Dissolution Behavior of Multiester Surfactants in CO ₂ Potentially for Carbon Geological Storage: Experimental Measurements and Molecular Dynamics Simulations

CO₂ flooding is a promising method for greenhouse gas emission reduction and enhancing oil recovery rate. Due to the significantly higher production of CO₂ miscible flooding compared to nonmiscible flooding, enhancing the affinity between CO₂ and surfactants is the key to achieving efficient injection into the formation, promoting the formation of miscible flooding, and improving the geological sequestration efficiency of CO₂. Experimental studies were conducted to investigate the impact of the quantity of CO₂-philic ester groups and the hydrophobic tail chain length on the dissolution performance in CO₂. Combined with molecular dynamics simulations, the interactions between each atom of the surfactants and CO₂ atoms were analyzed in depth, and the correlations between the surfactant molecular structures and their CO₂ dissolution performance were established. The microscopic mechanism was revealed at the molecular scale. The results indicate that the solubility of surfactants strongly depends on the quantity of CO₂-philic groups and the length of the hydrophobic tail chain. CO₂-philic ester groups can enhance the affinity of surfactants for CO₂. However, when the number of ester groups exceeds eight or the tail chain length exceeds C₁₂, their solubility can decrease due to internal spatial effects and conformational limitations. The simulation further reveals that the intermolecular interactions of ester groups with CO₂, along with Lewis acid-Lewis base (LA-LB) interactions, are key factors enhancing the dissolution performance. After adding the multiester surfactants, the uniformity of the mixture of CO₂ and white oil was significantly improved, indicating that this surfactant can effectively promote the mixture of oil and gas, thereby enhancing the oil recovery rate. This study provides an important basis for the optimal design of surfactant structures, which is helpful to realize efficient miscible flooding and promote the development of CO₂ geological storage technology.