gas contents for gas res ource calculations, gas production simulations, and the prediction of outbu rsting in coal seams. (C) 1999 Elsevier Science Ltd. All rights reserved.