PROPERTIES OF BLACK OILS - Petroleum Reservoir Fluid Property Correlations - page 124

Table 3-28. Statistics of data set used to evaluate the specific gravities of free gases in the
reservoir at reservoir pressures below bubblepoint pressures (2,453 lines of data from 313 reservoir
fluid studies with worldwide origins)
Laboratory measurement Minimum Median Mean Maximum

Reservoir gas specific gravity, γ gR 0.559 0.776 0.788 1.530

Reservoir pressure, p , psia 311.0 1,214.7 1,488.1 7,200.0

Reservoir temperature, T R , ºF 70.0 186.0 183.2 327.0

Solution gas-oil ratio at p b , R sb , scf/STB 101.0 738.0 721.8 2,424.0

Stock-tank oil gravity, API , ºAPI 11.6 39.6 37.5 63.0

The evaluation led to an ARE of -0.17% and an AARE of 3.76%.
McCain and Hill said that use of the resulting values of reservoir free
gas specific gravities in the correlation equations given in chapter 2 gave
values of gas z-factors and gas formation volume factors within ±2% of
measured values of these properties.
The data of table 3-28 were sorted into 10 subsets, each with
approximately 250 lines of data. The values of ARE and AARE calculated
with these subsets of data are shown in figures 3-45, 3-46, and 3-47.
The data were also sorted on reservoir pressure and reservoir
temperature. The ARE and AARE were consistent across the full ranges
of these conditions and plotted very much like figure 3-45.
The large increase in AARE at high values of solution gas-oil ratios in
figure 3-46 and the large increases in ARE and AARE at high values of
reservoir gas specific gravities in figure 3-47 indicate that equation (3.25)
should not be used to estimate free gas specific gravities for volatile oils.