This property of a fluid indicates its resistance to flow. It is a dynamic property, in that it can be measured only when the fluid is in motion. Viscosity, therefore, is simply the ratio at any shear rate of the shear stress to the shear rate. There are two expressions of viscosity, absolute (or dynamic) viscosity, u, and kinematic viscosity, y. These expressions are related by the following equation:
Fluid viscosity changes with temperature. Liquid viscosity decreases with increasing temperature, whereas gas viscosity decreases initially with increasing temperature and then increases with further increasing temperature.
The best way to determine the viscosity of a crude oil at any temperature is by measurement. If the viscosity is known at only one temperature, Figure 3-9 can be used to determine the viscosity at another temperature by striking a line parallel to that for crudes “A,” “C,” and “D.” Care must be taken to assure that the crude does not have its pour point within the temperature range of interest. If it does, its temperature-viscosity relationship may be as shown for Crude “B.”
Solid-phase high-molecular-weight hydrocarbons, otherwise known as paraffins, can dramatically affect the viscosity of the crude sample. The cloud point is the temperature at which paraffins first become visible in a crude sample. The effect of the cloud point on the temperature-viscosity curve is shown for Crude “B” in Figure 3-9. This change in the temperature viscosity relationship can lead to significant errors in estimation. Therefore, care should be taken when one estimates viscosities near the cloud point.
The pour point is the temperature at which the crude oil becomes a solid and ceases to flow, as measured by a specific ASTM procedure (D97). Estimations of viscosity near the pour point are highly unreliable and should be considered accordingly.
