Geo-storage of hydrogen in organic-rich shales: Multicomponent selectivity of organic nanopores

Abstract

The gas adsorption capacity of kerogen is commonly obtained through experimental isothermal adsorption procedures. These experiments employ mathematical models called adsorption isotherms, which bridge the gap between empirical data and theoretical frameworks to offer a continuous representation of the underlying processes in the depleted shale formation with some fraction of resident adsorbed natural gas. However, an exploration of the adsorption behavior of H2, in the presence of CH4, C2H6, C3H8, and CO2 in organic-rich porous media of varying pore size remains an unaddressed area of study. Considering this, in this study, we conduct a molecular simulation of the adsorption and diffusion behavior of hydrogen (H2), methane (CH4), ethane (C2H6), propane (C3H8), and carbon dioxide (CO2) within a representative kerogen organic nanopore structure. In particular, we studied hydrogen adsorption behavior in organic-rich shale formations in varying nanopore size and multi-gas components. We also evaluate the suitability of different adsorption models (Langmuir, Langmuir-Freundlich, Toth) for depicting the adsorption characteristics of single species. For the multicomponent mixture (H2/CO2/CH4/C2H6/C3H8), we apply Langmuir ratio correlation model (LRC). The findings indicate C3H8 demonstrates the highest adsorption potential, while CH4 ranks lowest, followed by H2, CO2 and C2H6 exhibit intermediate potential. Furthermore, the adsorption of each species is hindered by organic nanopores of 1 nm, as opposed to 2 nm nanopores. For multicomponent adsorption, hydrogen exhibits minimal presence at equilibrium, which can be attributed to its higher diffusivity compared to other gases. Notably, the Langmuir, Toth, and Langmuir-Freundlich models exhibit ability to accurately depict the overall adsorption trends single-component. However, for the multicomponent mixture (H2/CO2/CH4/C2H6/C3H8), the LRC is found reasonable effective to capture the adsorption.