Viscosity and colloidal properties of concentrated crude oil-in-water emulsions

Abstract

A significant fraction of petroleum reserves in the United States, Venezuela, and Canada are heavy crudes. Water continuous emulsions can be used to transport these highly viscous crudes in pipelines. Many crudes form emulsions with alkaline water due to saponification of natural acids. The viscosity of these anionic emulsions varies with continuous phase ionic strength due to electroviscous effects. Experimental studies were undertaken to determine the effect of NaOH and NaCl concentration on the viscosity, particle diameter, and zeta potential of emulsions of a Boscan heavy crude from Venezuela and a Cold Lake heavy crude from Canada. Oil-in-water emulsions could be formed only for a narrow range of NaOH concentrations. This range was approximately 0.010 to 0.080 mmole/g oil for the Boscan crude and 0.010 to 0.050 mmole/g for the Cold Lake crude. The properties of emulsions exhibited extrema at NaOH concentrations approximately equivalent to the total acid number (TAN) of each crude. This occurred because the maximum surfactant concentration obtainable from the natural acids was reached at the TAN equivalence point. High shear rate viscosity, stability, and zeta potential increased with NaOH addition below this concentration, while particle diameter decreased. Addition of excess NaOH or of NaCl above the equivalence point caused decreases in viscosity and increases in particle diameter at equivalent total sodium ion concentrations. For a given crude, variation of emulsion viscosity with electroviscous effects could be explained by zeta potential and particle diameter changes. However, large differences in the magnitude of electroviscous effects for different crudes were observed. These differences could not be explained by relative zeta potentials, particle diameters, or TAN's. Also, electroviscous effects and the stability of emulsions to aging were modified by addition of tall oil. Saponifiable acids present in tall oil differ from those found in crude oils. These two observations indicate the importance of specific surfactant chemistry on emulsion viscosity and electroviscous effects.