Phase Inversion of Pickering Emulsions by Electrolyte for Potential Reversible Water-in-Oil Drilling Fluids

Pickering emulsions show great potential for the petroleum industry, especially those that can realize emulsion inversion in basic conditions. Developing a simple and green approach to control emulsion inversion in basic conditions is highly desirable for harsh conditions and complicated operations. Here we report novel phase inversion of a Pickering emulsion through the controlled wettability of silica nanoparticles (NPs) by N-(2-((2-aminoethyl)­amino)­ethyl)­octadecenamide (C₁₈PDA) with Na₂CO₃ salt addition. A transitional phase inversion from an oil-in-water (O/W) emulsion to a water-in-oil (W/O) emulsion occurs with increasing C₁₈PDA surfactant concentration for a given amount of Na₂CO₃ salt. A subsequent phase inversion from a W/O emulsion to an O/W emulsion then occurs with increasing Na₂CO₃ salt concentration at a low amount of C₁₈PDA surfactant, and demulsification occurs when the Na₂CO₃ salt concentration is above 94 mM. The stable W/O Pickering emulsions are formed by virtue of adsorption of the C₁₈PDA surfactant onto the silica surfaces, which imparts hydrophobicity to the inherently hydrophilic silica NPs and facilitates strong flocculation of the silica particles in the polar solution. However, the hydrophobicity of the silica NPs is reduced by salinity and is suitable for creating O/W emulsions due to the reduced interactions between the NPs and the C₁₈PDA surfactant. The possible mechanism for emulsion inversion was investigated by characterizing turbidity, surface tension, contact angle, Fourier transform infrared spectroscopy with thermogravimetric analysis, zeta potentials, etc. The rheological properties of the reversible Pickering emulsion show that there is a smooth transition at the transitional inversion point, which differs from conventional surfactant-stabilized emulsions, in which the viscosity decreases to the minimum at the transitional inversion point. The emulsion phase inversion was also found to be conducive to improving the efficiency of filter-cakes cleanup. We anticipate this smart system to be easily operated and environmentally friendly, making it potentially applicable to real oil fields.