An Experiment and Molecular Dynamics Simulation of Synergistic Foaming between a Surfactant and CO₂ and the Structure–Activity Effect

CO2 foam fracturing in tight and shale reservoirs is a revolutionary technique for commercially viable production. Nevertheless, the screening of foaming agents used in CO2 foam fracturing fluid and the understanding of foaming mechanisms have not been sufficiently investigated. This study aimed to provide a comprehensive method for evaluating and selecting an optimized foaming agent for CO2 foam fracturing fluid integrating macroscopic and microscopic approaches through laboratory experiments and molecular dynamics simulations. The relationship between the molecular structure of the foaming agent and its corresponding foaming effect was elucidated by taking the interaction between CO2 and the foaming agent into account. Foam evaluation experiments indicated that the anionic surfactants exhibited superior foaming capacity and inferior stability compared to zwitterionic and non-ionic surfactants. The molecular dynamics simulation results demonstrated that the foaming mechanism of the CO2 foaming agent relied on the equilibriums between CO2-surfactant, CO2-water, and surfactant–water interactions. At the same time, it was found that if the molecular structure of the surfactant contained functional groups that could produce hydrogen bonding with CO2, the stability of the foaming effect improved to a certain extent, but the foaming volume was not obvious. The classic hydrophilic–lipophilic balance (HLB) theory was not applicable when screening the CO2 foaming agents. It was found that the ionic surfactants with CO2-philic groups and linear structures were suitable as the main foaming agents for CO2 foam fracturing fluids, while non-ionic surfactants with significant steric hindrance were suitable as auxiliary foaming agents. This study provides valuable guidance for selecting cost-effective foaming agents on-site and adds to the understanding of the relationship between the molecular structure of foaming agents and their foaming effects.