(589d) Measurement and Modeling of Competitive Sorption of Methane/Ethane Mixtures on Marcellus Shale – Isotherms and Kinetics

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 bar and 35 bar, respectively, have been performed using a thermogravimetric method in the temperature range (40-60 ^oC). 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 facilitate investigation of depletion dynamics (e.g. via application of the Extended Langmuir Model), excess sorption measurements must be converted to absolute sorption. This conversion requires accurate estimates of the adsorbate density. Here we investigate the performance of several of such methods proposed for evaluating the adsorbate density. We demonstrate that the equilibrium constant and hence the adsorption and desorption kinetic constants are sensitive to the choice of the adsorbate density model and conclude by providing recommendations for selection of adsorbate density modeling. Lastly, we apply the best-fit adsorbate density method to model the sorption dynamics of pure and binary gas mixtures on to the shale.

To the best of our knowledge, this is the first time that systematic measurements of CH₄, C₂H₆ pure and binary mixture 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.