Enhancing the Stability and Practical Application of CO₂-Responsive Foam with Nanoparticles: The Role of Salt Ions

This study aimed to comprehensively understand the impact of salt ions on the stability and practical application of CO2-responsive foam with nanoparticles. In this paper, we assessed how varying salt concentrations affect the stability of C12A-N20 foam, produced by the nanoparticle N20 coupled with the CO2-responsive surfactant C12A. Key factors such as surface tension, viscoelastic modulus, and bubble liquid film were analyzed. The effect of salt on oil recovery was further examined through sandpack displacement experiments. The results showed that at a salt ion concentration of less than 1.0 × 105 mg·L–1 (transition salt concentration), the impact on foam was minimal, which can meet the actual field application requirements. Na+ and Ca2+ had a slightly greater influence than Mg2+ on the interfacial characteristics of the dispersion. The addition of nanoparticles to the C12A solution reduced the expansion rate of the dark spot by an order of magnitude and significantly increased the stability of the liquid film. Adding salt ions to the C12A-N20 dispersion prevented the gas–liquid interface from stably absorbing the nanoparticles, thus decreasing the liquid film stability. In the displacement experiment, after adding 10 wt % salt ions, the foam effectively increased flow resistance and maintained a high-pressure differential. However, when the transition salt concentration was exceeded, the foam became unstable, the blocking effect deteriorated, and the recovery rate decreased. This indicates that C12A-N20 foam performs best in environments lower than the transition salt concentration. The results provided valuable insights into the influence of salt ions on nanoparticle-coupled responsive foams and their practical application.