Critical Properties Estimation From an Equation of State
Authors: Whitson C.H. 
Published: December 2007

Whitson, C.H., Consultant

Member SPE-AIME

This paper describes a new method for calculating critical properties of petroleum fractions used as input to a cubic equation of state (EOS). The method differs from existing methods in that it forces the EOS to match measured values of boiling point and molar volume (molecular weight divided point and molar volume (molecular weight divided by specific gravity) for each petroleum fraction.

PVT predictions are made with the proposed method using the Peng/Robinson EOS for selected reservoir fluids reported in the literature. Saturation pressure and saturated density are calculated with the EOS and compared with experimentally determined values. Heptanes-plus (C7+) fractions are characterized using the proposed method and results indicate that both proposed method and results indicate that both volumetric and phase behavior are improved. Reservoir fluids used in the study represent a wide range of compositions with PVT properties reported at temperatures ranging from 38–120 C.

A new method is suggested for matching experimental saturation pressure with an EOS. Using the proposed method for calculating critical properties, the boiling point of the heaviest properties, the boiling point of the heaviest petroleum fraction is adjusted until mixture petroleum fraction is adjusted until mixture saturation pressure is matched. A near-linear relation exists between boiling point of the heaviest fraction and saturation pressure. It is suggested that this method has more physical meaning than the common practice of adjusting methane binary interaction coefficients.

Proposed methods can be used with any cubic EOS. Critical properties are presented graphically for the Peng/Robinson and Soave/Redlich/Kwong equations. A generalized form of the two-constant cubic EOS is proposed, and necessary expressions for phase and volumetric calculations me given. The critical-property method is diagrammed schematically to facilitate programming. It can be easily incorporated into existing PVT software already based on an EOS.

Introduction

Cubic equations of state are used to calculate volumetric and phase behavior of petroleum reservoir fluids. input data required by an equation of state (EOS) usually includes critical pressure, critical temperature, and acentric pressure, critical temperature, and acentric factor of each component in the mixture. For pure compounds these properties are known. Critical properties must be estimated for the petroleum properties must be estimated for the petroleum fractions making up heptanes-plus. Having defined critical properties for all the components in a mixture the EOS can be used to predict PVT properties. properties. This paper proposes a new method for calculating critical properties of petroleum fractions. The method requires normal boiling point and molar volume for each petroleum point and molar volume for each petroleum fraction. The EOS chosen is forced to fit the boiling point and molar volume by adjusting critical pressure and critical temperature. Acentric factor is estimated by an empirical correlation using boiling point and molar volume; this correlation is general and does not depend on the EOS.

Whitson reviews the most common empirical correlations used for estimating critical properties of petroleum fractions. He studies the properties of petroleum fractions. He studies the effect of C7+ characterization on EOS predictions and concludes that none of the existing correlations gives consistently better PVT predictions. Observations from this earlier work predictions. Observations from this earlier work led to the idea that an alternative approach could be used for defining critical properties of individual petroleum fractions based on the EOS used for mixture calculations.

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