Application of the Bergman-Sutton Method for Determining Blend Viscosity

Summary As the petroleum industry accesses more low-gravity-oil resources, modification of viscosity by blending lighter hydrocarbons has become a necessity in order to attain bulk properties that will flow though a pipeline. In the more conventional oil reservoirs, the need to estimate the viscosity of oil blends occurs when reservoir fluids are contaminated with oil-based muds or when production streams from different reservoirs or fields are commingled in a single pipeline. Several methods have appeared in the literature for estimating blend viscosity. All of these methods require a measured viscosity for each component of the blend. The number of viscosity measurements is compounded when the viscosity of the blended mixture is required at several temperatures. Of the viscosity correlations published, the Bergman and Sutton method has the widest range of temperature and oil API gravity and has been consistently demonstrated to provide accurate results over these conditions. This method requires the component specific gravity, the Watson characterization factor (Watson K factor), and temperature to estimate viscosity. By using the proper mixing rules, an estimate of blend viscosity can be made with comparable or improved accuracy over the "best" published methods without the need for individual-component viscosity measurements. A database of 2,059 blend-viscosity measurements from more than 800 mixtures was created to compare the accuracy of the various methods. Viscosity measurements of the individual components in the blends studied exceeded 7,600 data points. A diverse group of mixtures, ranging from light alkane or aromatic pure components to bitumen, diesel, biodiesel, condensate, crude, and assay fractions, was included in this database. Blends comprised the typical binary mixtures but ranged up to a maximum of eight components in the mixture.