Summary

This paper presents the new general cubic equation of state (EOS) forpredicting phase and volumetric behavior of natural multicomponent systemsunder pressure and temperature conditions that conform to reservoir engineeringprocesses. A new effective method to calculate the critical pressure of C5+ or C7+ fractions in reservoir fluids is presented. The pressure of C5+ or C7+fractions in reservoir fluids is presented. The method is of great importancefor successful simulation at pressures greater than 25 MPa and can be used withany cubic EOS. The results of the mathematical simulation are described and theeffects of porous media on phase behavior for natural multicomponent systemsand of capillary pressure phase behavior for natural multicomponent systems andof capillary pressure on dew- and bubblepoint parameters are investigated.

Introduction

Development of numerical models to simulate EOR processes involving thecomplex phase behavior of natural multicomponent systems at high pressures isan important problem in reservoir engineering. Most of these models use an EOSto predict vapor/liquid equilibrium (VLE) and volumetric phase behavior.

In the last 20 years, numerous methods to predict the thermodynamicproperties of pure compounds and mixtures from a cubic EOS were published.properties of pure compounds and mixtures from a cubic EOS were published. Among these, the Soave-Redlich-Kwong (SRK) and Peng-Robinson (PR) methods areused most frequently in the petroleum industry for VLE prediction. The PR EOSgenerally yields better liquid density estimates than the SRK EOS. PR EOSgenerally yields better liquid density estimates than the SRK EOS. In manycases, these methods give good results except for individual substances andtheir mixtures at pressures greater than 30 MPa. This work describes a methodfounded on a new cubic EOS that improves the prediction of the phase behaviorand volumetric properties of natural systems and their components at highpressures.

A typical problem in the use of an EOS is the difficulty in describingpetroleum fractions constituting C5+ or C7+. These fractions significantlypetroleum fractions constituting C5+ or C7+. These fractions significantlyaffect EOS predictions. The values of critical pressure, critical temperature, and acentric factor are required by most EOS'S. Several correlations exist forestimating critical properties of petroleum fractions. The most commonempirical correlations use specific gravity and boiling point as correlatingvariables. Whitson reviewed the most common of these empirical correlations. Hestudied the effect of C7+ characterization on EOS predictions and concludedthat none of the existing correlations gives consistently better EOSpredictions than the others. This paper suggests an alternative method forcalculating critical pressure for use in EOS. The method enhances VLE andliquid density predictions for the pseudocomponents representing C5+ or C7+fractions in reservoir fluids.

For some time, whether VLE predictions made on data measured in PVT cells orin EOS calculations correctly represents the behavior of natural mixtures inporous medial has been questioned. Many investigators studied this problem bothexperimentally and theoretically. The results of experimental studies by Trebinand Zadora and Tindy and Raynal indicated that the presence of a porous mediumhas a significant influence on the VLE behavior of hydrocarbon mixtures. On theother hand, results reported in Refs. 7 through 11 indicated that the presenceof a porous medium has little effect on VLE relationships. These contradictoryfindings are discussed in the final part of this paper, along with the effectsof capillary pressure on dew- and bubblepoint parameters.

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