On the porous structure of coals: Evidence for an interconnected but constricted micropore system and implications for coalbed methane recovery

An experimental and theoretical study of adsorption and diffusion of carbon dioxide and methane in coals of widely varying rank was carried out. Low-pressure adsorption isotherms of CO2 were obtained and analyzed using Dubinin's theory of volume filling of micropores. High-pressure adsorption isotherms of CH4 were obtained and analyzed using tracer pulse chromatography in conjunction with an appropriate adsorption/diffusion model. A preliminary Xe-129 NMR analysis of chemical shifts experienced by xenon atoms in particles of different sizes is also reported. The heretofore undocumented and/or underestimated effects of activated diffusion of CO2 at 273-298 K complicate the elucidation of the true microporous structure of coals, especially its dependence on coal rank. Activated diffusion of both CO2 and methane at room temperature does not allow reliable estimates of coalbed gas content to be made. A model of an interconnected network of pores which includes randomly distributed, numerous and ultramicroporous constrictions (at any size scale) is consistent with all these experimental and theoretical findings.