For live crude oils, dissolved gas acts to reduce viscosity from the value observed for dead oils. The reduction in viscosity can significantly impact pressure drop and flow rate and must be properly accounted for by any viscosity model. This article provides a detailed review of existing correlation methods and offers state-of-the-art solutions necessary for oil field operations in 2007 and beyond.
A total of 21 methods for calculating saturated oil viscosity have been identified from the literature. A large database consisting of data from oil PVT reports and literature sources has been compiled in order to evaluate these methods. This data represents over 12,000 measurements from 1849 samples. The gases dissolved in the oils contain varying amounts of non-hydrocarbons and the oils provide a representative crossection of the expected range in API gravity and Watson characterization factor found in crude oils from producing regions located around the world. A detailed analysis of correlation performance has been conducted which reveals inaccuracies in many of the previously published methods. As a result, a new method is proposed which is consistent across a wide range of parameters and offers increased accuracy over existing methods.
Viscosity is the single most important parameter for calculating the pressure drop of fluids flowing through pipe or porous media. The viscosity of the crude oil is affected by the composition of the oil (specific gravity and characterization factor), solution gas-oil ratio, pressure and temperature. Three distinct areas are identified: dead or gas free, saturated, and undersaturated. Fig. 1 shows the viscosity behavior of a crude oil with each area identified. Starting from a dead condition, free gas in contact with the oil dissolves into the oil with increasing pressure until all of the gas is in solution at the bubblepoint. As the gas dissolves in the oil, the density and viscosity of the oil are reduced. Above the bubblepoint, viscosity increases in the undersaturated region due solely to the effect of pressure on the oil. Bergman and Sutton5,6 have provided a detailed review of the dead and undersaturated regions including accurate methods to determine viscosity in each region.