Synthesis of core–shell nano-silica fluids with emulsifying and viscosity-increasing capabilities for enhanced oil recovery under high salinity and high water cut

Nanofluid flooding has emerged as a promising technology for enhanced oil recovery; however, the challenge of achieving effective emulsification and viscosity enhancement under harsh reservoir conditions, such as high salinity and high water cut, remains a significant bottleneck. To address this, we designed and synthesized a novel amphiphilic core–shell nano-silica by grafting polyacrylic acid (PAA) chains and sulfated glucosamine groups onto the silica surface. This structural modification significantly improved the dispersion stability and salt tolerance of the nanoparticles. Subsequently, a high-performance PSD (P: PAA; S: silica; D: D-type glucosamine sulfate) nanofluid was constructed by compounding PSD with a carboxylate surfactant (AEC-9). This study systematically investigates the oil–water–rock interfacial behavior and displacement mechanism of the PSD nanofluid under high-salinity conditions. The results indicate that the PSD/AEC-9 nanofluid can reduce oil–water interfacial tension from 14.0 to 0.93 mN/m (reduced by 93.4%) and alter the surface wettability from oil-wet toward water-wet. Benefiting from the molecular synergistic effect between PSD nanoparticles and AEC-9 surfactant, they form a dense adsorption layer at the oil–water interface, which not only significantly prolongs the interfacial relaxation time but also endows the water-in-oil emulsion with great stability and viscosity-increasing ability. Microscopic visualization and core displacement experiments further confirm that the fluid effectively expands the sweep volume and mobilizes residual oil through the synergistic mechanism of emulsion plugging-wettability reversal, demonstrating great potential for chemical flooding in complex high-salinity reservoirs.