Methane and carbon dioxide adsorption in kerogen models using molecular simulations

Felipe Perez and Deepak Devegowda

Mewbourne School of Petroleum and Geological Engineering, The University of Oklahoma

Shales are sedimentary rocks composed of (1) a mineral matrix and (2) organic matter. Both media may contribute to the storage of hydrocarbons and non-hydrocarbon species in unconventional reservoirs. In this work, we present a molecular study of methane and carbon dioxide adsorption in realistic 3D models of overmature kerogen that were constructed using a molecular dynamics implementation of a simulated annealing process. The models account for the chemical functionality of kerogen with such degree of maturity, and the densities are consistent with experimental data reported in the literature. These models serve as rigid frameworks for the study of adsorption of methane and carbon dioxide using (1) grand canonical Monte Carlo simulations for methane, and (2) configurational biased/continuous fractional component Monte Carlo simulations for carbon dioxide. Visualization of total uptake of gas molecules exposes the pore geometry of the models and aids to determine the mechanisms of adsorption as a function of fugacity. Our results show that carbon dioxide has approximately 1.5 times the adsorptive capacity of methane in overmature kerogen.