Page 118 - PVT Property Correlations

96 PVT Property Correlations

separator conditions. In the case of two or three-stage separation, the average
gas specific gravity must be calculated as a volume-weighted average of the
gases released from the different separators and stock-tank. The second set
of correlations (Nassar et al., 2013) requires the use of reservoir temperature,
primary separator gas gravity, stock-tank API gravity of oil, and separator
conditions. Since specific gravity of secondary separator gas and stock-tank
gas are not usually available, we find Nassar et al. (2013) correlations to be
easier to implement in practice.
In general, the MBO correlations can yield significant error (in the order
of 20% absolute average percent error or slightly more when compared with
laboratory observed values) in the case of highly rich gas condensate fluids
and near critical volatile oils. These errors may be due to the volatility of the
fluids. It is always advisable to use these correlations only if an EOS model
is not available for the fluid.

HANDLING PVT PROPERTIES FOR GAS CONDENSATE
PVT properties for gas condensate fluids can be determined from PVT labo-
ratory experiments performed on a representative fluid sample. Other texts
(e.g., Ahmed, 2016; Whitson and Brule, 2000) explain gas condensate labo-
ratory experiments in full detail. The other available option to calculate the
PVT properties of gas condensate fluids is to use correlations.
Gas condensate specific gravity can be obtained by use of the techniques
presented in Chapter 4, Wet Gases. The critical properties (critical pressure
and temperature) for the gas condensate can be computed with the techniques
presented in Chapter 3, Dry Gases. Critical properties correlations either use
the gas specific gravity or the gas composition. Some of the important critical
properties correlations are given in Appendix B, Gas Correlations Formulae.
In general, for gas condensates, the use of more recent critical properties corre-
lations is recommended over the older ones. The older correlations were devel-
oped from fluid databases that were dominated by dry gas samples. Appendix
D, Gas Correlations Range of Applicability, contains the ranges of applicabil-
ity of the major correlations. One can consult the Appendix for the best corre-
lation to apply according to the validity ranges of input data.
The PVT properties such as single-phase z-factor, gas formation volume
factor, gas density, gas compressibility, and gas viscosity can be computed
with the techniques presented in Chapter 3, Dry Gases. Many of these corre-
lations are given in Appendix B, Gas Correlations Formulae.
The PVT properties that are specific to gas condensates (e.g., dew point
pressure, two-phase z-factor, and CGR) are calculated using the techniques
presented in this chapter. Tables 5.2 and 5.3 list the available dew point
pressure correlations from specific gravity and from gas composition,
respectively. Tables 5.4 and 5.5 summarize the two-phase z-factor and
CGR correlations. Table 5.6 shows the gas condensate specific gravity