PVT Correlations for Middle East Crude Oils

Summary. Empirical equations for estimating bubblepoint pressure, oil FVF at bubblepoint pressure, and total FVF for Middle East crude oils were derived as a function of reservoir temperature, total surface gas relative density, solution GOR, and stock-tank oil relative density. These empirical equations should he valid for all types of oil and gas mixtures with properties falling within the range of the data used in this study. properties falling within the range of the data used in this study. Introduction

PVT correlations are important tools in reservoir-performance PVT correlations are important tools in reservoir-performance calculations. The major use of PVT data is in carrying out material-balance calculations.

In 1947, Standing published correlations for determining the bubblepoint pressure and FVF from known values of temperature, solution GOR, gas relative density, and oil API gravity. A total of 105 experimentally determined data points on 22 different crude oil and gas mixtures from California were used in deriving the correlations. Standing reported an average relative error of 4.8% for the bubblepoint pressure correlation and an average relative error of 1.17% for the FVF correlation.

In 1980, Glaso presented correlations for calculating bubblepoint pressure, oil FVF, and total FVF from known values of temperature, solution GOR, gas relative density, and oil API gravity. A total of 45 oil samples, mostly from the North Sea region, were used in obtaining the correlations. Glaso reported average relative errors of 1.28%, -0.43%, and -4.56% for the bubble point pressure, the bubblepoint oil FVF, and the total FVF correlations, pressure, the bubblepoint oil FVF, and the total FVF correlations, respectively.

Reviews of other empirical PVT correlations were presented by Sutton and Farshad in 1984.

Standing used a graphic method and Glass used both a graphic method and linear regression analysis in the development of their PVT correlations. The graphic estimation and curve-fitting, PVT correlations. The graphic estimation and curve-fitting, however, do not lead to the best estimate. Therefore, this study developed the correlations using only linear and nonlinear multiple regression analyses to obtain the highest accuracy.

This paper deals with PVT correlations exclusively for samples of Middle East crude oils. However, they should he valid for all types of gas/oil mixtures with properties falling within the range of data used in this study. Moreover, this study evaluates the accuracy of Standing's and Glass PVT correlations, which are shown in Table 1. Error analyses were done for this study and also for Standing's and Glass correlations to compare their degree of accuracy. Finally, nomographs for bubblepoint pressure, bubblepoint oil FVF, and two-phase total FVF were constructed on the basis of the developed empirical correlations.

PVT Data PVT Data The PVT analyses of 69 bottomhole fluid samples from 69 Middle East oil reservoirs were made available for this study. The experimentally obtained data points were 160 each for the bubblepoint pressure, Pb, and bubblepoint oil FVF, Bob, correlations, and pressure, Pb, and bubblepoint oil FVF, Bob, correlations, and 1,556 for the total FVF, Bt, correlation. The ranges of the data used are shown in Table 2.

PVT Correlations PVT Correlations The correlations for bubblepoint pressure, bubblepoint oil FVF, and two-phase total FVF were developed by use of the linear and nonlinear multiple regression analyses shown in the Appendix. Bubblepoint Pressure. The following general relation of bubblepoint pressure of an oil and gas mixture with its fluid and reservoir properties was assumed:

(1)

Table 3 shows the 160 experimentally determined bubblepoint pressures obtained from PVT analyses of 69 different Middle East pressures obtained from PVT analyses of 69 different Middle East oil/gas mixtures. The nonlinear multiple regression analysis was used to develop the following relation:

(2)

where Pb = bubblepoint pressure, Rs = solution GOR, gammag = dissolved gas relative density (air = 1), gammag = stock-tank oil relative density (water = 1), and T = absolute temperature.

Bubblepoint Oil FVF. Oil FVF at bubblepoint pressure can he derived as a function of solution GOR, average gas relative density, oil relative density, and temperature as fellows:

(3)

The following empirical equation was developed by use of the nonlinear multiple regression analysis and a trial-and error method based on the 160 experimentally obtained data points shown in Table 3:

(4)

where B is an intermediate oil FVF value.

The bubblepoint oil FVF correlation (Eq. 4) was further refined by applying the linear regression analysis on the same data.

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