No abstract

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 C 20+ only, with estimation of the C n+ 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. 370À430 °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), 1393À1401], who highlighted thermal instability of heavy oils from around 370 °C. To that end, in this study, a pyrolysis model spanning the n-alkanes (nC 14 H 30 ÀnC 80 H 162 ) 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 (nC 10 H 22 ÀnC 75 H 152 ) 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.

International audienc

The purpose of this research work is to determine the maximum single carbon number (SCN) which can be reliably quantified using High Temperature Gas Chromatography (HTGC) analysis of heavy oil hydrocarbons, accounting for (i) thermal cracking risk and (ii) the non/incomplete elution. To that end, an in-house coupled numerical Pyrolysis-GC model has been developed, capable of calculating the degree of elution and of simulating the migration, partitioning, and pyrolysis conversion of a mixture of 11 heavy n-alkanes spanning the range from nC 14 H 30 to nC 80 H 16 throughout the GC column. On the basis of this model and using a commonly used column configuration and temperature program, two conclusions have been made: (i) half of the mass injected of nC 80 thermally decomposed before nC 70 has eluted, suggesting a possible coelution of both nC 70 and the pyrolysis products of nC 80 and therefore making the HTGC analysis of nC 70 and heavier n-alkanes no longer reliable, and (ii) alkanes heavier than nC 70 take progressively longer to elute completely from the column, compromising the resolution of the peaks, i.e., nC 70 takes 2.5 min and nC 80 takes 8.5 min. Moreover, nC 80 remained 12.9 min in the isothermal plateau before complete elution, implying that the nC 80 peak will be overlooked and masked by the FID plateau signal, in combination with column bleed products. Therefore, in the case study the maximum reliable SCN which can be quantitatively analyzed with HTGC will be the lighter components than nC 70 .

No abstract