Improved PVT Correlations For UAE Crude Oils
Authors: Almehaideb R.A. 
Published: December 2007

Current empirical correlations to evaluate crude oil PVT properties are based on data from different parts in the world and are more accurate for use with crudes from the same basins for which the data was correlated. Standing correlation, for example, used California crudes. Glaso's used North Sea crudes. Al-Marhoun's used Middle East crudes. Petrosky and Farshad used Gulf of Mexico crudes, and Dokla and Osman produced a correlation based on UAE offshore crude oil samples. A few other correlations were based on a very large number of samples from different regions. The most widely used of these is the Vasquez and Beggs correlation. The universal correlations, however, are less accurate than regional correlations, when they are available, because of the scatter involved by using a large number of data.

The UAE fields are very significant and constitute around 10% of reserves worldwide. In this work, experimental PVT measurements from around 13 reservoirs located in the UAE were used to test the validity of commonly used PVT correlations. The data then was used to construct a new set of empirical correlations which were also tested against the experimental values. Results indicate that the new correlations reduced the error involved in estimating the bubble point pressures and the oil formation volume factor to within less than half the range of error associated with commonly used correlations when compared to experimental values. The physical properties correlated include the oil formation volume factor at the bubble point, the bubble point pressure, the oil compressibility (in order to get the oil formation volume factor above the bubble point pressure), and the oil viscosity at the bubble point and above the bubble point.

Introduction

Accurate estimates of PVT properties for crude oil are essential, especially for new reservoirs so that quick estimates can be made of Oil Initially in Place (OIIP), and the prevalent natural reservoir forces. This knowledge can then be used to define the most suitable method to recover oil from a particular reservoir, and the amounts recoverable under primary, secondary, or enhanced methods. Also, design of surface facilities, like separators and treatment facilities require accurate knowledge of PVT properties.

The bubble point pressure, the oil formation volume factor, and the oil viscosity are all fundamentally functions of composition, pressure, and temperature. However, since composition is not usually available initially, and even when it becomes available, does not fully define the hundreds of components that crude oil is made of, the effect of composition on the above mentioned PVT properties is typically approximated by using the surface oil and gas gravities and the producing gas oil ratios, which are readily available to the engineer. Another two correlating parameters, namely the reservoir pressure and temperature, are usually measured early in the lifetime of a reservoir. Using these five field measurements, the engineer can use the PVT correlations to estimate the bubble point pressure, the formation volume factor, and the viscosity for the crude oil at the bubble point. Also, the engineer may use the PVT correlations to construct tables for the solution gas to oil ratio and viscosity under any combination of pressure and saturation pressure, the values of which are necessary to estimate the future performance of any reservoir.

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