Adsorption based realistic molecular model of amorphous kerogen

Abstract

This paper reports the results of Grand Canonical Monte Carlo (GCMC)/molecular dynamics (MD) simulations of N₂ and CO₂ gas adsorption on three different organic geomacromolecule (kerogen) models. Molecular models of kerogen, although being continuously developed through various analytical and theoretical methods, still require further research due to the complexity and variability of the organic matter. In this joint theory and experiment study, three different kerogen models, with varying chemical compositions and structure from the Bakken, were constructed based on the acquired analytic data by Kelemen et al. in 2007: ¹³C nuclear magnetic resonance (¹³C-NMR), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near-edge structure (XANES). N₂ and CO₂ gas adsorption isotherms obtained from GCMC/MD simulations are in very good agreement with the experimental isotherms of physical samples that had a similar geochemical composition and thermal maturity. The N₂/CO₂ uptake by the kerogen model at a range of pressure shows considerable similarity with our experimental data. The stronger interaction of CO₂ molecules with the model leads to the penetration of CO₂ molecules to the sub-surface levels in contrast to N₂ molecules being concentrated on the surface of kerogen. These results suggest the important role of kerogen in the separation and transport of gas in organic-rich shale that are the target for sequestration of CO₂ and/or enhanced oil recovery (EOR).