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Experimental data are presented for the viscosity, pour point, and wax appearance temperature of a stabilized, waxy North Sea crude oil treated by 12 different commercial wax crystal modifiers, all of which may potentially act both as wax deposition inhibitors and pour point depressants. The viscosity data cover the temperature range from 40 to 5 °C. In general the studied chemicals only marginally influence the wax appearance temperatures whereas the majority has a pronounced effect on pour points and apparent viscosity. The viscosity data suggest that the inhibitors, probably by some kind of steric hindrance, "inactivate" wax components within a certain range of molecular weight by preventing them from building of network structures. It is shown that this effect can be modeled by assuming a lowering of the melting temperatures of the affected range of wax molecules.

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Viscosities of 18 North Sea oils (API gravity 23.8 to 47.6) have been measured at temperatures between 40 and 0 °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 precipitation is most extensive, the oils typically behave like pseudoplastic or viscoplastic fluids. A shear-rate-dependent viscosity model is presented. It is based on a correspondence between viscosity and volume fraction of precipitated wax and further uses the Casson rheological fluid model. It contains 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 absolute 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 points) were predicted with an average absolute deviation of 47%.

Most crude oils contain high molecular weight components, which at low temperatures may precipitate as a wax phase. This may cause plugging of pipes and numerous other problems. This paper presents a solid-liquid equilibrium based model for the description of wax formation. The model for the Gibbs energy contains a contribution based on Flory's theory of multicomponent polymer solutions and a contribution from a metastable subcooled state which oil mixtures may attain. The latter is formulated in terms of the surface tension of the wax phase. Experimental wax appearance points (temperatures), WAP's, are reported for 17 different stabilized North Sea crude oils. The values predicted by the new model are in very good agreement with the experimental WAP's.