Smart Pickering water-in-oil emulsion by manipulating interactions between nanoparticles and surfactant as potential oil-based drilling fluid

Smart Pickering emulsions show great promise for many applications in the oil and gas industry, especially those with pH-responsive behavior that allows them to be easily implemented. However, the pH-responsive emulsion systems reported thus far suffer from a narrow pH window in the stabilization of emulsions, which is less favorable for complicated reservoir conditions. Here we report a smart Pickering water-in-oil (W/O) emulsion system with a wide pH window, based on manipulation of the interactions between fumed silica nanoparticles (NPs) and amine surfactant. The pH-dependent interaction between NPs and amine surfactant was experimentally confirmed by FTIR spectroscopy, turbidity, surface tension, etc. The stable W/O emulsions generated by this system are formed within the range of pH that shows strong flocculation of nanoparticles due to the adsorption of surfactant onto the particle surfaces, which imparts a certain hydrophobicity to the NPs. Outside of the pH window, the similar charge of the NPs and the surfactant generates strong repulsion between them, so the fumed silica NPs cannot be endowed with such hydrophobicity. Due to their highly hydrophilic nature at such pH values, the NPs are not able to stably remain at the immiscible fluid interface, and thus the emulsion system is destabilized. The pH-responsive Pickering emulsion-based drilling fluid shows adjustable rheological properties and is reversible by a cyclic emulsification-demulsification process. We anticipate that this system can be switched "on" over a wide pH range with high stability when running underground and switched "off" in follow-up procedures and waste disposal at the surface.