Effects of Divalent Cations on the Interfacial Properties of Surfactant-Crude Oil Systems and Their Implications for Ion-Tuned Waterflooding.

The integration of ion-tuned waterflooding with surfactant flooding has emerged as a promising strategy for enhanced oil recovery (EOR), yet the mechanisms governing ion-surfactant interactions at fluid-fluid and fluid-solid interfaces remain insufficiently understood. This study systematically investigated the effects of divalent cations (Ca2+ and Mg2+) on the interfacial behavior and displacement performance of cationic, zwitterionic, and nonionic surfactants. Surfactant solutions were prepared with low-salinity (1000 mg/L) and high-salinity (50000 mg/L) CaCl2 and MgCl2 brines. Interfacial tension, dilatational modulus, and contact angle measurements were conducted to characterize fluid-fluid and fluid-solid interactions, followed by pore-scale displacement experiments in microfluidic devices containing dead-end pore structures. The results demonstrated that the influence of divalent cations strongly depended on the surfactant type and salinity. For the cationic surfactant, high-salinity CaCl2 significantly reduced the IFT, whereas MgCl2 exhibited a weaker IFT-reduction effect. Divalent cations progressively decreased the dilatational modulus of the cationic surfactant interface, indicating an enhanced interfacial mobility. In contrast, the zwitterionic surfactant exhibited minimal changes in IFT but showed an increased dilatational modulus upon salt addition, suggesting the formation of a more rigid interfacial film through ion-mediated intermolecular interactions. The nonionic surfactant displayed limited sensitivity to divalent cations with only moderate variations in IFT and interfacial rheology. Wettability measurements revealed that divalent cations altered the adsorption behavior of the cationic surfactant on quartz surfaces, while their influence on the zwitterionic and nonionic surfactants was comparatively weak. Microfluidic displacement experiments showed that higher oil recovery correlates with a balance between interfacial tension and dilatational modulus, particularly when their values are comparable. Statistical analysis further identified salinity as the dominant factor controlling oil recovery and revealed a significant synergistic interaction between selected surfactant species and salinity with the largest effect size. These findings provide mechanistic insights into ion-surfactant interactions and offer guidance for the design and optimization of ion-tuned surfactant flooding processes for enhanced oil recovery.