Measurement and modeling of the adsorption isotherms of CH4 and C2H6 on shale samples

Abstract

CH₄ and C₂H₆ are two common components in shale gas. Adsorption isotherms of CH₄, C₂H₆, and their binary mixtures on shale samples are significant for understanding the fundamental mechanisms of shale gas storage and the recovery of shale resources from shale reservoirs. In this study, the thermogravimetric method is applied to obtain the adsorption isotherms of CH₄, C₂H₆ and their binary mixtures on two typical shale core samples. A simplified local density theory/Peng–Robinson equation of state (SLD-PR EOS) model is then applied to calculate the adsorption of CH₄ and C₂H₆ on shale, and the efficiency of the SLD-PR EOS model is thus evaluated. The results show that C₂H₆ exhibits a higher adsorption capacity than CH₄ on shale samples, indicating the greater affinity of C₂H₆ to organic shale. As the molar fraction of C₂H₆ increases in the CH₄/C₂H₆ mixtures, the adsorption capacity of the gas mixtures increases, indicating the preferential adsorption of C₂H₆ on shale. Based on the predicted results from the SLD-PR EOS model, a reasonable agreement has been achieved with the measured adsorption isotherms of CH₄ and C₂H₆, validating the reliability of the SLD-PR EOS model for predicting adsorption isotherms of CH₄ and C₂H₆ on shale samples. In addition, the SLD-PR EOS model is more accurate in predicting the adsorption of CH₄ on shale than that of C₂H₆. This study is expected to inspire a new strategy for predicting the adsorption of hydrocarbons on shale and to provide a basic understanding of competitive adsorption of gas mixtures in shale reservoirs.