Optimal Separation of CO₂/CH₄/Brine with Amorphous Kerogen: A Thermodynamics and Kinetics Study

Carbon dioxide (CO₂)-enhanced oil recovery and sequestration are both processes that are associated with the separation and storage of gas in organic-rich shale formations. The current study investigates the applicability of kerogen, an amorphous and insoluble organic matter abundant in unconventional shale formations, for the separation of the mixture of gases (CO₂ and CH₄) in dry and wet (brine) conditions for an effective storage and injection operation. Here, through molecular dynamics, thermodynamics, and kinetics, we investigate the CO₂ transportation and adsorption behavior on three-dimensional kerogen molecular models from the Bakken, which contains nonperiodically arrayed functional groups. The diffusion/separation of CO₂ and CH₄ is probed subject to a varying range of concentrations as well as pressure from atmospheric to high (30 bar) and realistic temperatures (333-393 K) to represent an unconventional reservoir system. It is found that kerogen models from the Bakken would demonstrate an unprecedented CO₂ sorption selectivity over methane in the presence of brine (formation or interstitial water, a mixture of water and salt). Moreover, the concentration of brine shows a positive effect for CO₂/CH₄ selectivity that supports our goals of sequestration and enhanced production. Based on the quantitative results, the developed kerogen model is suggested as an appropriate framework for CO₂ sequestration and injection to further facilitate hydrocarbon-improved recovery in organic-rich shale reservoirs and promote sequestration in a major shale formation.