Molecular Assessment of Storage Capacity and Enthalpy of Adsorption in Organic-Rich Shale Gas Reservoirs

Storage capacity and differential molar enthalpy of adsorption (or isosteric heat of adsorption) are important parameters to understand the characteristic of heterogeneous materials and catalysts for chemical and energy industry applications. Langmuir isotherm and other single site, dual site, and multilayer isotherms are developed for the prediction of adsorption and enthalpy, with a main assumption of constant isosteric heat of adsorption. However, some experimental and simulation data showed some inconsistencies in terms of heat of adsorption. Exploiting molecular simulation, we provide a first principle estimate of gas hosting capacity and associated thermodynamic properties of nanopores as present in type IID kerogen. The adsorption capacity and enthalpy of adsorption of the organic matter was computed using the grand canonical ensemble combined with the fluctuation method. The data obtained were utilized to assess the predictive power of industry standard models such as the Langmuir isotherm and other single site, dual site, and multilayer isotherms with respect to adsorption and enthalpy. The obtained results suggest that the sorption and thermodynamic properties of kerogen nanostructures are best described by monolayer-multisite isotherms rather than multilayer models. In short, for an adsorption theory to be physically consistent, it should capture both adsorption and isosteric heat.