Laboratory-Scale Investigation of Sorption Kinetics of Methane/Ethane Mixtures in Shale

Natural gas produced from shales is composed primarily of methane (CH4), accounting for up to 87-96 mol %. In addition to CH, shale gas contains a host of secondary components, including nitrogen (N-2), helium (He), and hydrocarbons such as ethane (C2H6) and propane (C3H8). CH4 and the other hydrocarbons are thought to be stored in the adsorbed state in the micropores and mesopores of the shale, and as free gas in the (natural) fracture networks. Although convective transport and diffusive transport account for the short-term behavior during shale gas production, desorption is thought to dominate the long-term dynamics of shale gas production. The key objective of this study, therefore, is to investigate the sorption kinetics of methane/ethane mixtures in gas shales. Specifically, we study here the adsorption/desorption behavior of pure CH4 and C2H6, and their mixtures in a whole shale sample (cube) using thermogravimetric analysis (TGA). The choice of ethane is because it is, typically, the second largest component of shale gas and is thought to compete for the same adsorption sites as methane. To this end, we first determine the steady-state isotherms of the pure component gases and their binary mixtures, which are essential to predicting the gas storage capacity of the shale. We then study the dynamics toward equilibrium during the sorption process in order to better understand the role of desorption during the later times of shale gas production. We apply the well-established Langmuir approach to analyze and interpret the experimental dynamic sorption observations. Our experimental data predict a lag in ethane production relative to that of methane due to the preferential sorption of ethane on the shale. This provides for added insight into interpreting field-scale production data in terms of the produced gas compositions. The experimental observations and their analysis pave, therefore, a path toward improving the interpretation of production data from shale gas operations via an enhanced understanding of desorption dynamics (and subsequent mass transfer) of gas mixtures in shale.