A modified supercritical Dubinin–Radushkevich model for the accurate estimation of high pressure methane adsorption on shales

Abstract

Knowledge of supercritical methane adsorption behavior in shales is crucial for evaluations of total shale gas-in-place (GIP) resources in deep formations. The primary object of this study is to establish an optimized model for the adsorption of supercritical methane in shale. An adaptive L-SDR model, which is a modification of the supercritical Dubinin–Radushkevich (SDR) model in combination with the Langmuir model, was introduced to interpret the measured absolute adsorption isotherms. A series of shale gas high pressure methane adsorption isotherms were measured at 303.15 K for Mesozoic shale collected in the Kuqa Depression of the Tarim Basin. In addition, published methane adsorption data on Posidonia shales were also investigated. The absolute adsorption results were fitted using the Langmuir, SDR and L-SDR models, and the accuracy of each model was analyzed and compared using an average relative error analysis. It was found that the L-SDR model can improve predictions of supercritical methane adsorption compared with predictions from the Langmuir and SDR models at both low and high temperatures, and the results indicate that this model can effectively model supercritical methane adsorption on shale. The individual contributions of the Langmuir and SDR models to the overall adsorption capacity were investigated at multiple temperatures, and the results showed that the contribution of the Langmuir model became more significant with increasing temperatures.