PROPERTIES OF BLACK OILS - Petroleum Reservoir Fluid Property Correlations - page 122

Comments on reservoir oil viscosity correlation equations
The very large values of average absolute relative error in tables 3-23,
3-25, and 3-27 do not occur because the researchers proposing the
correlation equations are incompetent. Rather, they reflect the quality of
the reservoir oil viscosity data. Most of these data were measured using an
instrument called a rolling-ball viscosimeter . This device is simple and easy
to use, and thus the procedure is relatively inexpensive. Unfortunately,
in order to produce accurate results with this equipment, the tested
fluid must be a Newtonian fluid. Reservoir oils with high densities, and
thus high viscosities, have components (resins and asphaltenes) that
cause the oils to be non-Newtonian. Thus, the resulting data are very
inaccurate, and any correlation equation prepared using these poor
quality data cannot be any better than the quality of the data. The same
results can be observed in figures 3-33 through 3-44.
Figures 3-38 and 3-44 show that the better correlations fit the
measured oil viscosities reasonably well for oils with viscosities below
approximately 1 cp. Thus, the measured values of oil viscosities below
approximately 1 cp are most likely reasonably accurate. Reservoir oils
with larger viscosities apparently have very large measurement errors,
and these poor results contribute to inaccurate correlation equations.
Further, the use in engineering calculations of oil viscosities higher than
approximately 1 cp as measured with the rolling-ball instrument can
result in inaccurate results.
The relationships among oil density, oil formation volume factor,
and oil compressibility at pressures above the bubblepoint pressure
have already been discussed. Values of these properties are controlled
primarily by the expansion of the reservoir oil as reservoir pressure
declines for pressures above the bubblepoint pressure. This expansion
should also affect reservoir oil viscosities at pressures above the
bubblepoint pressure. So a correlation of reservoir oil viscosities
should take these relationships into account, most likely reservoir oil
density. Abu-Khamsin and Al-Marhoun are the only researchers that
have recognized this. 50 Although their work was somewhat successful,
their equation was devised only for use at bubblepoint pressure. Future
work should explore the relationships of reservoir oil viscosity with
reservoir oil density or even oil compressibility at pressures above the
bubblepoint pressure.