Methane and CO₂ Adsorption Capacities of Kerogen in the Eagle Ford Shale from Molecular Simulation

Over the past decade, the United States has become a world leader in natural gas production, thanks in part to a large-fold increase in recovery from unconventional resources, i.e., shale rock and tight oil reservoirs. In an attempt to help mitigate climate change, these depleted formations are being considered for their long-term CO₂ storage potential. Because of the variability in mineral and structural composition from one formation to the next (even within the same region), it is imperative to understand the adsorption behavior of CH₄ and CO₂ in the context of specific conditions and pore surface chemistry, i.e., relative total organic content (TOC), clay, and surface functionality. This study examines two Eagle Ford shale samples, both recovered from shale that was extracted at depths of approximately 3800 m and having low clay content (i.e., less than 5%) and similar mineral compositions but distinct TOCs (i.e., 2% and 5%, respectively). Experimentally validated models of kerogen were used to the estimate CH₄ and CO₂ adsorption capacities. The pore size distributions modeled were derived from low-pressure adsorption isotherm data using CO₂ and N₂ as probe gases for micropores and mesopores, respectively. Given the presence of water in these natural systems, the role of surface chemistry on modeled kerogen pore surfaces was investigated. Several functional groups associated with surface-dissociated water were considered. Pressure conditions from 10 to 50 bar were investigated using grand canonical Monte Carlo simulations along with typical outgassing temperatures used in many shale characterization and adsorption studies (i.e., 60 and 250 °C). Both CO₂ and N₂ were used as probe gases to determine the total pore volume available for gas adsorption spanning pore diameters ranging from 0.3 to 30 nm. The impacts of surface chemistry, outgassing temperature, and the inclusion of nanopores with diameters of less than 1.5 nm were determined for applications of CH₄ and CO₂ storage from samples of the gas-producing region of the Eagle Ford Shale. At 50 bar and temperatures of 60 and 250 °C, CH₄ adsorption increased across all surface chemistries considered by 60% and 2-fold, respectively. In the case of CO₂, the surface chemistry played a role at both 10 and 50 bar. For instance, at temperatures of 60 and 250 °C, CO₂ adsorption increased across all surface chemistries by 6-fold and just over 2-fold, respectively. It was also found that at both 10 and 50 bar, if too low an outgassing temperature is used, this may lead to a 2-fold underestimation of gas in place. Finally, neglecting to include pores with diameters of less than 1.5 nm has the potential to underestimate pore volume by up to 28%. Taking into consideration these aspects of kerogen and shale characterization in general will lead to improvements in estimating the CH₄ and CO₂ storage potential of gas shales.