Surface Tension Of Petroleum Mixtures

Abstract

Abstract A new correlation has been developed where the surface tension of a petroleum mixtures is related to the viscosity. The calculated results compare well with experimental observations atboth atmospheric and higher pressures. Accurate results are also obtained using a modification of the procedure of Lee and Chien. Introduction The surface tension is defined as the partial derivative of the surface free energy with respect to the dividing area between the gas and liquid phases. Proper estimation of the surface tension of oil-gas systems is important in a number of reservoir engineering calculations. The surface tension is required to predict the capillary pressure of the oil in a porous solid. The surface tension also influences the relative gas/liquid phase permeabilities. For pure components it has been experimentally observed(1) that there is an almost linear dependence between the reciprocal of the viscosity, η, and the logarithm of the surface tension, σ: (1) In ( J = In A + B / ρ ) A and B may for pure components be determined by a least TABLE 1. Results for the surface tension at atmospheric pressure of oils 1–7 and of gas condensate 1. Available in Full Paper. squares fit to experimental data relating the viscosity to the surface tension. For mixtures it is less obvious how to determine A and B. On the other hand it is desirable to be able to determine the surface tension from the viscosity. Viscosity data are more frequently available than are surface tension data. Also more reliable correlations exist for predicting the viscosity(2, 3) than is the case for the surface tension. 1. Experimental Data The surface tensions have been measured of eight North Sea reservoir fluids (7 oil mixtures and 1 gas condensate mixture) flashed to standard conditions (1 atm and 15 °C). The surface tensions were measured with an MOM Lauda Ring Tensiometer. The thermalization time was chosen long enough to allow surface equilibrium. For the temperatures reported, repeated measurements on the same sample showed no time effect. The viscosity measurements were performed with a Contraves Low Shear 30 Viscometer. The reported viscosities correspond to a shear rate of 100 S-I. The densities were measured with an Anton Paar-DMA 602H apparatus. The surface tension results for the 8 mentioned petroleum mixtures are presented in Table. 1. The corresponding results for viscosities are shown in Table 2. In Tables 3–10 molar compositions of the 8 petroleum mixtures are given. In addition to the data of Tables 1 and 2, pure component data for ethane, propane, n-butane, n-pentane, n-hexane, benzene, toluene(1) and cyclohexane(4) have been used to develop the procedure of the following section. Experimental surface tension data for a reservoir fluid(5) has been used to test the predictive capabilities of the calculation procedures at higher pressures. The results are given in Table 11 and the molar composition of the reservoir fluid in Table 12.