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 density.
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 traditional methods.
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.