An Accurate Method for Determining Oil PVT Properties Using the Standing-Katz Gas Z-Factor Chart

Summary 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 FVF 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 FVF 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.