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 heavier hydrocarbons are stored in an 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 transport phenomena of methane/hydrocarbon mixtures in shale-gas rocks.
We focus our attention on the pure components of methane and ethane and their binary mixtures, 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 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. A dynamic Langmuir-type sorption model is proposed, that allows us to isolate desorption kinetics from diffusive mass transfer. This, in turn, facilitates our modeling and interpretation of the experimental observations.
The experimental observations and their interpretation pave a path to improve the interpretation of production data from shale-gas wells by leveraging an improved understanding of desorption dynamics and mass transfer of natural gas mixtures in shale.