Ethoxy Chain Length Regulated Gemini Polymeric Surfactants for Enhanced Oil Recovery in High-Salinity Reservoirs

Abstract In high-salinity reservoirs, conventional polymer flooding systems often suffer from severe viscosity loss, leading to reduced oil recovery efficiency. To address this challenge, a series of Gemini surfactant-functionalized polymer with tunable ethoxy (EO) chain lengths (AASD-O7, AASD-O10, AASD-O13, and AASD-O15) were synthesized. The influences of EO chain length and ion-specific effects of Na+ and Ca2+ on rheological properties, interfacial behavior, aggregation characteristics, emulsification performance, and oil displacement efficiency were systematically evaluated. Molecular dynamics simulations were employed to elucidate the effects of EO chain length and salt ions on molecular association, salt tolerance, and oil displacement performance. The results demonstrated that EO chain length played a critical role in regulating the hydrophilic–hydrophobic balance and salt tolerance of the polymeric surfactants. Their viscosity, surface activity, and interfacial activity exhibited a pronounced nonmonotonic dependence on EO chain length, with AASD-O10 showing the optimal overall performance. Under high-salinity conditions, Na+ primarily enhanced hydrophobic association and chain entanglement through electrostatic shielding, whereas Ca2+ induced intermolecular ion-bridging networks via multipoint coordination with carboxyl groups, resulting in distinct ion-specific regulation mechanisms. Benefiting from the optimal balance between hydrophobic association and EO chain interactions, AASD-O10 maintained excellent interfacial activity in simulated formation water, reducing the oil–water interfacial tension to the order of 10–3 mN/m and forming stable oil-in-water (O/W) emulsions with a heavy-oil viscosity reduction rate over 99%. Microscopic displacement experiments demonstrated that all AASD polymeric surfactants outperformed conventional partially hydrolyzed polyacrylamide (HPAM), with AASD-O10 achieving the highest oil recovery of 76.67%, which was 22.37 percentage points higher than HPAM. Molecular dynamics simulations further revealed the intrinsic relationships among EO chain length, ion-specific interactions, molecular association structures, and macroscopic oil displacement performance. These findings provide new insights into the design of salt-tolerant polymeric surfactants and offer a promising strategy for enhanced heavy-oil recovery in high-salinity reservoirs.