Comparison of PVT Properties From Equation of State Analysis and PVT Correlations for Reservoir Studies

Abstract

Abstract Measured fluid properties from four reservoirs are used to compare property prediction results using an equation of state (EOS) method and different PVT correlations available in the literature. These fluid properties include oil bubble point pressure, oil formation volume factor, solution gas-oil ratio, gas formation volume factor, and gas and oil viscosities. It is shown that with properly characterized EOS analysis, even without the use of regression, one can match all the measured property values better than by using correlations. It is noted that one correlation can generally predict one or more of the parameters better than the other correlations. However, no one correlation can match all measured data consistently. Introduction PVT properties such as oil bubble point pressure, oil formation volume factor, solution gas-oil ratio, gas formation volume factor, and gas and oil viscosities are required for reservoir studies. However, they are not always available or only an incomplete data set may be available. Hence, engineers have to use either an equation of state (EOS) method or a set of correlations to complete the data set to conduct the particular study. The literature has many comparative studies of equations of State(1,2) and many papers on correlations for calculating PVT Properties(3–7). In this paper we compare results from one EOS analysis to a number of available correlations. The EOS used in this study is the Peng-Robinson EOS (PREOS), from a commercially available PVT package. The PREOS originally contained two parameters that represent the attractive pressure term and the thermal repulsive term respectively. To improve the volumetric phase behaviour prediction accuracy, a third parameter is usually added (PRF shifting factor). The PREOS is a semi-empirical equation, requiring some PVT property data to determine these parameters before one can use it for property predictions. PVT correlations are typically developed for fluid properties(3–7) in a geographic region, such as for California, Alaska, the Gulf of Mexico, and the Middle East, by fitting available regional data. The first set of correlations was derived by Standing(3) in 1942 for California oils and gases. The basic assumption was that the bubble point pressure is a function of dissolved gas-oil ratio, gravity of dissolved gas, density of stock-tank oil, and temperature. Later, other correlations were obtained by regression to very similar equations but using different data sets since the crudes from different reservoirs or regions have different properties. Therefore, these correlations may not be applicable to oils other than those used in deriving the regression. Furthermore, no one correlation provides all PVT properties required for a reservoir study. Hence, one always has to use different correlations for different properties. For example, in a recent study of Alaskan crude properties(4), the Glaso(16) correlation was used for bubble point pressure and the Standing correlation(3) was used for oil formation volume factor while the Beggs-Robinson(25) viscosity correlation was used for dead and live oil viscosity. The purpose of this paper is to compare EOS results with available PVT correlations using measured laboratory values as a reference and then to provide some guidelines in generating PVT properties for reservoir study.