A Simple and Efficient Approach for Improving the Prediction of Reservoir Fluid Viscosity
Authors: Xu D.-H. , Khurana A.K. 
Published: December 2007

Abstract
A simple and efficient approach has been developed for improving the predictions of gas and liquid phase viscosities of reservoir fluids. The proposed method was based on the Lohrenz-Bray-Clark (LBC) correlation and applied to the Peng-Robinson equation of state (EOS) as all example although it can be applicable to any EOS. It can significantly improve the predictions of gas and liquid phase viscosities of hydrocarbon mixtures, particularly of high viscosity fluids. The approach is quite suitable for equation of state (EOS) based reservoir simulators and can further improve the prediction of reservoir fluid viscosity with appropriate tuning of the EOS.

Introduction
Accurate predictions of gas and liquid phase viscosities of reservoir fluids are required in reservoir simulators, particularly in equation of state (EOS) based reservoir simulators. Many researchers have focused their attention on improving the prediction of gas and liquid viscosities. However, most of the methods from mathematically rigorous to completely empirical forms are only applicable to either gas phase or liquid phase within a limited range of composition, temperature and pressure. In reservoir simulations related to oil and gas production, viscosity prediction methods are required to be applicable to a wide range of hydrocarbon mixtures at various conditions. The Lohrenz-Bray-Clark (LBC) correlation is probably the most used one in reservoir simulation models. The correlation was based on the Jossi-Stiel-Thodos correlation and modified by Lohrenz et al. The correlation is related to a fourth-degree polynomial in the reduced density. The disadvantage of the correlation is that the predicted viscosity is very sensitive to the density which is normally determined by a cubic EOS and may be very inaccurate for high viscosity fluids. Alternatively Ely and Hanley presented an extended corresponding states model for predicting the viscosity of non-polar pure fluids and their mixtures. Pedersen and Fredenslund studied this method and proposed a new corresponding states method which is based on the principle of corresponding states with methane as the reference component. Later Aasberg-Petersen et al. further improved the model by introducing n-decane as the second reference component. Little and Kennedy used an analogy with the van der Waals EOS to correlate the viscosity of reservoir fluids. Lawal applied the Lawal-Lake-Silberberg EOS to correlate the viscosity as well.
In this study, a simple and efficient approach is proposed to improve the predictions of gas and liquid viscosities of reservoir fluids. The new method is based on the LBC correlation with the introduction of an exponential term and incorporating a cubic EOS. The method has been used to predict the viscosities of pure components, binary mixtures and reservoir fluids over a wide range of pressures and temperatures. The predicted results demonstrate that the new method can significantly improve the accuracy of viscosity predictions when compared with other published methods, particularly for high viscosity fluids. It is shown that the predictions can be further improved by appropriate tuning of the EOS.

Viscosity Model
As the Lohrenz-Bray-Clark (LBC) correlation is expressed as a function of the reduced density, the calculated viscosity is very sensitive to the density.
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