An Alkali-Free Biosurfactant Formulation for Ultralow Interfacial Tension under High-Temperature and High-Salinity Conditions

Abstract

Developing effective oil-flooding systems with ultralow interfacial tension (IFT) for enhanced oil recovery (EOR) in high-temperature and high-salinity reservoirs remains particularly challenging in the absence of alkali. To address this issue, a novel compound system comprising a biobased zwitterionic surfactant (TEHSQA) and a biosurfactant, surfactin (SF), was developed. The system was systematically characterized using a spinning drop tensiometer, Cryo-TEM, interfacial dilatational/shear rheometer, and QCM-D. The results demonstrated that the compound system exhibited excellent tolerance to high temperature and high salinity. Under alkali-free conditions, ultralow IFT was maintained even at 120 °C and in solutions containing up to 120 g/L NaCl or 18,000 mg/L Ca2+. Cryo-TEM observations revealed that the addition of SF induced a structural transition of aggregates from unilamellar to multilamellar vesicles. Interfacial rheology tests showed that the dilatational modulus increased from 16.53 to 21.41 mN/m, and the shear storage modulus (G′) increased from 2.70 × 10–4 to 4.07 × 10–3 N/m. Additionally, QCM-D measurements revealed a significant positive frequency shift (Δf). These findings indicate that the compound system is well adapted to high-temperature and high-salinity reservoirs without alkali. Furthermore, the compound system significantly enhanced interfacial viscoelasticity, effectively accelerated interfacial film formation, and promoted oil detachment. Overall, such attributes highlight the potential application of the proposed binary surfactant system for EOR flooding operations under harsh reservoir conditions.