C Characterization For Fluid Properties Predictions
Authors: Wu R.S. , Fish R.M. 
Published: March 2010

A simple iterative procedure has been developed for characterizing the C7+; fraction of a hydrocarbon fluid with only one pseudocomponent. This procedure applies the Peneloux, Rauzy and Freze shifting factors and Watson's heavy end correlations to the PengRobinson equation of state. The procedure requires the following experimental data:bubble point pressure;compositions of gas and liquid resulting from a room condition flash of theservoir fluid;flashed gas-oil ratio, andreservoir fluid density at pressures above the bubble point pressure.

When the equation of state is correctly tuned, the results match not only the reservoir fluid phase behaviour and fluid density, but also the phase behaviour of mixtures of reservoir fluid blended with hydrocarbon and carbon dioxide solvents.

Introduction

In order to predict the fluid properties and phase behaviour of hydrocarbon fluids, either in the presence or absence of solvents, people have been increasingly turning to equations of state (EOS). Unfortunately a great deal of misinformation about EOS usage has been created because of misunderstandings regarding their applications and limitations. As an example, it is sometimes believed that an EOS can be used to generate reservoir fluid properties such as density and viscosity without first tuning the EOS to laboratory data. This is not so. A reservoir fluid is a complex multicomponent mixture, the properties of which depend significantly upon the interaction of the various components. Because every reservoir oil has its own unique composition, these interactions vary from one oil to the next. Hence, experimental data is always required for the proper tuning of an equation of state. This data should include low-pressure gas and liquid analyses as opposed to high-pressure liquid analysis. As will be shown later, high-pressure liquid analyses are less accurate.

In order to employ an EOS, one must characterize the C7+ fraction of the reservoir fluid. In this context, characterization is defined as the determination of the critical temperature, critical pressure, acentric factor and the interaction parameters. There are two approaches to this characterization:the application of a continuous component model(1–3); andthe grouping of components into one or more pseudocomponents(4–13).

This report describes a technique which, using only one pseudocomponent, falls into the latter group.

Two procedures are used for determining C7+ pseudocomponent parameters. In one procedure these parameters are continuously adjusted until the EOS can match measured equilibrium data, e.g. bubble point pressure, gas-oil ratio, density and equilibrium compositions. This is a very tedious task. A common alternative procedure involves the determination of these parameters using correlations based on one or more measureable property, such as specific gravity, or molecular weight, or normal boiling point of the pseudocomponent. Although many correlations exist(4–13), there is little instruction available regarding the over-all procedure from data acquisition through to tuned EOS. Furthermore, little exists in the literature regarding the need to ensure that all aspects of the tuning procedure are internally consistent.