Phase inversion studies for the pipe flow of surfactant stabilized oil-water emulsions are reported in the present work. Salt water (3.5% NaCl w/v, pH=7.3) and a mineral oil premixed with a lipophilic surfactant (Exxsol D80 + 0.25% v/v of Span 80) were used as the test fluids. The addition of the lipophilic surfactant makes the mineral oil to resemble crude oil behavior. The oil-water mixtures were circulated with a low shear pump in a closed flow loop setup. Four different acrylic pipe diameters (16, 32, 60 and 90 mm) were tested. Water-in-oil stable emulsions were observed from low water fractions up to the inversion point. After inversion, unstable water-in-oil in water multiple emulsions were observed under different flow regimes. The observed regimes are homogeneously dispersed water-in-oil droplets in water, water in-oil droplets in water and water, water-in-oil emulsion bubbles in water and a stratified or eventually annular flow of a water-in-oil emulsion surrounded by water. These regimes depend on the mixture velocity and the local water fraction of the water-in-oil emulsion. The inversion process occurred at a high water fraction (90%). The phase inversion process at this high water cut was observed to occur due to two different conditions, by an increase of the dispersed phase fraction and by an increase of the mixture velocity (shear rate).

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