New Correlation for Estimating the Viscosity of Undersaturated Crude Oils

Abstract

Abstract A new general correlation for estimating the viscosity of undersaturated crude oils has been developed using 253 experimentally obtained oil viscosities on 41 different oil samples collected from two different unpublished sources. The developed correlation is derived from plotting (P-Pb) vs. (µoa - µob) on log-log paper. The plot reveals a series of straight lines of a constant slope equal to 1.11. It is found that the intercepts of the resulting lines can be accurately represented as a function of API gravity and solution gas-oil ratio at bubble point pressure. The present correlation shows excellent results and clearly outperforms the existing correlations, when tested against the present data bank (253 points) and against available data from the literature (137 points). Introduction Viscosity in general can be defined as the internal resistance of fluid to flow. Like other physical properties of liquids, the viscosity is sensitive to changes in pressure and temperature. Increasing temperature always causes a decrease in viscosity. Increasing pressure always increases viscosity above the bubble point. However, below the bubble point, an increase in pressure causes an increase in solution gas which in turn decreases viscosity. Crude oil viscosity is essential for both petroleum reservoir engineering and production design operations. The present work deals with the correlations of viscosity of undersaturated crude oils (above bubble point pressure). The common empirical correlations available for predicting this property are those of Beal and Vasquez. In 1946, Beal presented a graphical method derived as the rate of change of the undersaturated oil viscosity per unit pressure increase above the bubble point pressure and the bubble point crude oil viscosity. The disadvantages of Beal's correlation are:it is based on only 26 data points covering limited ranges of flow conditions, andno analytical expression is given for the correlation. Using only 11 data points, obtained from Figure 11 of Beal's work, Standing found that Beal's correlation can be approximated by the following equation, which fit the 11 points with average error = 1% and standard deviation = 4.64%.