PROPERTIES OF BLACK OILS - Petroleum Reservoir Fluid Property Correlations - page 89

Table 3-18. Statistics of data set used to evaluate reservoir oil density and oil formation volume
factor correlation equations at reservoir pressures greater than bubblepoint pressure (3,061 lines of
data from 399 reservoir fluid studies with worldwide origins)
Laboratory measurement Minimum Median Mean Maximum

Bubblepoint pressure, p b , psia 120.7 2,084.7 2,346.3 7,854.0

Pressure, p , psia 127.7 3,514.7 3,561.6 14,915.0

Temperature, T , ºF 70.0 187.0 192.7 320.0

Stock-tank oil gravity, API , ºAPI 11.6 37.6 36.4 57.7

Separator gas specific gravity, γ gSP 0.561 0.784 0.804 1.798

Solution gas-oil ratio at p b , R sb , scf/STB 12.0 594.0 636.9 1,808.0

Oil formation volume factor at p b , B ob , res bbl/STB 1.007 1.346 1.372 2.044

Reservoir oil density, ρ oR , lb/cu ft 32.16 45.32 45.71 60.62

The average relative error of all 3,061 lines of data was -1.44%, and
the average absolute relative error was 2.10% for equation (3.20). The
data set was sliced by stock-tank oil gravities, solution gas-oil ratios at
bubblepoint pressures, reservoir temperatures, and reservoir pressures,
each into eight segments of approximately equal size. The resulting
average relative errors and average absolute relative errors for all slices
were amazingly consistent, with average relative errors varying from
-2.53% to -0.15% and average absolute relative errors varying from 0.98%
to 2.97%. No trends of the errors with these variables were observed
across the full range of the data set.

Oil Formation Volume Factors

There is a change in slope at the bubblepoint pressure of a plot of
oil formation volume factors versus reservoir pressures. This slope is
positive for pressures less than bubblepoint pressure and negative for
pressures greater than bubblepoint pressure. An equation based on a
material balance of the mass of oil leaving the reservoir with the mass
of total surface fluids will be used for estimating values of this property
at pressures equal to and less than bubblepoint pressure. An equation
based on the definition of the coefficient of isothermal compressibility,
c ofb , will be used for calculating values of this property at pressures
greater than bubblepoint pressure. The selection of these equations
ensures that the relationship with pressure is continuous, i.e., the values
of oil formation volume factor coincide at the bubblepoint pressure.