Adsorption Models for Methane in Shales: Review, Comparison, and Application

Finding an optimized adsorption model to estimate the true adsorbed quantity of methane in shale at reservoir conditions is fundamental for estimating the gas-in-place (GIP), and developing an accurate shale gas transport model. However, describing true methane adsorption behavior in shale is challenging because the density or volume of the adsorbed phase cannot be measured directly using current technology. There are several models available to describe the observed adsorption isotherms and extrapolate the true adsorbed quantity of methane, but a consensus model has not been reached by researchers. This work first revisits available absolute and Gibbs excess adsorption models for describing methane in shales. It then compares nine available adsorption models to assess the efficacy of each model, in describing both high pressure and low pressure methane adsorption isotherms in shales. Three aspects of the adsorption model are compared: (1) the goodness-of-fit of each adsorption model, (2) interpretation of the observed test phenomena, and (3) predicted isotherms beyond test data. Comparison results show that even though the goodness-of-fit for each model is comparable, the dual-site Langmuir model is still superior to other available models in interpreting observed phenomena and extrapolating adsorption isotherms beyond test data. The successful application of the dual-site Langmuir model therefore lays the foundation for accurately estimating and extrapolating the deep gas resource and differentiating the accurate ratio of the adsorbed phase to bulk gas for use in shale gas transport models. This study also clarifies some inappropriate concepts and methods routinely used by the shale gas community.