Separation Kinetics of Oil/Water Emulsions Stabilized by Nanoparticles

Abstract

A state-of-the-art, Portable Dispersion Characterization Rig (P-DCR) is used to investigate the effect of nanoparticles on oil-water emulsion formation and stabilization. Mineral oil and distilled water are used as the test fluids and separation profiles are obtained from the experiments using a sophisticated software. Spherical silica nanoparticles of average primary particle size of 20 nm were selected as the emulsifying agent, since silica is commonly found in the produced fluids. Nanoparticles of different wettabilities were used to investigate their effect on separation kinetics of solid stabilized emulsions in terms of solid particle concentration, wettability, initial dispersion phase, water-cut, and shearing time. In one series of experiments the emulsions were prepared with intermediate-wet nanoparticles. Both simple water-in-oil (W/O), as well as multiple oil-in-water-in-oil (O/W/O) emulsions were observed. Faster separation occurred when the particles were initially dispersed in oil. Increased nanoparticle concentration, as well as shearing time typically resulted in slower emulsion separation rates. Another series of experiments was performed with hydrophobic and hydrophilic nanoparticles. Very fast separation rates were observed when using hydrophilic silica nanoparticles and 25% water-cut regardless of solid concentration. However, when the water-cut was increased to 50% and 75% very stable emulsions were produced. Emulsions prepared using hydrophobic particles were the most stable across all water-cuts. For the case of 25% water-cut, no water coalescence was observed for a wide range of oil-wet nanoparticle concentrations. Oil creaming was promoted as the concentration of solids decreased, and the emulsions remained oil continuous and highly resistant to water coalescence even for very low solid concentration (100 ppm), resulting in a dispersed phase volume fraction as high as 93%. The effect of nanoparticles on the properties of pure fluids, namely, density, viscosity and surface/interfacial tension is also reported. The main findings of the study include the following: Nanoparticles, even at low concentrations, can significantly decrease separation rates of oil and water emulsions. The Portable Dispersion Characterization Rig (P-DCR) is recommended as an effective way to measure emulsion stability in the field.