Covalently Hybridized Amphiphilic Core–Shell Nanofluids for Enhanced Oil Recovery in Moderate-Salinity Reservoirs: Balancing Colloidal Stability and Interfacial Activity

The deployment of amphiphilic nanomaterials in reservoir environments is often hampered by the stability-activity trade-off, where physical desorption leads to structural failure. To overcome this, we developed a covalent core–shell nanohybrid (DNM) by anchoring a sulfonate-rich AM/AMPS copolymer shell onto a rigid KH570-modified silica core via in situ polymerization. This covalent grafting strategy effectively mitigates the shell detachment issues prevalent in physically adsorbed systems. The resulting “rigid-core/flexible-shell” architecture exhibits a salinity-adaptive behavior, achieving optimal dispersion at ∼20,000 mg/L TDS through a balance of electrostatic and Hofmeister effects. While the interfacial tension reduction is moderate (∼0.29 mN/m), the DNM significantly alters rock wettability from oil-wet (119.3°) to water-wet (41.3°) via a structural wedge mechanism. Dynamic core flooding demonstrated a 35% enhancement in oil recovery, driven by the synergistic coupling of the “Log-jamming” effect for mobility control and interfacial stripping for oil mobilization. These findings confirm that covalent hybridization provides a robust molecular solution for mobilizing residual oil in moderate-salinity reservoirs.