PROPERTIES OF RESERVOIR GASES - Petroleum Reservoir Fluid Property Correlations - page 29

temperatures of 300ºF to 400ºF and pressures of 10,000 to 30,000 psia.
The ARE was 1.312% and the AARE was 1.312%. No particular trends in
ARE or AARE with either temperature or pressure were observed.
The effect of nonhydrocarbon components is unknown. However,
equations (2.3)-(2.8) predict reasonably accurate values of z-factors
for fairly large values of nonhydrocarbon content, as can be seen in
tables 2-2 and 2-3. Thus, these equations can be used with reasonable
confidence for HPHT conditions until sufficient data are available to
prepare HPHT correlations.
Recently some z-factor data for four gases with carbon dioxide
contents of 0, 5, 10, and 20 mol%, pressures up to 20,000 psia, and
temperatures of 300ºF and 400ºF were presented in graphical form. 41
The hydrocarbon portions of these gases were 96 mol% methane, with 3
mol% ethane and 1 mol% propane. The Piper et al. and DAK equations
[equations (2.3)-(2.8)] gave z-factors that fit these data to within 1% for
pressures from 3,000 to 20,000 psia.
Recommended correlation equations/procedures for estimation
of gas z-factors at HPHT
Thus equations (2.3)-(2.8) are recommended for estimating gas
z-factors at HPHT until additional HPHT data using naturally occurring
petroleum gases become available.
Gas viscosities at HPHT
Viswanathan modified the LGE gas viscosity equations for HPHT
conditions by fitting them to the NIST pure methane gas viscosity
values. 42 The equations produce values that fit the NIST pure methane
gas viscosity values (300ºF to 400ºF and 10,000 to 30,000 psia) with an
ARE of -0.707% and an AARE of 0.707%. The data were sorted and split
by temperature and also by pressure, and no particular trends in ARE or
AARE with either were noted.
Again, the effect of nonhydrocarbon components is not known.
However, the densities required in the Viswanathan viscosity correlation
equations are calculated with the Piper et al. and DAK equations, so
any nonhydrocarbons present in the gas should not seriously affect
the results. These equations can be used with some confidence until
sufficient data are available to prepare correlations based on naturally
occurring petroleum gases for use at HPHT.