Establishing the Maximum Carbon Number for Reliable Quantitative Gas Chromatographic Analysis of Heavy Ends Hydrocarbons. Part 2. Migration and Separation Gas Chromatography Modeling

Abstract

Reservoir fluid characterization by high-temperature gas chromatography (HTGC) extends the range of single carbon number (SCN) groups in oil analyses by temperature programming up to 450 °C. However, the reliability of HTGC analyses is questionable for two main reasons: first, possible pyrolysis of the injected oil inside the GC column which could induce overestimation of light and intermediate fractions; and second, possible incomplete elution of heavy fractions, which in turn would induce under-estimation. The former has been treated in the first paper of this series,(1) which focused on predicting the pyrolysis temperature of n-alkanes (nC14H30–nC80H162) at GC conditions. The latter is the focus of this second paper which introduces a gas chromatography migration and separation model for the n-alkane range nC12H26–nC62H126 in an HT5 column, using as main input the in-house distribution factors derived from isothermal GC retention time measurements. On the basis of the developed model, the concentration and velocity of the above n-alkanes were determined at every point and time throughout the GC column, for typical temperature-programmed analyses. Retention times were then predicted, and validated against experimental values, with an overall relative error within 2%. This study gives an insight into the components’ behavior throughout the GC column, allowing preliminary assessment of elution, by proposing a new approach for determining the non/incomplete elution of every component by introducing: the degree of elution, defined as the amount of component which has been eluted in relation to the amount injected. Thus, the degree of elution of each of the heavy n-alkanes studied in this work: (nC12H26–nC62H126) has been calculated for a typical temperature program. This new approach can be applied, in order to determine the analytical conditions required for ensuring maximum elution of a given component, with the possibility of improving the practice of HTGC by optimizing the separation process.