An aqueous nanofluid containing superhydrophobic silica nanoparticles with a high surface activity and an average size of 7 nm was used to enhance the water injection of a low-permeability well. The mechanism for the aqueous nanofluid to enhance water injection was discussed. Findings indicate that the silica aqueous nanofluid can greatly increase the effective water permeability even after injecting water for 2100 pore volumes. This is because the hydrophobic silica nanoparticles can be well adsorbed onto the surface of the porous channels to cause hydrophilic to hydrophobic transformation. Both the hydrophobic capillary force and adhesion work contribute to increasing water injection; and in particular, there is a critical point in the pressure-permeability curves for the rock cores with different wettabilities. Only above the critical point, the hydrophobic rock core exhibits a higher effective water permeability than that of the hydrophilic one, which is imperative for drag reduction. Moreover, the hydrophobic rock core surface has a remarkable self-cleaning ability and can reduce the expansion ratio of clay and inhibit the formation of scale in association with the increase of effective porosity via decreasing the hydration film amount. This approach, highlighting the important role of wettability alteration in increasing water injection, could potentially promote the application of a silica aqueous nanofluid in enhanced oil recovery.

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Different dosages of hexamethyldisilazane (denoted as HMDS), a silane coupling agent, were adopted to modify nanosilica (denoted as NS) to afford a series of HMDS-NS nanoparticles with different hydrophilic-lipophilic balance governed by the amount of surface hydroxyl. The amounts of the hydrophilic hydroxyl of the as-prepared HMDS-NS nanoparticles and their water contact angles were measured, and their dispersing behavior in water and oil was examined in relation to their transfer behavior therein. Moreover, the effects of the as-prepared HMDS-NS nanofluids on the oil–water interfacial tension as well as the oil recovery were investigated based on interfacial tension measurements and simulated rock core flooding tests. Findings indicate that the hydrophilic-lipophilic balance of HMDS-NS nanoparticles highly depends on the amount of the surface hydroxyl, and the surface hydroxyl amount can be well adjusted by properly selecting the dosage of HMDS modifier. Besides, the transfer behavior of HMDS-NS nanoparticles in oil and water is closely related to their hydrophilic-lipophilic balance, and they can greatly reduce the oil–water interfacial tension and increase the oil recovery by 7.7–11.1% as compared with conventional water flooding. This is because the surface grafting of the hydrophobic segments of HMDS leads to a significant increase in the hydrophobicity of nanosilica, thereby changing the wettability of oil on the sand surface and favoring the stripping of oil droplets. Particularly, the HMDS-NS nanofluid obtained with 2[Formula: see text]wt.% of HMDS modifier has a water contact angle of 83.6∘ and can dramatically reduce the oil–water interfacial tension from 20.22[Formula: see text]mN/m to 0.28[Formula: see text]mN/m, showing desired hydrophilic-lipophilic balance and potential for enhanced oil recovery associated with chemical flooding.