Extended Analysis for Gas Condensate Systems

Abstract

Summary Equation of State (EOS) predictions for gas condensate systems require extended analysis beyond the heptanes plus (C7+) fraction. In the absence of experimental data, several schemes have been proposed to extend these compositional data based on the observation that a single straightline relationship exists between log of mole percent and molecular weight for these pseudocomponents or single carbon number (SCN) fractions. An examination of compositional analysis for gas condensate systems showed a discontinuity in the relationship between mole percent and molecular weight at C8 and C13. As a result, two straight lines are needed for a more accurate description of SCN composition; one from C8 to C12, and the other from C13 and beyond. When applied, this new universal observation gives an improved prediction of SCN composition. An average absolute deviation of less than 6.0% between the predicted and experimental composition was obtained using parameters from two straight lines. From a single straightline relationship, this difference was as high as 36.0%. This new observation provides the basis for defining the partial experimental analysis required for applying extended models for a more accurate description of SCN composition. For the logarithmic distribution, a partial analysis to C20+ is required to define the change in slope at C13 and beyond. For the three parameter gamma distribution function, a partial analysis is required up to C14 and splitting can be applied from C14+ and beyond. These widely used models are not suitable for extending the C7+ fraction.