Effect of precipitated wax on viscosity - A model for predicting non-Newtonian viscosity of crude oils

Viscosities of 18 North Sea oils (API gravity 23.8 to 47.6) have been measu red at temperatures between 40 and 0 degrees C and shear rates ranging from 30 to 500 s(-1). Precipitated wax has a pronounced effect on the viscosity and rheological behavior of these oils. At low temperatures where wax prec ipitation is most extensive, the oils typically behave like pseudoplastic o r viscoplastic fluids. A shear-rate-dependent viscosity model is presented. It is based on a correspondence between viscosity and volume fraction of p recipitated wax and further uses the Casson rheological fluid model. It con tains a Newtonian and two shear-rate-dependent terms. The Newtonian term is similar to the type of viscosity models used for oil/water emulsions. The model correlates 713 measured viscosity data points with an average absolut e deviation of 48%. The model has been tested on three oils not included in the data basis. The non-Newtonian viscosities of these oils (176 data poin ts) were predicted with an average absolute deviation of 47%.