Gas-Saturated Bitumen Density Predictions Using The Volume-Translated Peng-Robinson Equation Of State
Authors: Kokal S.L. , Sayegh S.G. 
Published: March 2010

This paper presents a method to predict the gas'saturated densities of heavy oils and bitumen at reservoir conditions using a modified Peng-Robinson equation of state. The method is based on the volume translation concept by which volumetric predictions are improved without affecting the phase behaviour calculations. Significant improvement in volumetric predictions was achieved with the translated equation of state (less than 1% deviation for 22 data sets) when compared to the original Peng-Robinson equation of state results (18% to 22%).

Introduction

The use of light hydrocarbon gases, carbon dioxide, nitrogen, Etc., for enhanced oil recovery from petroleum reservoirs has received much attention in recent years. Steam-flooding and gas injection are two of the popular methods for recovering heavy oils and bitumen from the reservoirs. During gas injection the gas or solvent reduces the oil viscosity, making the oil mobile and enhances oil recovery. It also promotes swelling and increases oil density.

In designing gas injection schemes and in many other reservoir engineering situations, a method to predict the gas-saturated crude oil density is needed for the calculation of crude oil swelling and formation volume factors. This has led to prediction methods and correlations which are generally applicable to specific cases and often requires a knowledge of molar volumes and solubilities of the components in the mixture at reservoir conditions. Many of these correlations are in graphical form and are not suitable for computer calculations.

Furthermore, these correlations are empirical in nature and can involve large errors when extrapolated beyond the range of variables covered in the study. Another disadvantage with this method is that many correlations were derived for a specific gas, while in practice the injected gas can be mixed with other gases rendering the correlations inapplicable for those situations.

There are four approaches which have been used in calculating the saturated liquid densities (Marra et al. (2)): partial molar volumes, ideal-solution, corresponding states principle and the equation of state (EOS) methods. The first two methods have several disadvantages some of which have been discussed above. A review of these methods was made by Marra et at.(1) who also proposed a correlation for the density of crude oil saturated with CO2. Their correlation is purely empirical and its validity beyond the range of data it was correlated with is unknown.

The method of corresponding states has been used extensively for the prediction of saturated liquid densities (Reid et al (2)). Some of the work in this area are by Racket(J). Spencer and Danner(4,5), and Hankinson and Thomson(6,7). These methods were developed specifically for calculating liquid densities and provide accurate results over a wide range of temperatures and pressures. Recently Teja and Sandler(8) proposed an extended corresponding states method for the prediction of liquid densities. They applied their method to LNG and to CO2/crude oil systems with good results. One disadvantage of this-method is that a different method must be used for cases where mixture gas density is also required simultaneously and this can lead to a discontinuity near the critical region of the mixture.