Novel SiO2 nanoparticles (NPs) costabilized by a low-molecular-weight ligand (steric stabilization) and a zwitterionic surfactant (electrostatic stabilization) were developed for enhanced oil recovery purpose in sandstone reservoirs with high salinity and elevated temperatures. According to our experiments, the proposed NPs were not sensitive to either monovalent or divalent cations, whose size remained around 10.0 nm in API brine within 8 weeks at 25 degrees C and 4 weeks at 60 degrees C. Using the zwitterionic surfactant with NPs reduced the NP dosage required to induce wettability alteration and the possibility of severe permeability damage. The presence of NPs in return effectively decreased the surfactant adsorption loss on rocks, reduced the surfactant concentration needed to produce a low interfacial tension, and rendered the oil-wet solid surface toward a more water-wet condition beneficial for water imbibition and oil displacement. Core flooding tests showed that the nanofluid composed of 800 mg/L of zwitterionic surfactant and 2000 mg/L of lab-synthesized SiO2 NPs was able to recover extra 3.12 and 5.39% original oil-in-place from Berea sandstone cores in the tertiary recovery mode after surfactant flooding or pure NP flooding, respectively. In addition, thanks to the weak interactions between the zwitterionic surfactant and the surface-modified SiO2 NPs, flexible adjustments can be made to their concentration ratios to customize the desirable nanofluid formulas intended for specific applications.