Abstract

Since the heavy components in the petroleum mixtures have the strongest effect on the characteristics of the fluids, critical properties must be estimated for the petroleum fractions making up heptanes-plus (C7+). A number of procedures for characterization of the C7+ fractions have been suggested. Most correlations are empirical equations giving a best fit of graphical correlations based on experimental data. In this study, critical properties of C7+ fractions were estimated by using some of these correlations. However, each correlation resulted in different values for these critical properties. It's difficult to make a decision, about which correlation is the best, thus a new method is proposed. Equilibrium constants (Kvalues) calculated by evaluating Constant Volume Depletion (CVD) data (measured K-values) and by using vapour-liquid equilibria (calculated K-values) were compared with the changes of critical properties of heavy components till the optimum critical properties are reached.

A phase equilibria calculation of a gas-condensate system with Peng-Robinson equation of state was done by satisfying the condition of chemical equilibrium. A genetic algorithm was used to determine the optimum critical properties of heptanesplus (C7+) fraction. The effects of population size, maximum number of generation, crossover probability and mutation probability on maximum fitness were studied. Higher maximum generation number means higher chance to reach maximum fitness value. Population size is also important but crossover and mutation probabilities have greater effects on fitness. In order to determine the upper and lower limits of the variables that were optimised in genetic algorithm, a characterization study was performed. The correlations of critical properties of C7+ fraction are functions of molecular weight (MW) and specific gravity © of C7+ mainly. For this reason, any small change in these parameters affects the range of critical temperature and pressure of C7+.

Introduction

To be able to use an equation of state on oil or a gascondensate mixture the critical temperature, Tc, the critical pressure, Pc, and the acentric factor, w, must be known for each component of the mixture1. During the development of the application of EOS's to naturally occurring hydrocarbon mixtures, it has become clear that insufficient description of heavier hydrocarbons (e.g., heptanes and heavier) reduces the accuracy of PVT predictions. Volatile oil and gas-condensate volumetric phase behavior is particularly sensitive to composition and properties of the heaviest component2.

In this study, several correlations has been used to estimate critical properties of heptanes plus fraction3–9. Each correlationresulted in different values for these critical properties. Whitson (1984) indicated that relatively small differences in critical properties and acentric factor can result in significant differences in EOS predictions. He also suggested that C7+ characterization has a significant influence on EOS predictions of reservoir fluid behavior. It is difficult to make objective conclusions about which C7+ characterization schemes are best. A different approach to characterization is needed. A genetic algorithm was used to determine the optimum critical properties of C7+ fraction. Critical properties obtained by correlations have been used to define the boundaries needed

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