The rheological behaviour of waxy crude oils is a crucial parameter in the design of pipelines for transportation of crude oil and down-stream processing equipment. Most of these types of crude oils exhibit a combination of time-independent and time-dependent rheological properties. Hence, the measurement and modeling of the flow properties of these oils has been found to be difficult. The process of gel formation is a function of many variables, of which the most important are thermal history, shear rate and composition.
Experiments were performed with prepared samples of paraffinic mixtures, using dodecane or hexadecane as the solvent and a paraffin wax of known composition as the solute. The rheological measurements were performed using a concentric cylinder viscometer. Additional experiments for the wax appearance temperature (WAT) were performed using videomicroscopy, differential scanning calorimetry (DSC), viscometry and a visual method. The effects of mixture composition and shear rate on the apparent viscosity were studied at different temperatures. The dependence of viscosity on composition and shear rate was found to be quite significant; however, the effect of cooling rate was not as appreciable.
At higher temperatures, generally above 30–40 °C, most crude oils behave as simple Newtonian liquids. Below a certain critical temperature (known as wax appearance temperature or WAT), which is unique for a given crude oil, wax crystals start to appear and become suspended in the Newtonian base liquid. As the temperature is reduced further, wax crystals grow in size and precipitate from the solution. The precipitated wax may adhere to cold surfaces, which may give rise to problems during storage, transportation, handling and processing. Moreover, as more wax precipitates with a further lowering of temperature, the rheological behavior becomes distinctly non-Newtonian.1 The non-Newtonian behavior exhibited by these waxy crude oils ranges from time-independent characteristics such as Bingham plasticity and pseudoplasticity to more complex time-dependent characteristics such as thixotropy2,3. Ultimately, the interaction of wax crystals gives rise to an interlocking structural network, resembling polymer gels, that possesses a flow limit or yield stress.4
Because of the presence of gel structure, waxy crude oils display complex rheology, and at least eight parameters affecting their flow behavior have been identified.5 Of these, the more important variables are thermal history, shear history, aging and composition. The effect of these variables has been investigated by many researchers over the years; but some conflicting conclusions have been found.6
In order to understand the rheological phenomena associated with the flow of crude oils, it is necessary to understand their time-dependent rheological behavior. The time-dependent behavior observed is largely due to the breakdown of the crystalline structure formed during cooling. Although the exact nature of the structure is not well defined, it is believed that it is a result of the formation of crystalline complexes by the paraffin, asphaltene and resin constituents. In general, the breakdown of the structure with shear produces a reduction in the apparent viscosity. Recovery after shear is generally slight; a complete recovery is obtained only after reheating and recooling.7
