(613f) Adsorption, Desorption and Flow Phenomena of Methane-Ethane Binary Gas Mixtures in Shale Powder and Whole Core Samples

Methane (CH₄)is the largest component of the natural gas produced from shale rocks. Although methane accounts for 87-96 mol% of the gas from these rocks, other components including hydrocarbons such as ethane (C₂H₆) and propane (C₃H₈), nitrogen (N₂) and helium (He) are also found in the gas produced. Methane and the various hydrocarbons are stored in the adsorbed state in the micro and mesopores of the shale rocks, and as free gas in the fracture networks. Though convective and diffusive transport accounts for the short-term behavior in gas production, desorption is thought to dominate the long-term dynamics of shale-gas generation.

The key objective of this study is the investigation of adsorption/desorption and diffusive and convective transport phenomena of methane/hydrocarbon mixtures in shale-gas rocks. In this talk, in particular, we focus our attention on the binary CH₄/C₂H₆mixture, since ethane is typically the second largest component in shale gas, and is thought to compete for the same adsorption sites in shale-gas rocks as methane. We study the adsorption/desorption behavior of this mixture (and its individual components) in ground and whole shale rock samples using thermogravimetric analysis (TGA). The sorption isotherms generated are important to predict the gas storage capacity of the shale samples, while the study of adsorption/desorption dynamics/kinetics help us understand the role of desorption during the later times of gas production. The combination of experiments with whole and ground samples allow us to isolate desorption kinetics from diffusive and convective mass transfer. This, in turn, facilitates our modeling and interpretation of the experimental observations.

The extended Langmuir model and the more sophisticated multicomponent potential theory of adsorption (MPTA) approach are used to model the experimental observations, and together, they provide for improved predictive capabilities of the shale gas equilibrium and dynamic characteristics. In this work, we also study the dynamics of shale-gas production from full-diameter shale-gas cores, under realistic field-scale pressure, temperature, and confining-pressure conditions. Depletion experiments from such cores storing CH₄/C₂H₆ mixtures at various compositions (simulating a model shale-gas) are carried-out, during which the pressure of the core, the flow of gas, and its composition are all continuously monitored and measured. Our experimental observations clearly indicate a lag in ethane production relative to methane due to the preferential sorption of ethane on the shale samples. These experiments, and related analysis and modeling, nicely compliment the studies with the ground and the cube shale samples, and help one to better understand the interplay between adsorption/desorption and transport phenomena that take place during shale-gas production from these rocks.