May 4, 2007
ProMax uses the arithmetic average method for calculating mixed liquid viscosity because that method is used by all of the pipeline pressure drop models included in the program (see Brill and Beggs, 1991) except the OLGAS correlations. However, the viscosity of oil/water mixtures is extremely difficult to predict. In Brill and Mukherjee, SPE, Multiphase Flow in Wells, 1999, they say: “Studies have shown that Eq. 3.18 [mL = mofo + mwfw, where m is viscosity and f is the fraction of oil or water] often is not valid for the viscosity of two immiscible liquids, such as oil and water..… The viscosity of a dispersion or emulsion has been found to depend mainly on the determination of which phase is continuous. The apparent liquid viscosity then will be governed primarily by the viscosity of the continuous phase, because this is the phase that predominates at the pipe wall where most of the friction losses occur. Other factors, such as the dispersed-phase viscosity and the droplet-size distribution of the dispersed phase, also are important. For some oil/water systems, the viscosity of the liquid mixture can be several times greater than the oil viscosity when the continuous phase is oil but the water fraction is approaching the point where an inversion of the dispersion or emulsion will occur…. The inversion point of an oil/water mixture occurs at water fractions ranging from 0.2 to 0.5 with inversion taking place at lower water fractions when oil viscosities are high..….Although Eq. 3.18 is the most common way to treat the apparent viscosity of an oil/water mixture, a more accurate method is to use the oil viscosity when oil is the continuous phase and the water viscosity when water is the continuous phase…. An even better alternative is to conduct flow tests on actual crudes and water to determine the rheological characteristics and the probable inversion point.” So this tells us that there is no totally accurate way to determine the viscosity of oil/water mixtures short of taking physical samples and testing.