Effect of the Water Content on the Adsorption of CO2 and CH4 in Calcite Slit Nanopores: Insights from GCMC, MD, and DFT

It is significant to understand the adsorption mechanisms of shale gas (CH₄) and CO₂ in shale formations to enhance CH₄ recovery rates and enable geological CO₂ storage. This study provides a comprehensive investigation into the adsorption behaviors of CO₂ and CH₄ within dry and hydrous calcite nanopores, utilizing a combination of grand canonical Monte Carlo simulations, molecular dynamics simulations, and density functional theory calculations. In dry calcite slits, the calculated results for the adsorption capacity, density profile, and isosteric heat of CO₂ and CH₄ reveal that CO₂ possesses a stronger adsorption affinity, making it preferentially adsorb on the pore surface compared to CH₄. In hydrous calcite slits, calculating the adsorption capacity and density profile of CO₂ and CH₄, the results show that the gas adsorption sites become progressively occupied by H₂O molecules, leading to a substantial decrease in the adsorption capacity of CO₂ and CH₄. Furthermore, by analysis of the adsorption energy and electronic structure, the reason for the reduction of gas adsorption capacity caused by H₂O is further revealed. This work has a deep understanding of the adsorption mechanisms of shale gas and CO₂ in calcite and can offer valuable theoretical insights for the development of a CO₂-enhanced shale gas recovery technology.