An important component of hydrocarbon storage in coalbed methane and organic-rich shales is sorption within organic matter. Natural gas sorption isotherms measured in these systems may include a combination of adsorption onto the internal surface area and absorption within the organic matter. The focus of the current work is the modeling of adsorbed fluids.
There have been multiple models proposed for modeling single- and multi-component adsorption on coal and shale, of which the most popular remain the simple Langmuir model and its extension to multi-components. In this work, we first review various approaches used for modeling adsorption on coal and organic-rich shale, including newer approaches such as the 2D Equation-of-State (2D-EOS) method. We discuss extensions of simple approaches, such as the Langmuir and Dubinin-Radushkevich (D-R) equations, to modeling supercritical, single-component fluids. The applications of these models to coal and shale datasets will be demonstrated. An important finding of the current work is that the simple Langmuir/D-R models are mostly adequate for modeling supercritical, single-component adsorption on coals and shales, provided that certain adjustments are made to account for supercritical fluid properties, such as adsorbed-phase density. The 2D-EOS model was found to be superior for modeling high-pressure CO2 excess adsorption on shale, however.
Several multi-component adsorption models are used to predict binary component adsorption (CH4-CO2) on shale and to investigate CO2 selectivity over CH4 for two shale systems as a function of pressure and gas composition. This information is useful for designing enhanced recovery operations in dry shale. A dataset containing heavy hydrocarbon adsorption on shale was also examined to determine the importance of adsorption in "liquid-rich?? systems. The change in selectivity of heavier hydrocarbons with addition of CO2 was examined using binary gas adsorption modeling; it was found that CO2 could reduce heavy hydrocarbon component selectivity which provides a possible mechanism for enhanced recovery.
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