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Absolute viscosity provides a measure of a fluid’s internal resistance to flow. For liquids, viscosity corresponds to the informal notion of "thickness". For example, honey has a higher viscosity than water.
Any calculation involving the movement of fluids requires a value of viscosity. This parameter is required for conditions ranging from surface gathering systems to the reservoir. Correlations for the calculation of viscosity can be expected to evaluate viscosity for temperatures ranging from 35 to 300°F.
Fluids that exhibit viscosity behavior independent of shear rate are described as being Newtonian fluids. Viscosity correlations discussed in this page apply to Newtonian fluids.
Factors affecting viscosity
The principal factors affecting viscosity are:
- Oil composition
- Dissolved gas
Typically, oil composition is described by API gravity only. The use of both the API gravity and the Watson characterization factor provides a more complete description of the oil. Table 1 shows an example for a 35° API gravity oil that points out the relationship of viscosity and chemical makeup recalling a characterization factor of 12.5 is reflective of highly paraffinic oils, while a value of 11.0 is indicative of a naphthenic oil. Clearly, chemical composition, in addition to API gravity, plays a role in the viscosity behavior of crude oil. Fig. 1 shows the effect of crude oil characterization factor on dead oil viscosity. In general, viscosity characteristics are predictable. Viscosity increases with decreases in crude oil API gravity (assuming a constant Watson characterization factor) and decreases in temperature. The effect of solution gas is to reduce viscosity. Above saturation pressure, viscosity increases almost linearly with pressure. Fig. 2 provides the typical shape of reservoir oil viscosity at constant temperature.
Viscosity calculations for live reservoir oils require a multistep process involving separate correlations for each step of the process. Dead or gas-free oil viscosity is determined as a function of crude oil API gravity and temperature. The viscosity of the gas saturated oil is found as a function of dead oil viscosity and solution gas-oil ratio (GOR). Undersaturated oil viscosity is determined as a function of gas saturated oil viscosity and pressure above saturation pressure.
Figs. 3 and 4 summarize all of the dead oil viscosity correlations described in Tables 2 and 3. The results provided by Fig. 4 show that the method proposed by Standing is not suited for crude oil with gravities less than 28°API. Al-Kafaji et al.‘s method is unsuited for crudes with gravities less than 15°API, while Bennison’s method, developed primarily for low API gravity North Sea crudes, is not suited for gravities greater than 30°API.
Comparison of different methods
Fig. 5 provides an annotated list of the most commonly used correlation methods for calculating viscosity. The results illustrate the trend for dead oil viscosity and temperature. As temperature decreases, viscosity increases. At temperatures below 75°F, the method of Beggs and Robinson significantly overpredicts viscosity while Standing’s method actually shows a decrease in viscosity. These tendencies make these methods unsuitable for use in the temperature range associated with pipelines. Beal’s method was developed from observations of dead oil viscosity at 100 and 200°F and has a tendency to underpredict viscosity at high temperature. Dead oil viscosity correlations are somewhat inaccurate because they fail to take into account the chemical nature of the crude oil. Only methods developed by Standing and Fitzgerald take into account the chemical nature of crude oil through use of the Watson characterization factor. Fitzgerald’s method was developed over a wide range of conditions, as detailed in Tables 2 and 3, and is the most versatile method suitable for general use of the correlations listed in that table. Chapter 11 of API Technical Data Book - Petroleum Refining includes a graphic showing the area of applicability for Fitzgerald’s method.
Andrade’s method is based on the observation that the logarithm of viscosity plotted vs. reciprocal absolute temperature forms a linear relationship from somewhat above the normal boiling point to near the freezing point of the oil, as Fig. 6 shows. Andrade’s method is applied through the use of measured dead oil viscosity data points taken at low pressure and two or more temperatures. Data should be acquired at temperatures over the range of interest. This method is recommended when measured dead oil viscosity data are available.
Bubblepoint oil viscosity methods
Correlations for bubblepoint oil viscosity typically take the form proposed by Chew and Connally.  This method forms a correlation with dead oil viscosity and solution GOR where A and B are determined as functions of solution GOR.
Figs. 7 and 8 shows the correlations for the A and B parameters developed by various authors. Fig. 9 shows the effect of the A and B correlation parameters on the prediction of viscosity. This plot was developed with a dead oil viscosity value of 1.0 cp so the effect of solution GOR could be examined. Correlations proposed by Labedi,  Khan et al.,  and Almehaideb do not specifically use dead oil viscosity and solution GOR and were not included in this plot.
Correlations for undersaturated oil
When pressure increases above bubblepoint, the oil becomes undersaturated. In this region, oil viscosity increases nearly linearly with pressure. Tables 6 and 7 provide correlations for modeling undersaturated oil viscosity. Fig. 10 presents a visual comparison of the methods.
|μob||=||bubblepoint oil viscosity, m/Lt, cp|
|μod||=||dead oil viscosity, m/Lt, cp|
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