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Oil formation volume factor
The oil formation volume factor (FVF) relates the volume of oil at stock-tank conditions to the volume of oil at elevated pressure and temperature in the reservoir. Values typically range from approximately 1.0 bbl/STB for crude oil systems containing little or no solution gas to nearly 3.0 bbl/STB for highly volatile oils.
Correlations for calculating FVF
Tables 1 and 2 summarize thirty correlations for saturated crude oil systems that have been identified in the literature. For saturated systems, gas is liberated as pressure is reduced below the bubblepoint. This results in a corresponding shrinkage in oil volume, as shown for all of the methods in Fig. 1. The rather large number of correlations preclude the identification of individual methods. The results show a relatively narrow range of oil FVF values determined by all of the correlation methods.
These correlations determine FVF based on the following function.
Solution GOR accounts for the largest change in FVF. Increases in temperature, crude oil gravity, and gas gravity provide a small increase in FVF.
Statistical analysis of correlation performance
Recent studies provide statistical analyses for bubblepoint oil FVF correlations and provide recommendations based on their findings; however, none of these references examines the full set of correlations. Al-Shammasi compiled a databank of 1,345 data points from the literature that was combined with 133 data points from the GeoMark Research database to yield a total of 1,478 data points. These data were used to rank the accuracy of the oil FVF correlations. The ranges and distribution of these data can be found in Table 3 and Fig. 2. Table 4 summarizes correlation performance. The results are sorted by absolute average relative error, which provides a means to rank the methods.
Impact of gravity and GOR
The data were further grouped to examine the impact of crude oil gravity and GOR on consistency of the correlations. Methods proposed by Al-Marhoun, Al-Shammasi, Farshad et al., and Kartoatmodjo and Schmidt showed reliability over a wide range of conditions. The author has experienced good results from both the Standing and Glasø correlations, although they may not have ranked highly with this data set. Fig. 3 summarizes these methods.
Cautions in use of correlations
The correlations were tested against the other parameters used in the derivation of the methods:
- Crude oil API gravity
- Gas gravity
Several methods use multiple equations valid for specified ranges of crude oil gravity. Discontinuities, which are summarized in Fig. 4, can result from the use of this technique to develop a correlation. Furthermore, FVF should increase with increasing API gravity. Fig. 4 shows methods that exhibit nonphysical results.
FVF should increase with increasing solution gas gravity. Fig. 5 shows that a number of correlations predict results opposite to this trend. Correlations listed in Figs. 4 and 5 should be used with caution to avoid problems associated with discontinuities or nonphysical behavior. Limitations imposed by data used in the correlation’s development should be followed.
|Bob||=||oil formation volume at bubblepoint pressure, bbl/STB|
|T||=||temperature, T, °F|
|γAPI||=||oil API gravity|
|γg||=||gas specific gravity, air=1|
|Rs||=||solution GOR, scf/STB|
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- Frick, T.C. 1962. Petroleum Production Handbook, Vol. II, Chap. 18-19. Dallas, Texas: Society of Petroleum Engineers Cite error: Invalid
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