Molecular characterization of carbon dioxide, methane, and water adsorption in micropore space of kerogen matrix

Abstract

Carbon dioxide and methane interaction with organic matter mostly represented by kerogen heavily impacts carbon storage and enhanced gas recovery in shales. Kerogen pores are dominated by micropore network connecting mesopores and understanding adsorption in kerogen is a key element for estimation of shale capacity to retain and release fluids. In this work, molecular dynamics and Monte Carlo simulations were engaged to study adsorption of carbon dioxide, methane, and water in immature type II-A kerogen. Fluid adsorption induces volumetric swelling and pore space expansion with water imposing the largest impact compared to carbon dioxide and methane. The adsorption isotherms show that water demonstrates the highest uptake (up to 6.2 mmol/g) which is attributed to strong water-kerogen energy interaction and the tendency to form large water clusters. On the other hand, methane and carbon dioxide show similar adsorption (up to 1.5 mmol/g) in the micropore space of kerogen. The analysis of cluster size distributions implies a large extent of a disconnectivity in micropores with adsorbed carbon dixoide and methane, while micropores demonstrate continuity with adsorbed water.