Pipe flow of water-in-crude oil emulsions: Effective viscosity, inversion point and droplet size distribution

The present work is a comparative study of the pipe flow of water-in-crude oil emulsions. The pipe flow of emulsions based on six different crude oils (viscosities from 4.8 to 23.5 mPa s) and salt water (3.5% NaCl w/v, pH=7.3) were investigated experimentally using a small scale flow loop. The formation of the emulsions was induced by the flow shear itself by circulating the oil–water mixture in a closed loop system (pipe ID 2.2 cm). The effective viscosity of the emulsions as a function of the water fraction was calculated from pressure drop measurements. The phase inversion points and the in-situ droplet size distributions were also measured and a SARA analysis was made on all crudes. The point of inversion was observed to be fluid dependent. Not all fluids could be circulated through the inversion point due to prohibitive large pressure drops. All fluids turned to laminar flow conditions as the water cut increases towards the inversion point. The emulsions viscosities of six crudes show a similar increase with water cut up to about 30% where larger differences are observed. The effective viscosities were not sensitive to the mixture velocities (0.4–1.0 m/s for constant water cut). Higher shear rates resulting from increasing the mixture velocity lead to the formation of smaller droplets. As the amount of dispersed water increases and gets closer to the inversion point larger droplets show up. The contribution in volume of these coarse droplets was also observed to be important. The experiments with the six crudes illustrate the complexity of oil–water flows and the challenges in understanding the coupling of surface chemistry and fluid flow.