This paper evaluates several empirical PVT correlations for application inthe Gulf of Mexico. Ideally, fluid properties are determined experimentally inthe laboratory; however, these data are not always available. Correlations areconsequently used to determine values for bubblepoint pressure, solution GOR, FVF, and viscosity. These values are necessary to compute oil reserves or forcalculations involving flow through pipes or porous media. A review of thecorrelations is provided, along with the results of calculations made on 31individual crude oil samples.
The calculation of reserves in an oil reservoir or the determination of itsperformance requires a knowledge of the fluid's physical properties at elevatedpressure and temperature. Of primary importance are properties at elevatedpressure and temperature. Of primary importance are bubblepoint pressure, solution GOR, and FVF. In addition, viscosity must be determined forcalculations involving the flow through pipes or porous media. Ideally, theseproperties are determined from laboratory studies designed to duplicate theconditions of interest. Experimental data quite often are unavailable, however, because adequate samples cannot be obtained or because the producing horizondoes not warrant the expense of an in-depth reservoir fluid study. In thesecases, PVT properties must be determined by analogy or through the use ofempirically derived correlations. During the last 42 yews, several correlationshave been proposed for determining PVT properties. The most widely used treatoil and gas as a two-component system with each component having a fixedcomposition. Only the specific gravity and relative amount of each component, the pressure, and the temperature are used to characterize the oil's PVTproperties. Crude oil systems from various oil-producing regions were used inthe development of the correlations. These crude oils exhibit regional trendsin chemical composition that categorize them as paraffinic, naphthenic, oraromatic. Because of the differences in composition, correlations developedfrom regional samples that are predominantly of one chemical base may notprovide satisfactory results when applied to crude oils from other regions. This paper examines the fluid properties and correlations shown in Table 1 todetermine their properties and correlations shown in Table 1 to determine theirapplicability in the offshore Gulf of Mexico region.
For years, field engineers have used empirical correlations in lieu oflaboratory data for determining the fluid properties necessary for calculatingreserves, reservoir performance, and equipment design. Until recently, the onlyguide for selecting a correlation was offered by Chierici et al., who suggestedthe use of Lasater's correlation for crude oils with a gravity >15 API[less than 0.97 g/cm3] and Standing's correlation for crude oils with a gravityless than 15API [ >0. 97 g/cm3 ]. In 1983, Ostermann et al. provided a morecomplete evaluation of empirical correlations for determining the PVTproperties of Alaskan crude oils. Eight samples were analyzed, PVT propertiesof Alaskan crude oils. Eight samples were analyzed, and it was found that thecorrelations proposed by Glaso (bubblepoint pressure), Standing (oil FVF), and Beggs and Robinsons (dead and gas-saturated oil viscosity) were the mostaccurate. At this time, similar analyses have not appeared in the literaturefor other oil-producing regions. The effective use of the correlations lies inan understanding of their development and a knowledge of their limitations. Thefollowing presents a review of the correlations' development. The equationsthat form these correlations are provided in the Appendix.
Standing published correlations for determining the bubblepointpressure and FVF of a gas-saturated oil from known values of temperature, solution GOR, and oil and gas gravities. In all, 105 experimentally determineddata points on 22 different crude-oil/natural-gas mixtures from California wereused to arrive the correlations. The gases present in the mixtures were free ofN2 and H2S but CO2 Was present in a few samples at concentrations less than 1mol%. All the data were obtained in the laboratory with a two-stage flashseparation designed to duplicate average field conditions in California. Table2 shows the ranges of these data. Standing reported an average error of 4.8 %and 106 psi [731 kPa] for the bubblepoint-pressure correlation and an averageerror of 1.17% for the FVF correlation. The latter is more general and theformer and will provide satisfactory results for a wider variety of crudeoils.
Lasater presented a bubblepoint-pressure correlation in 1958. ALRAN of 158 experimentally measured bubblepoint pressures tom 137 independentcrude oil systems from Canada, western and midcontinental U.S., and SouthAmerica was used in its development. The natural gases associated with thesecrudes were essentially free of nonhydrocarbons. Lasater used Henry's law toderive a "bubblepoint-pressure factor" and correlated it with the molefraction of gas in solution to obtain the curve shown in Fig. 1. The molefraction of gas in solution is calculated from values of solution GOR, oilspecific gravity, and molecular weight. Because an oil's molecular weight isgenerally an unknown quantity, Lasater provided Fig. 2, which relates molecularweight to oil gravity. This relationship corresponds with that observed fromcrude oils with a UOP characterization factor of 11.8. Table 2 shows the rangesof data used by Lasater in deriving the correlation. Lasater reported amaverage error of 3.8% between measured and calculated bubblepointpressures.
Vazquez and Beggs
In 1976, Vazquez and Beggs presented relationships fordetermining the solution GOR and FVF of a gas-saturated crude oil. In total,6,004 data points were used in the development of these correlations. The datawere separated into two groups because of variations in the volatility of crudeoil. The first group contained oils with gravities less than 30API [0.88g/cm3]. The second group contained oils with gravities less than 30API [lessthan 0.88 g/cm3]. Table 3 gives the number of data points and ranges data foreach group.
Vazquez and Beggs found gas gravity to be a strong correlating parameter inthe development of the solution GOR correlation. parameter in the developmentof the solution GOR correlation. Because gas gravity is dependent on theconditions under which the gas is separated from the oil, Vazquez and Beggsdeveloped a correlation to normalize gas gravity to a separation pressure of100 psig [690 kPa]. This pressure was chosen because it was felt to psig [690kPa].