Insight into the molecular level detachment process of crude oil from the calcite surface by CO2 and CO2/N2 gas injection

CO2 flooding and CO2/N2 mixed flooding have demonstrated great potential in enhancing oil and gas recovery. However, the molecular-level mechanistic details, especially within complex systems involving specific mineral surfaces like calcite, require further clarification. To clarify these processes, we employed molecular dynamics simulations to examine oil phase adsorption on calcite and the microscopic interactions among gas, oil, and calcite walls. The findings reveal that the highly polar compounds, like benzoic acid and n-heptanoic acid, exhibit stronger and more stable adsorption to the calcite surface, making them less prone to removal by pure CO2. In contrast, less polar hydrocarbons like n-heptane and n-dodecane form oil films that are more easily stripped. Additionally, during the mixed oil phase adsorption, polar compounds tend to localize closer to the wall. The gas phase (CO2, CO2/N2) displaces oil molecules primarily through competitive adsorption on calcite and dissolution of oil molecules. The interaction between gas and oil is stronger than that between oil and calcite, highlighting a key condition for effective oil detachment. CO2 exhibits stronger adsorption on calcite and greater oil solubility than N2. Accordingly, the interaction between the gas and oil phases, along with the oil stripping efficiency, increases with the CO2 content. van der Waals energy dominates interactions between CO2/N2 and hydrocarbons, as well as between calcite and these hydrocarbons. In contrast, electrostatic energy governs interactions between calcite and polar compounds. These findings offer molecular-level insights to guide gas injection strategies for enhanced oil recovery in carbonate formation.