Formulation study on blended salt-tolerant surfactant nanoscale emulsions with tunable hydrophilic heads for application in high-salinity reservoirs

Recent studies indicate that surfactant application in high-salinity reservoirs is limited by the poor solubility and weak interfacial activity of conventional formulations. Hydrophilic-head modification improves brine compatibility, while blending different types of anionic surfactants with complementary headgroup properties enables precise tuning of head composition for better oil–water balance. However, systematic studies on hydrophilic-head-modified binary systems remain scarce. To evaluate salt-tolerant formulations, a structured workflow was applied. Binary blends of G5 and the propylene oxide unit (PO)-head-modified G9 were screened by varying blend ratios to achieve ultralow interfacial tension (IFT). Interfacial and wettability tests under high-salinity and elevated-temperature conditions determined operating ranges and rock–fluid interactions, while emulsion stability and droplet morphology were analyzed via microscopy and zeta-potential measurements. Core flooding with reservoir cores validated enhanced oil recovery performance. The designed binary system achieved ultralow-IFT values down to 1.86 × 10−4 mN/m, far outperforming single components. Hydrophilic-head modification improved solubility and salt tolerance, while the complementary interfacial roles of G5 (linear alkylbenzene sulfonate (LAS)-type) and G9 (PO-head-modified) enhanced dispersion-anchoring behavior at the interface. Wettability alteration toward a more water-wet state and stable oil-in-water emulsions with narrow droplet distributions confirmed favorable interfacial conditions. Core flooding demonstrated an additional 12.2% recovery after waterflooding, aligning with interfacial and emulsion results. The findings verify that combining a PO-head-modified surfactant with a LAS-type surfactant provides an effective, thermally stable formulation for high-salinity reservoirs in Kazakhstan and offers a rational framework for surfactant design in challenging environments.