A properly understanding of how oil mixtures behaves as they flows in a pipeline is a key factor for piping design. Facilities engineers from oil companies currently use commercial pipeline simulators in order to perform pressure and temperature gradients predictions. Nearly all the simulators apply the general mixing rule when they need to calculate the viscosity of a certain oil mixture. As viscosity depends both on molecular characteristics and molecular interaction, and oil is a complex mixture of components, it is not surprising that this rule gives poor results. Besides, non-Newtonian behavior makes things more complex. This paper discusses experimental results of mixing at lab several different oils and measuring their viscosity. Each oil had its own composition and rheological behavior. Some mixtures were prepared in order to reproduce real situations. Once the mixtures viscosities were obtained they were introduced as input data in a commercial pipeline network and nodal analysis software. Then, pressure and temperature gradients were calculatedUsing the experimental mixture viscosityUsing the general mixing rule viscosity estimated by the software Results showed, in some cases, dramatic differences between the two alternatives. Higher deviations were obtained where the mixtures were composed of crudes of different properties. In order to improve the accuracy other mixing rules were tried and some of them fit better. A proper understanding of crude oil properties seems to be essential for successfully applying complex simulators in pipeline design. Introduction Viscosity of liquid mixtures have been studied for a long time(1) but, unfortunately, this knowledge has not always been taken into account in the development of commercial pipeline simulation software. Viscosity has a molecular origin, and it is highly dependent on the molecular interaction. That is the case of complex multiphase and multicomponents fluids such as oil. If an engineer needs to design an oil pipeline, there are several commercial software available to predict pressure and temperature drop. It is only necessary to introduce a viscosity curve as an input to the soft. But the problem is presented when several pipes, which transport different oils, form the system to design. Most of this technical software makes use of the general mixing rule as the main alternative to calculate the mixture viscosity. This paper presents a study of viscosity mixtures of several crude oils. In each case individual crude oil viscosities were measured at a particular shear rate and shear stress, and different temperatures. The viscosity mixture measurements were made at similar conditions, and the same values were calculated using a commercial pipeline network and nodal analysis software. The results show that there is no satisfactory equation, which can be applied to all the practical cases. The simulator generally overestimates the pressure drop through the pipeline. Therefore this could carry out economical problems. Theory The general mixing rule is currently used to obtain an average value of many physical properties, including viscosity. The rule states that the property of a mixture is obtained by the weight average of the properties of each component. However, this equation is only valid when the fluids that are mixed do not interact with each other. That is not the case of oil mixtures. Besides it is not valid when dealing with emulsions or water-oil flow. These particular situations (also found in pipeline design) are not in the scope of this work.

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