Synergistic Mechanisms of Surfactants and Monovalent Ions for Enhanced Oil Recovery Through Interfacial Properties and Microfluidic Study

In oil and gas development, the oil displacement efficiency of single surfactants is inherently constrained. While synergistic interactions between salt ions and surfactants can enhance displacement performance by modulating interfacial properties and wettability, the underlying mechanisms remain insufficiently understood. This study systematically investigated the synergistic effects of two monovalent salts (NaCl, KCl) and four surfactants through macroscopic characterization of interfacial property and microfluidic displacement experiments using microfluidic device with dead-end structures. The results show that salt type and concentration significantly influence interfacial dynamics. The four selected surfactants exhibit gel-like behavior through molecular self-assembly in aqueous solutions, and their synergistic interaction with salt ions enhances oil displacement efficiency by modulating interfacial characteristics. High-salinity solutions reduce interfacial tension, with CTAB exhibiting a concentration-dependent decrease, while NP-10 behavior is governed by both surfactant and salt concentrations. The presence of Na+ generally resulted in lower IFT, improved interfacial viscoelasticity, and more favorable wettability alteration compared to K+. One-way analysis of variance confirmed that salt type is the main factor affecting recovery rate (p < 0.05). Notably, 0.2% CTAB+50,000 mg/L NaCl combination achieved the highest recovery rate owing to an optimal balance between interfacial adsorption, film viscoelasticity, and wettability alteration. This investigation elucidates the mechanisms driving surfactant–salt synergism and proposes an optimized surfactant and salt formulation to enhance oil recovery through tailored interfacial properties.