Predicting Phase Behavior of Condensate/Crude-Oil Systems Using Methane Interaction Coefficients
Authors: Katz D.L. , Firoozabadi A. 
Published: December 2007

Introduction
This paper extends the Peng-Robinson-AGA procedure for computing phase behavior to include gas-condensate and crude-oil systems at reservoir conditions. Interaction coefficients from methane to the C6+ groups were correlated with liquid density. With these coefficients, the behavior of gas condensate systems may be predicted reliably.

Introduction
Prediction of phase behavior of complex hydrocarbon Prediction of phase behavior of complex hydrocarbon systems has improved gradually. Recent papers using equations of state for predicting phase fugacities of constituents have extended their usefulness to higher boding components. Using extended analysis with boiling points of higher boiling hydrocarbon groups and their points of higher boiling hydrocarbon groups and their densities and molecular weights provides a basis for improvement when predicting phase behavior. At the high pressure of reservoir conditions, properties of these C6+ groups are important.
Using the Peng-Robinson equation of state has the advantage of improving accuracy when calculating liquid density. When used by Bergman et al. with extended gas analyses, the equation gave good prediction of dew points and liquid yield at pipeline conditions. Bergman et points and liquid yield at pipeline conditions. Bergman et al. used interaction coefficients but did not completely explore their usage since phase behavior of condensates at pipeline conditions appeared rather insensitive to the C6+ interaction coefficients.
This paper reports the extension of the Peng-Robinson-AGA procedure for phase prediction of Peng-Robinson-AGA procedure for phase prediction of condensates and natural-gas/crude-oil systems at reservoir conditions. New interaction coefficients were found necessary for methane and C6+ constituents (especially the highest boding group, such as C20+) so that experimental behavior could be matched by prediction. Steps for developing interaction parameters are presented. Then, agreement between measured and predicted phase behavior up to pressures of 64 121 kPa (9,300 psia) is shown for several crude oil and condensate systems.

Calculations of Phase Behavior
The equation of state, using the relationship of fugacity to partial molal volumes, is used to compute the fugacity of partial molal volumes, is used to compute the fugacity of each component in both liquid and gaseous phases. At equilibrium the fugacity of each component in liquid equals its fugacity in the gaseous state.
Basic parameters for each constitutent are the critical properties, normal boding point or vapor pressure. For properties, normal boding point or vapor pressure. For hexane and heavier groups present in natural-gas and crude-oil systems, average normal boiling point, liquid density, and molecular weight are used instead of measured properties available for pure components.
For systems containing various molecular families, the interaction coefficients between the primary constituents must be included to account for the degree of compatibility. Here methane and higher boiling C6+ groups are the principal components for which interaction coefficients principal components for which interaction coefficients appear significant.
Since properties of C6+ groups in crude oil and condensates are needed here, generalized properties are given in the absence of measured values.
Properties of Hexanes and Heavier Groups Properties of Hexanes and Heavier Groups Average boiling points, liquid densities, and molecular weights were reported by Bergman et al. from C6 through C15, based on analysis of 26 condensates and naturally occuring liquid hydrocarbons.
JPT
P. 1649