Pipeline Flow of Heavy Crude Oil Emulsions

Abstract

Abstract Using a 53 mm test pipeline, emulsions containing up to 67% Lloydminster heavy crude oil in water have been transported in laminar and turbulent flow. Pressure drops, droplet size distributions, viscometry and velocity distribution measurements were made. The results showed that a homogeneous model was appropriate for emulsions with median droplet sizes near 30 microns. Introduction Pipeline flow of oil in water emulsions occurs widely in field operations and has also been studied systematically in laboratory conditions. Zakin et al.(1), used oils with specific gravities between 0.78 and 0.923 in emulsions formed in a colloid mill. The emulsions were non-Newtonian at oil concentrations of 50% by weight and above. Turbulent flow pressure drops were between 8% and 26% lower than values predicted using their laminar flow behaviour. These predictions used the Dodge and Metzner(2) correlation for turbulent non-Newtonian friction which has been largely supplanted by those of Wilson and Thomas(3) and Thomas and Wilson(4). Pal and Rhodes(5) used a mineral oil of comparatively low viscosity and inferred effective viscosities from the laminar and turbulent flow pressure gradients, assuming Newtonian fluid behaviour. With 45% and 55% oil in water emulsions, laminar flow effective viscosities were significantly lower than those in turbulent flow. The droplet size was unknown, however. With heavy crudes, inversion(6,7) of an oil-in-water emulsion to a water-in-oil emulsion can occur during pipeline flow with a catastrophic increase in pipeline pressure drop if the oil wets the pipe wall. On the other hand Layrisse et al.(8), found stable emulsions for a wide range of concentrations (to 65% oil) and shear rates with droplets whose mean diameters ranged between 6 and 86 ?m. Good agreement was found between viscosities inferred from pipeline pressure drops and values measured with Couette viscometers. The rheology was strongly dependent on droplet size, however, and non-Newtonian behaviour was evident for smaller diameters, low shear rates and high oil concentrations. Turbulent flow pressure drops appeared to be within 20% of the predicted values. In contrast with these results, Wyslouzil et al.(9), found pipeline pressure drops to be significantly lower than values predicted using measured viscosities, in both the laminar and turbulent flow regions. The disparity was attributed to droplet migration away from the wall, producing a region of depleted concentration. With prolonged recirculation in a closed loop, an instability occurred which resembled the inversion reported by Gillies et al.(6) The initial droplet size in the Wyslouzil experiments was in the range of 5 – 10 µm. Gillies and Shook(7) have reported experiments conducted with high oil concentrations in laminar pipe flow. Differences between the apparent viscosities in pipe flow and those measured in a Couette viscometer were also attributed to droplet migration and evidence for this was obtained in the form of velocity distributions measured with a pitot tube. In contrast with the droplets in the Wyslouzil et al. experiments, the droplets were too large to allow their diameters to be determined photographically.