Competitive Sorption of Methane/Ethane Mixtures on Shale: Measurements and Modeling

As the primary mechanism of gas storage in shale, sorption phenomena of CH4 and other hydrocarbons in the micropores and mesopores are critical to estimates of gas-in-place and of the long-term productivity from a given shale play. Since C2H6 is another important component of shale gas, besides CH4, knowledge of CH4-C2H6 binary mixture sorption on shale is of fundamental significance and plays a central role in understanding the physical mechanisms that control fluid storage, transport, and subsequent shale-gas production. In this work, measurements of pure component sorption isotherms for CH4 and C2H6 for pressures up to 114 and 35 bar, respectively, have been performed using a thermogravirnetric method in the temperature range (40-60 degrees C), typical of storage formation conditions. Sorption experiments of binary (CH4-C2H6) gas mixtures containing up to 10% (mole fraction) of C2H6, typical of shale-gas compositions, for pressures up to 125 bar under the aforementioned temperature conditions have also been conducted. To the best of our knowledge, this is the first time that systematic measurements of CH4 and C2H6, both pure and in binary mixtures, sorption on the Marcellus shale have been conducted, thus providing a comprehensive set of CH4-C2H6 competitive sorption data, which can help to improve the fundamental understanding of shale-gas storage mechanisms and its subsequent production. In the study, the multicomponent potential theory of adsorption (MPTA) approach is utilized to model the sorption data. The MPTA model is shown capable in representing the pure component sorption data, and also provides reasonable predictive capability when applied to predict the total sorption for CH4-C2H6 binary mixtures in shale over a range of compositions and temperatures.