Adsorbed methane makes up a large portion of the total shale gas-in-place (GIP) resource in deep shale formations. In order to accurately estimate the shale GIP resource, it is crucial to understand the relationship between the adsorbed methane quantity and the free methane quantity of shale gas in shale formations (under high pressure conditions). This work describes and accurately predicts high pressure methane adsorption behavior in Longmaxi shale (China) using a dual-site Langmuir model. Laboratory measurements of high pressure methane adsorption (303-355 K and up to 27 MPa) are presented. Our findings show that for depths greater than 1000 m (> 15 MPa) in the subsurface, the shale gas resources have historically been significantly overestimated. For Longmaxi shale (2500-3000 m in depth), classical approaches overestimate the GIP by up to 35%. The ratio of the adsorbed phase compared to the free gas has been significantly underestimated. The methods used herein allow accurate estimations of the true shale GIP resource and the relative quantity of adsorbed methane at in situ temperatures and pressures representative of deep shale formations.

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.

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Thermodynamic analyses of high pressure methane adsorption in shale are rarely reported because of the lack of a reliable approach for obtaining the true adsorption uptake from observed adsorption isotherms and the routinely used, oversimplified Clausius-Clapeyron (C-C) approximation. This work extends our previously proposed dual-site Langmuir adsorption model to calculate the isosteric heat of adsorption analytically from the observed adsorption isotherms for high pressure methane adsorption isotherms on Longmaxi shale from Sichuan, China (up to 27 MPa and 355.15 K). The calculated isosteric heat of adsorption considers both the real gas behavior of bulk methane and the adsorbed phase volume, which are neglected in the CC approximation. By this method, the temperature dependence as well as the uptake dependence of the isosteric heat can be readily investigated, where the former cannot be revealed using the CC approximation. The influence of the adsorbed phase and the gas behavior (real gas or ideal gas) on the isosteric heat of adsorption are also investigated, which shows that neglecting either the real gas behavior or the adsorbed phase volume always results in an overestimation of the isosteric heat of adsorption. In the Henry's law regime of low pressure and low adsorption uptake (and up to a surface occupancy of < 0.5), the isosteric heat of adsorption of methane on Longmaxi shale is approximately constant at 15-17 kJ/mol, but then decreases significantly at higher pressures. This work therefore justifies the method to obtain the true isosteric heat of adsorption for high pressure methane in shale, which lays the foundation for future investigations of the thermodynamics and heat transfer characteristics of the interaction between high pressure methane and shale.

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