Viscosity Prediction of Non-Newtonian Waxy Crude Heated at Various Temperatures

Abstract

Viscosities are important parameters for design and operation of crude pipelines. The heating temperature is the major factor affecting viscosities of waxy crude below the wax appearance temperature. Below the abnormal point, waxy crude exhibits non-Newtonian flow behavior with the viscosity dependent on the shear rate. Both of these make determination of the non-Newtonian viscosities of waxy crude a very time-consuming job. On the basis of the model for predicting non-Newtonian viscosity of waxy crudes as a function of temperature and precipitated wax, an approach to predict non-Newtonian viscosity of waxy crude heated to various temperatures has been developed only based on a few measurements. The accuracy of prediction by this approach has been verified by 468 viscosity data from the Daqing crude heated at various temperatures. The totally average relative deviation between the measured and predicted viscosity is 9.42%.

Influence of Wax Precipitation on the Impedance Spectroscopy of Waxy Oils

“…Crude oil is a complex hydrocarbon mixture whose constituents can usually be fractionated into four categories: saturates, aromatics, resins, and asphaltenes. , Besides the resin and asphaltene fractions, which are known to affect the viscosity of the crude oil, saturated compounds, such as paraffin wax, also have a significant impact on the flowability of the crude oil, especially at low temperatures. − It should be noted that the waxy components of crude oil can be further classified into two types: macrocrystalline and microcrystalline waxes based on their molecular structures . Macrocrystalline wax components include n -alkanes with carbon numbers ranging from 17 to 55+ and microcrystalline wax components include high-molecular-weight iso- and cyclo-alkanes. , At temperatures above the wax appearance temperature (WAT), wax molecules are dissolved in oil, whereas at temperatures lower than the WAT, waxes crystallize and remain suspended in the oil, causing a significant increase in the viscosity of the crude oil. − Moreover, the wax-in-oil suspension can lose its flowability entirely with as less as approximately 2–5 wt % of solid dispersed in the oil. − Loss of crude oil flowability due to wax precipitation can cause flow assurance problems during oil production and transportation, ,− which are usually expensive to remediate.…”

Newtonian and Non-Newtonian Viscosity of Waxy Crude Oils and Condensates: A Noncompositional Model

“…It seems that the suspensions viscosity models, which are mostly based on the Einstein’s model, apply the volume fraction of dispersed phase instead of mass fraction. ,, The literature survey shows that most of the viscosity models of the waxy petroleum liquids conform with this rule so far. ,,, On the other hand, consideration of the crucial role of wax precipitation amount prediction in the viscosity modeling causes one to consider the mass fraction instead of volume fraction, because the mass fraction of wax precipitation is obtained directly by the thermodynamic modeling or experimental measurement. − Therefore, this work is intended to relate the mass fraction to the volume fraction of dispersed phase for a suspension and extend this relation to the wax precipitation.…”

“…Under conditions without the presence of wax, established at temperatures above WAT, the oil exhibits Newtonian behavior. The Arrhenius equation is widely used for the prediction of Newtonian fluid viscosity under temperature variation. ,,,, In this work, the following model is presented to predict the viscosity in the Newtonian region based on the Arrhenius equation: …”

“…There are some main categories to model the viscosity of petroleum fluids in the Newtonian and non-Newtonian regions. For Newtonian behavior, these include the Arrhenius equation ,,,,,,− and the corresponding state model; , for non-Newtonian behavior, the Einstein equation, ,, Equation of State (EoS)-based models (e.g., friction theory and expanded fluid (EF) models − ), and conventional non-Newtonian models or modified version of them , (e.g., Herschel–Bulkley or Casson models , ) are mostly used. In the Newtonian region, the Arrhenius equation accurately predicts the viscosity variation, with respect to the temperature, while most of the Arrhenius-based models are adjusted for a specified oil rather than having the generality over a wide range of oil API gravity.…”

“…Also, the lack of generality is seen for the Einstein-equation-based models for the non-Newtonian modeling. , On the other hand, about the being compositional or noncompositional viscosity model, the corresponding state model for the Newtonian region and the friction theory and EoS-based models for the non-Newtonian behavior are basically compositional, in that it may be a disadvantage when the composition of oil is not available. Also, in some models, for the prediction of apparent viscosity of an oil at a specified temperature related to its wax precipitation percent, the apparent viscosity measurement should be performed at least for two or three points to obtain model parameters for a specified oil. , Thus, most of these models lose their generality property because they are not extendable to other oils and, in addition, for a specified oil, the prediction of rheological behavior is impossible without the rheological measurement. − …”