Page 234 - PVT Property Correlations

204 PVT Property Correlations

problem of estimating the best correlation(s) to use. Many practicing engi-
neers follow a completely arbitrary process to select PVT correlations.
Others may follow more structured approaches such as those discussed in the
following section.

SELECTION OF PVT OIL PROPERTIES CORRELATIONS

Numerous PVT correlations exist for each oil (and gas) property. The number
of oil correlations available exceeds that of gas correlations (Appendix A:
Oil Correlations Formulae and Appendix B: Gas Correlations Formulae).
Oil and gas PVT correlations can usually be grouped into correlation
families. Researchers of PVT correlations usually develop a separate correla-
tion for each PVT property from their PVT laboratory-measured data.
For selection of correlation(s) to use for a specific fluid, the following
approaches are available: (1) use of geographical location, (2) use of the
type or obvious characteristics of the fluid, (3) use of correlations based on
application, (4) use of data ranges and ranking, and (5) use of an expert
system or set of rules.

Correlation Selection Based on Geographical Location

Many oil PVT correlations were developed with data sets that belong to a
specific region. Among the commonly known geographical correlations are
Al-Marhoun (1988) correlations for Saudi Arabian crudes; Standing (1947)
for California crudes; Petrosky (1990) for Gulf of Mexico crudes; Hanafy
et al. (1997) for Egyptian crudes; and Omar and Todd (1993) for Malaysian
crudes. Appendix C, Oil Correlations Range of Applicability, gives the appli-
cation ranges of each correlation and identifies the geographical origin of
each correlation, where applicable.
In absence of laboratory-measured PVT data, many engineers use specific
correlation families for crude oils based on geographical origin. Although it
is logical to assume that oil in different reservoirs in a particular geographi-
cal location is of the same origin, it is erroneous to assume that this approach
is consistently reliable. The geographical PVT correlation families may have
been developed from PVT data collected from relatively shallow reservoirs.
Deeper reservoirs developed at a later stage usually contain more volatile oil
and richer gas condensate fluids. New correlations are required for the fluids
more recently discovered in these traditional regions when sufficient data
becomes available. Therefore, caution is advisable in use of the geographical
correlation approach.
In addition, Al-Shammasi (2001), in a large comparison study, showed
that geographical location-based correlations are not guaranteed to work.