Improved K-value Correlation for UAE Crude Oil Components at High Pressures Using PVT Laboratory Data

Abstract

Abstract In this paper, 732 high-pressure K-values obtained from PVT analysis of 17 crude oil and gas samples from a number of petroleum reservoirs in the UAE are used. Material Balance techniques are used to extract the K-values of crude oil and gas components from the Constant Volume Depletion (CVD) and Differential Liberation (DL) tests for the oil and gas samples respectively. These K-values are then correlated and the resulting correlation compared with published correlations. Comparisons of results show that currently published correlations give poor estimates of K-values for nonhydrocarbon and hydrocarbon components, while the proposed new correlation improved significantly the average absolute deviation for non-hydrocarbon and hydrocarbon components. The average absolute error between experimental and predicted K-values for the new correlation was 20.5% compared with 76.1% for the Whitson and Torp correlation, 84.27% for the Wilson correlation, and 105.8 for the McWilliams correlation. Additionally, the bubble point and dew point pressures are calculated for these 17 samples and compared with experimental values. The average absolute error in the saturation pressures for the new correlation was 6.08 % compared with 56.34 % for the Wilson correlation, 57.84 % for the Whitson and Torp correlation, and 9.28 % for the Peng-Robinson equation of state with default parameters. 1. Introduction Equilibrium ratios play a fundamental role in the understanding of phase behavior of hydrocarbon mixtures. They are important in predicting compositional changes under varying temperature and pressure in reservoirs, surface separations, and production and transportation facilities. In particular, they are critical for reliable and successful compositional reservoir simulation. Equilibrium ratios, more commonly known as K-values, relate the vapor mole fractions, yi, to the liquid fraction, xi, of a component i in a mixture,Equation 1 In a fluid mixture consisting of different chemical species at high pressure, K-values are dependent on pressure, temperature, and the composition of the mixture. This extra dependency on the fluid composition, for high-pressure systems compared to low-pressure systems, has limited our ability to predict high pressure K-values empirically and shifted the emphasis for preferred methods to using the more sophisticated equations of state approach. The objective of this work is to evaluate the published empirical correlations that could possibly be used for computing K-values for high-pressure systems, namely the Wilson, Whitson &Torp, and the polynomial equation of McWilliams, using the experimental K-values extracted through material balance techniques from PVT tests performed on UAE petroleum samples, and to develop a new correlation for UAE crudes formulated using the multivariable regression technique. To test the merits of the new correlation, a comparison is performed for predictions from all correlations with experimental K-values and with measured bubble and dew point pressures.