PROPERTIES OF BLACK OILS - Petroleum Reservoir Fluid Property Correlations - page 49

Only three of the sets of correlation equations (Velarde et al., Casey-
Cronquist, and Carlson) permit entering a bubblepoint pressure from an
outside source. 15 All the other correlations require that the bubblepoint
pressure predicted by that particular set of equations be used. So the
Velarde et al. and Casey-Cronquist correlations were evaluated twice. The
correlations were evaluated first with values of bubblepoint pressures
from the laboratory reports and then with values of bubblepoint
pressures from the best available bubblepoint pressure correlation. 16 It
is obvious that an accurate value of bubblepoint pressure, if available,
will improve the quality of the results.
Velarde et al. used approximately one-third of these data in preparing
their correlation equations; Vazquez-Beggs apparently used most of
these data as well as additional data in their research. 17 The other authors
apparently did not use any of these data in preparing their correlations.
In order to determine if the recommended equations are valid
throughout the full range of possible reservoir conditions and oil
properties, the data described in table 3-9 were sorted into 10 groups
of approximately 500 lines of data each. The groups were sorted by
reservoir temperatures, reservoir pressures, and stock-tank oil gravities.
Comparisons of these subsets of the data with five different correlations
are given in figures 3-4, 3-5, 3-6, and 3-7. The Velarde et al. correlations
are clearly the best and are effective throughout the full range of the
data. The Standing and Vazquez-Beggs equations were selected for these
comparisons as they seem to be the most popular in the petroleum
industry. 18 The use of a rearrangement of the Glaso bubblepoint
pressure equation for calculation of solution gas-oil ratios at pressures
below bubblepoint pressure is a misapplication of Glaso's work but
is included here because this seems to be a common practice in the
industry. 19