Page 64 - PVT Property Correlations

42 PVT Property Correlations

properties of the more common hydrocarbon and nonhydrocarbon compo-
nents normally found in reservoir fluids.
Critical pressures and temperatures are not usually measured for hydro-
carbon mixtures or for the plus fraction in a composition experiment and
must therefore be estimated from correlations. Critical pressure and tempera-
ture correlations require that either the specific gravity of the gas or the full
composition be known. If the critical properties are estimated for a mixture
(rather than a pure component), they are termed pseudocritical pressure and
pseudocritical temperature.
If the gas-specific gravity is known or estimated, several correlations can
be used for computing pseudocritical pressure and temperature. Examples of
these correlations include Standing (1977) and Sutton (2007). If impurities
exist (nonhydrocarbon components such as CO 2 ,H 2 S, and N 2 ), a correlation
such as Wichert and Aziz (1972) will be used to correct the pseudocritical
properties for the presence of nonhydrocarbons. Note that the Wichert and
Aziz correlation requires the composition of the nonhydrocarbons. Other cor-
relations that correct directly for the presence of nonhydrocarbons also exist
(e.g., Piper et al., 1993; Elsharkawy et al., 2000).
If the full composition of the gas is known, other sets of correlations can
be used to estimate the pseudocritical pressure and temperature for the gas
mixture. The most straightforward are Kay’s rules (Kay, 1936). These mix-
ing rules are calculated as follows:
nc
X
p pc 5 y i p ci ð3:17Þ
i51
nc
X
T pc 5 y i T ci ð3:18Þ
i51
Calculation of pseudocritical properties from composition requires that
critical pressure and temperature be known for the plus fraction. Numerous
correlations exist for calculating the pseudocritical properties for the plus frac-
tion such as C7 1 (Riazi, 2005). These correlations depend on different
inputs. The common inputs are molecular weight and/or specific gravity of the
C7 1 (both are usually measured in the PVT laboratory for the plus fraction)
and normal boiling temperature. In general, for dry gases, the C7 1 mole frac-
tion is a small value. Therefore, the weight of C7 1 critical properties is also
small in the summations given by Eqs. (3.17) and (3.18). Any C7 1 critical
properties correlations can therefore be used. Wichert and Aziz (1972) corre-
lation can then be used to correct for nonhydrocarbon impurities.
Other correlations that use composition and correct for nonhydrocarbon
impurities also exist (e.g., Piper et al., 1993; Elsharkawy et al., 2000). These
correlations and others are given in Appendix B. Their ranges of applicabil-
ity are given in Appendix D.