A Corresponding States Correlation for Calculating Gas-Condensate Phase Equilibria
Authors: Emanuel Alan S. 
Published: December 2007

A correlation has been developed for calculating the phase behavior of gas-condensate systems at reservoir conditions. The correlation is based on the principle of corresponding states and has been coded for an IBM 7094. Experimental K-values were determined for several gas-condensate systems at reservoir conditions to evaluate various semiempirical parameters of the correlation. The approximate range of application of the correlation is 150 to 300F and 1,500 to 6,000 psi.

Introduction

The rapid development of digital computers during the past several years has made feasible the calculation of hydrocarbon phase behavior by methods based on rigorous thermodynamic principles. Good correlations have been developed for low to moderate pressures, but these techniques have not yet been extended successfully to reservoir fluids at high pressures. Consequently, the determination of phase behavior of oil and gas systems at reservoir conditions is still based almost entirely on generalized data correlations or on experimental studies of the fluid in question. While these methods have been used successfully many times, they do have inherent limitations that restrict their applicability. Generalized correlations, such as the NGSMA K-charts, are limited to the range of pressure, temperature and components for which pressure, temperature and components for which the data were determined. The accuracy of these correlations is often questionable because the effect of total system composition is not well defined. Experimental studies offer a reliable method for determining phase behavior, but usually the studies are costly and time consuming.

Recently, Leland and coworkers presented a new approach to calculating phase behavior from the principles of corresponding states. Corresponding states methods determine the thermodynamics properties of a given system by comparison with a reference substance whose properties are known. The accuracy of data properties are known. The accuracy of data approach depends on close chemical and structural similarity between the reference substance and the system in question and between components within the system itself. For high accuracy, it is usually necessary to correct for chemical and structural dissimilarities. In principle, however, the corresponding states method should be no less accurate at high pressures than at low pressures, provided reference substance properties are known. provided reference substance properties are known. This paper describes an empirical modification of the basic correlation proposed by Leland, et al. for the specific purpose of calculating the phase behavior of gas-condensate fluids at reservoir conditions. The modified correlation, which has been Programmed for an IBM 7094, may be used for either approximate or precise determination of fluid behavior depending on the amount of analytical and, experimental data available for the system.

BASIC THEORY

The basic theory of the corresponding states phase equilibria correlation was first published by phase equilibria correlation was first published by Leland, Chappelear and Gamson. Subsequently, Leland, Chappelear and Leach published methods for improving me accuracy of the original theory. The aim of the correlation is to calculate the K-value of each component of a given system as a function of pressure, temperature, and over-all composition, where

..........................................(1)

Once the K-values are known, the phase behavior may be determined directly by an appropriate flash calculation.

The basic equation for calculating component K-values was taken from the work of Joffe. For any component i of a mixture, the K-value is given by

..........................................(2)

SPEJ

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