Pore Accessibility of Methane and Carbon Dioxide in Coals

Two Australian coals were heat-treated, and the accessibility of the pore space to CH4 and CO2 was investigated. Samples heat-treated at 573 and 673 K exhibit larger adsorption/desorption hysteresis and smaller surface areas (measured by CO2 adsorption at 273 K) than untreated samples. For samples heat-treated at 773 K, however, the surface area increased by 50% and the hysteresis was lower, compared to untreated samples. These results demonstrate that volatile hydrocarbons at pore mouths are the cause of energy barriers that prevent adsorbing molecules from passing through. A conceptual model is proposed to illustrate changes in activation energy at constricted pore mouths. Also, the results suggest that both adsorption and desorption isotherms should be measured to determine kinetically inaccessible pore spaces in order to correctly estimate CH4 recovery and CO2 storage capacity. The results have importance to the problem of estimating CH4 recovery and CO2 storage capacity for CO2 geosequestration as part of a CO2-enhanced coal bed methane recovery operation.