Distribution characteristics and mechanisms of CH₄ and CO₂ adsorption in montmorillonite nanopores in water-bearing and saline environments

Background Carbon capture, utilization, and storage (CCUS) has emerged as a significant way to reduce greenhouse gas emissions under the guidance of the strategic goals of peak carbon dioxide emissions and carbon neutrality. With the continuous exploitation of unconventional hydrocarbon resources such as shale oil and gas, varying water-bearing and salinity conditions exert significant impacts on the occurrence stability and seepage behavior of CO₂, thereby reducing the efficiency and safety of CO₂ storage. Therefore, a thorough understanding of the competitive behavior and distribution patterns of CO₂ and other gases in shale nanopores under varying water-bearing and salinity conditions has become a critical scientific issue in the optimization of CO₂ storage and flooding processes.

Methods To reveal the impacts of the water-bearing and saline environments on the occurrence and competitive behavior of typical gases, this study constructed slit-shaped nanopore models of montmorillonite under different water-bearing and salinity conditions through molecular dynamics simulations. Accordingly, it systematically analyzed the adsorption configurations, density distributions, diffusion behavior, and variations in interaction energy of CH₄ and CO₂ under various conditions.

Results and Conclusions  CH₄ and CO₂ exhibited significant differences in occurrence and migration behavior under dry, water-bearing, and saline conditions. In the dry system, CO₂ formed adsorption layers along montmorillonite surfaces, while the density of adsorbed CH₄showed a bimodal distribution, with a maximum reaching up to 1.78 g/cm³. After water was added to the dry system, water layers covered the montmorillonite surfaces, weakening gas-mineral interactions. Consequently, CO₂ and CH₄ migrated toward the nanopore center. Upon NaCl addition, CO₂ formed secondary adsorption layers at the interfaces, with the peak density of adsorbed CO₂ under a NaCl mass fraction of 20% recovering to about 17% of the peak value in the dry system. With an increase in salinity, both CO₂ and CH₄ displayed decreased self-diffusion coefficients, especially CO₂, suggesting that CO₂diffusion was more significanly restricted. While water films weakened the adsorption potential on the montmorillonite surfaces, Na+ enhanced the interfacial re-adsorption of CO₂ through electrostatic shielding and hydration clustering. The results of this study reveal the molecular-scale CO₂ retainment in water-bearing and saline envionments.