Application Of The Developed Redlich-Kwong Equation Of State To Predict The Thermodynamic Properties Of Condensate Gases
Authors: Peneloux Andre , Jain Corentin , Behar Emmanuel 
Published: December 2007

A new characterization procedure for the heavy fraction is used with PENG-ROBINSON (PR) and DEVELOPED REDLICH-KWONG (RKD) equations of state to predict the thermodynamic behavior of condensate gases. The computed properties show that RKD model can be considered as an improvement of previous work with PR equation, mainly for evaluating the compositions of equilibrium phases.

INTRODUCTION

Most natural gases contain small amounts of components heavier than n-pentane. in some cases, significant amounts of the hexanes and heavier components are present in the reservoir effluent. Published phase equilibria data have shown the dependence of phase behavior on the chemical nature of hydrocarbons present, mainly aromatics. In most instances, a compositional analysis of the hexanes and heavier fractions is incomplete or unavailable, but phase equilibria predictions still must be made for reservoir engineering purposes.

A new characterization procedure for the C7+ fraction is proposed in this work. The only datum used is the average molar weight of this fraction, which is considered to be a pseudo-binary made of a 'light' n-alkane and a 'heavy' n-alkyl-benzene in such proportions that the molar weight of their mixture is equal to that of the C7+ fraction.

The compositions thus obtained are used with different equations of state to predict the PVT characteristics and the molar compositions of the produced and residual fluids, under laboratory conditions simulating the evolution of the reservoir effluent during depletion, as well as the retrograde liquid-deposit curve.

A new non-cubic equation of state, called DEVELOPED REDLICH-KWONG (RKD) has been proposed elsewhere for predicting high pressure liquid-vapor equilibrium. All the above mentioned thermodynamic properties of condensate gases determined with RKD as well as SOAVE and PENG-ROBINSON equations of state, are compared to laboratory data. It is thus possible to evaluate the degree of accuracy of these different compositional models for application to reservoir engineering calculations with natural gases.

THE DEVELOPED REDLICH-KWONG EQUATION (RKD)

A new non-cubic equation of state of the REDLICH-KWONG type has been presented elsewhere 1. This expression was obtained from the original REDLICH-KWONG equation of state by assuming that the attraction parameter depends not only on temperature, but also on density. The generalization of this equation by a corresponding states correlation enables it to be applied over wide ranges of temperature, pressure and hydrocarbon molecular weight.

The RKD equation has the formEquation (1)

the parameters a, b, Î3 and Î´ are functions of temperature.

In order to use this equation in compositional models needed to compute the thermodynamic behavior of petroleum fluids, we established a three parameter corresponding states correlation. We can thus represent the properties of the non- or slightly polar components which play an important role in the petroleum industry knowing their critical parameters : Tc, Pc and their acentric factor Ï?.