Extended Analysis for Gas Condensate Systems

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. Introduction With the increasing emphasis on liquid natural gas (LNG), natural gas liquids (NGLs) and liquid condensates during the last 15 years, gas condensate reservoirs became increasingly important. A combination of laboratory studies, such as Chromatographic; true boiling point (TBP); and pressure, volume, temperature (PVT) analyses became necessary for characterizing these reservoir fluids and evaluating their volumetric performance at various pressure depletion stages. An accurate description of pseudocomponent compositions is an integral part of the reservoir fluids characterization process. For gas condensate systems, these data are applied with Equations of State (EOS) to evaluate gas and condensate reserves and production for field development and surface facility design. The evaluations rely on a tuned EOS formulated from adjustment of SCN compositions. Good quality compositional data require minimal adjustment for obtaining the best match between predicted and experimental phase behavior data. Very often the required extended compositional data are unavailable experimentally and are generated from mathematical relationships. Literature (Ahmed 1989; Danesh 1998; Pedersen et el. 1989) has shown that a plot of SCN composition against molecular weight produces a continuous exponential relationship for gas condensate systems. This observation also led to a generally accepted representation of a single straightline relationship between log of mole percent and molecular weight for these SCN fractions. Based on this observation, very useful functional approaches called "splitting" schemes (Whitson 1983; Pedersen et al. 1984) were devised to describe the composition of these SCN fractions in the absence of experimental data. Although splitting schemes are applied from the C7+ or last available plus fraction, a review by Danesh (1998) stated that a partial analysis is first required followed by the application of these schemes. To date, literature has not specified the SCN or last plus fraction for terminating a partial analysis. From an examination of compositional analysis for gas condensate systems, this paper describes a different universal trend from the single straightline relationship between log of mole percent and molecular weight. Also, the last plus fraction is defined for terminating a partial analysis. A total of 22 compositional data sets to C20+ were examined. Six of these were generated experimentally from separator samples taken in Trinidad (Hosein 2004) and 16 were taken from PVT lab reports generated from samples taken worldwide.