Establishing the Maximum Carbon Number for Reliable Quantitative Gas Chromatographic Analysis of Heavy Ends Hydrocarbons. Part 1: Low-Conversion Thermal Cracking Modeling
Résumé
Reservoir fluid characterization by gas chromatography (GC) has an impressive capability of detection and quantification of a wide range of single carbon number (SCN) groups in oil analyses. However, some researchers prefer to report analyses to C20+ only, with estimation of the Cn+ fraction distribution obtained using various correlations. Conversely, other researchers prefer to extend GC analysis to the highest possible SCN group using high-temperature gas chromatography (HTGC), with programming to ca. 370-430 °C. However, the reliability of extended GC analyses to high carbon number fractions is questioned because of a possible overestimation of light and intermediate fractions in the original oils caused by thermal decomposition products. The thermal stability of heavy hydrocarbons at the above HTGC conditions has been a major concern for some authors based on the results of thermogravimetric analysis (TGA) published by Schwartz et al. [Schwartz, H. E.; Brownlee, R. G.; Boduszynski, M. M.; Su, F. Anal. Chem. 1987, 59 (10), 1393−1401], who highlighted thermal instability of heavy oils from around 370 °C. To that end, in this study, a pyrolysis model spanning the n-alkanes (nC14H30-nC80H162) at low conversion has been developed and applied to mixtures at the GC column pressure and oven temperatures up to 450 °C. On the basis of this model, the minimum SCN, which could possibly be at risk of thermal cracking at some commonly used HTGC temperature programs, has been obtained by comparing the retention time of n-alkane standard mixtures (nC10H22-nC75H152) and the minimal pyrolysis time at the same SCN range of equimolar, heavy, and light mixtures at different dilutions in He and some low isoconversion pyrolysis curves. Finally, this study gives the first insight into the limitation in the practice of GC and introduces a new approach for calculating the minimum SCN not suffering pyrolysis inside a particular GC column.