Adjustment of Hydrophilic-Lipophilic Balance of Hexamethyldisilazane-Modified Nanosilica for Enhanced Oil Recovery

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.