PROPERTIES OF BLACK OILS - Petroleum Reservoir Fluid Property Correlations - page 103

Recommended equation for estimation of oil formation volume
factors at reservoir pressures greater than bubblepoint pressure
Although the ARE values reported in figures 3-29 through 3-32 have
mixed results, equation (3.22) was consistently best on AARE. Further,
use of equation (3.22) with B ob from equation (3.21a) ensures that the
values of oil formation volume factors are continuous as reservoir
pressure decreases through the bubblepoint pressure.
The expansion of the oil in the reservoir as pressure decreases from
initial pressure to bubblepoint pressure is the controlling factor for the
changes in the values of reservoir oil densities, oil formation volume
factors, and oil compressibilities. Thus, there are relationships among
these properties for pressures greater than bubblepoint pressure.
The use of equation (3.20) for oil density and equation (3.22) for oil
formation volume factor, both tied to oil compressibility, equation
(3.13), preserves these relationships. Note that equations (3.20) and
(3.22) are not correlations; they follow directly from the definition of
oil compressibility.
Viscosities of Reservoir Oils
There is a change in slope at the bubblepoint pressure of a plot of
reservoir oil viscosity versus reservoir pressure. This slope is negative
for pressures less than bubblepoint pressure and positive for pressures
greater than bubblepoint pressure. Two sets of correlation equations
will be used for estimating values of this property for pressures less than
bubblepoint pressure and pressures greater than bubblepoint pressure.
Care must be taken in the selection of these equations such that the
relationship is continuous, i.e., the values of oil viscosity coincide at the
bubblepoint pressure.
Evaluation of proposed correlation equations for reservoir oil
viscosities at reservoir pressures equal to bubblepoint pressure
Statistics of the data set used to evaluate reservoir oil viscosity
correlation equations for reservoir pressures equal to bubblepoint
pressure are shown in table 3-22.
An evaluation of the published correlation equations for reservoir
oil viscosities at reservoir pressures equal to the bubblepoint pressure of
the reservoir oil is given in table 3-23.