Tuning Nanofluid Performance via Structure-Driven Interfacial Modulation: Surfactant–Nanoparticle–Alkali–Salt Synergy for Low Salinity Enhanced Oil Recovery

Nanotechnology-driven enhanced oil recovery (EOR) is gaining momentum due to its potential to simultaneously reduce oil–water interfacial tension (IFT) and alter rock wettability. However, designing a chemically stable and performance-optimized nanofluid under reservoir-relevant salinity conditions remains a critical challenge. This study investigates the synergistic effect of silica nanoparticle (SNP) and structurally distinct anionic surfactants single-tail [sodium dodecyl sulfate (SDS)] and double-tail [dioctyl sodium sulfosuccinate (AOT)] dispersed in low salinity water (LSW) formulated using monovalent (NaCl, referred to as LSW 1) and divalent (CaCl2, referred to as LSW 2) salts, for spontaneous imbibition based EOR in Berea sandstone. The novelty of this work lies in systematically dissecting the influence of ion valency, surfactant tail architecture, and alkaline stabilization (NaOH, pH 10) on nanofluid stability, IFT behavior, and oil recovery performance, providing mechanistic insights for tuning nanofluid formulations. Experimental results show that divalent ions significantly destabilize SNP suspensions, while alkali addition improves zeta potential (−50 to −53 mV) and extends dispersion stability up to 10 days. AOT, due to its double-tail configuration, achieved nearly a 2-fold IFT reduction compared to SDS and, when combined with SNP, reduced IFT to ∼0.99 mN/m. Spontaneous imbibition tests revealed that oil recovery increased from 8% with LSW 1, to 19% with LSW 1 + AOT, and further to 29% with SNP-stabilized nanofluids were used. These findings demonstrate that tailored surfactant–nanoparticle–alkali formulations can significantly enhance oil displacement by optimizing interfacial behavior and fluid–rock interactions, especially under ion-specific low salinity conditions. The study provides a robust design framework for next generation nanofluid-based EOR systems.