An accurate technique for estimating oil pressure/volume/temperature (PVT)
properties using the Standing-Katz Z-factor chart has been developed. Prior
work to derive tuned gas-pseudocritical-property values that provide consistent
and accurate compressibility factors has been extended to oils.
Data from 1,099 worldwide oil PVT reports with 11,960 density measurements
have been analyzed. The oils studied have API gravities ranging from 10.6 to
63, with gas/oil ratios ranging from 5 to 4,631 scf/STB. Bubblepoint formation
volume factors (FVFs) range from 0.98 to 4.58 bbl/STB. Saturation pressures
ranged from 60 to 10,326 psia at temperatures ranging from 50 to 332°F with
pressure differentials to 18,491 psi.
The resulting work shows that oil pseudocritical properties can be
correlated with molecular weight, as with gases. The resulting relationships
can be used to determine saturated- and undersaturated-oil density accurately.
Oil PVT properties, such as formation volume factor and isothermal
compressibility, can then be derived from fundamental relationships with
Because the new method correlates properties with the molecular weight of
the wellstream, the chemical nature of the oil (e.g., whether naphthenic or
paraffinic) is considered. Existing correlations (30 for bubblepoint oil
formation volume factor and 17 for isothermal compressibility) have been
identified in the literature. These methods are tested against the database
along with the new method to determine accuracy and recommended procedures.
Isothermal oil compressibility has proved difficult to correlate historically,
with errors typically in excess of 30%. The proposed method significantly
reduces the error in calculated isothermal compressibility in comparison to
This paper uses a unified approach to determine oil PVT properties using
methods typically reserved for gas. Equations involving the Z factor can be
applied to either single-phase-gas or single-phase-oil systems to calculate
density and isothermal compressibility.
Since publication in 1942, the Standing and Katz (SK) gas Z-factor chart
(Standing and Katz 1942) has become a standard in the industry. Several very
accurate methods have been developed to represent the chart digitally. The
Dranchuk and Abou-Kassem (1975) (DAK) method has been shown to provide accurate
and consistent results over a wide range of conditions extrapolated beyond the
data originally used in its development. The DAK method uses a form of the
Benedict-Webb-Rubin equation of state (EOS) to fit 1,500 points of selected
digital Z-factor-chart data originally published by Poettmann and Carpenter
(1952). Data for isotherms less than 1.15 required smoothing.
© 2008. Society of Petroleum Engineers
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- Original manuscript received:
28 June 2006
- Meeting paper published:
24 September 2006
- Revised manuscript received:
21 May 2007
- Manuscript approved:
13 August 2007
- Version of record:
25 April 2008