Page 178 - PVT Property Correlations

152 PVT Property Correlations

FIGURE 7.5 RFT example.

Oil Viscosity
Dynamic or absolute viscosity is defined as resistance to flow exerted by a
fluid (usually measured in centipoise, cp). Viscosity of oil is highly affected
by the oil composition in addition to amount of solution gas and temperature.
It is a measure of the magnitude of internal friction between fluid layers as
they move relative to each other. High viscosity is usually associated with
low GOR oils; lower viscosity is usually seen with high GOR oils. We gen-
erally expect wells with lower viscosity to have higher flow rates, and reser-
voirs with lower viscosity oils to recover more reserves. Static (or kinematic)
viscosity is obtained by dividing the dynamic viscosity by the density of the
fluid.
Fig. 7.6 is a typical oil viscosity plot versus pressure (measured at reser-
voir temperature). The behavior of viscosity is similar to that of density. The
figure shows that oil viscosity decreases with reduction in pressure, due to
reduced friction between fluid layers with the reduction in pressure. The vis-
cosity of oil records a minimum value at the bubble point. With the contin-
ued depletion of reservoir pressure and the release of free gas, the reservoir
oil becomes heavier (i.e., more viscous) due to loss of lighter components to
the free gas. Viscosity values of black oils can differ significantly from one
oil to another.