Effect of kerogen maturity on the adsorption capacity of CO₂ and CH₄: A molecular investigation

The adsorption characteristics of shale under a wide range of pressure and temperature conditions is essential for successful exploitation and development. In this work, we present a molecular investigation of the single and binary component adsorption behavior of methane and carbon dioxide based on a representative kerogen nanostructure to aid the assessments of the predictive power of theoretical adsorption models. In particular, we elucidate the applicability of the Langmuir, Tóth, and Langmuir-Freundlich model for single component methane and carbon dioxide systems, and explicate the non-revised Langmuir, revised Langmuir, extended Freundlich, Tóth, and Langmuir Ratio Correlation (LRC) for an equimolar binary mixture of methane/carbon dioxide system, respectively. It was also explained how the maximum adsorption capacity can be calculated directly from the model parameters, reducing the complexity of the nonlinear regression problem, and provide analytical solutions for the adsorption behaviour of pure/binary systems at different pressure regime. The key findings obtained indicated that the Tóth and the Langmuir-Freundlich isotherms are found best fit to the single species adsorption behavior, while Tóth and LRC models captured acceptable binary adsorption behavior. Importantly, the LRC isotherm is the only framework capable of describing a selectivity reversal. This study outcomes are useful to enhance understanding on prediction capability of adsorption isotherms that will lead to allow ranking of potential sequestration sites as a function of predicted CO₂ storage capacity.