Molecular Dynamics of Acid Waste Gas Replacing Oil in Hydrated Mineral Nanopores: Implications for Shale Oil Recovery and Acid Gas Storage

Acid waste gas (H₂S–CO₂ mixture) has great potential in enhancing oil recovery (EOR) and achieving gas storage (GS) in water-bearing shale reservoirs. The process of oil replacement by acid gas and pure CO₂ in hydrated nanopores was compared through molecular dynamics. The EOR and GS microscopic mechanisms were revealed, and the various factors that affect the acid gas-EOR and GS performances were analyzed. The results show that the water film in illite and composite nanopores promotes the migration of desorbed oil toward the middle of nanopores, while the water mass formed in kerogen nanopores retains gas and induced free oil reflux. Therefore, the acid gas-EOR performance of the former and latter are positively and negatively correlated with water content, respectively. The acid gas storage rate in illite and composite nanopores initially increases and then decreases with the increase of water content because the thickened water film increases the gas penetration resistance and gradually occupies the GS space. Only the acid gas storage stability in illite nanopores is positively correlated with water content because the water film prevents the stored gas from escaping outward and compressing the gas adsorption layer. Acid gas outperforms pure CO₂ in both EOR and GS performances in illite and composite nanopores because H₂S enhances the competitive adsorption and inhibits the formation of water columns on walls. The acid gas-EOR and GS performances in hydrated nanopores are positively correlated with the H₂S proportion and buried depth. Although an increase in the nanopore diameter improves the oil recovery, it weakens the GS performance of acid gas. This study provides a theoretical basis for the implementation and optimization of acid gas injection in hydrated shale oil reservoirs.