Establishing the Maximum Carbon Number for Reliable Quantitative Gas Chromatographic Analysis of Heavy Ends Hydrocarbons. Part 1: Low-Conversion Thermal Cracking Modeling

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.

Reactive Transport and Its Implications on Heavy Oil HTGC Analysis – A Coupled Thermo-Hydro-Chemical (THC) Multiphysics Modelling Approach

“…The large number of species in the reduced free-radical pyrolysis model developed in [23] has imposed a need to develop a reduced molecular pyrolysis model, comprising 11 n-alkanes (nC 14 H 30 ,nC 16 In this work, a "class" molecular mechanism has been obtained after applying the following three rearrangements to our reduced molecular mechanism model [7]:…”

“…Refer to [23], to understand the thermal cracking original kinetic and mechanism model development, and refer to [7] to understand the detailed explanation of the kinetics and mechanism reduction procedure from molecular mechanism model to a "class" molecular mechanism.…”

“…6. Their thermal cracking is modelled in the gas phase only, using a reduced and optimised kinetics and mechanism developed previously [7] and validated against a detailed mechanism of the heavy oil mixture developed initially [23].…”

“…Initially, for every component studied, the position of the zone's centroid in the next time step (x i+1 ), is calculated, using Snijders [23,27] approach Eq. (8) (see ref.…”

Establishing the Maximum Carbon Number for Reliable Quantitative Gas Chromatographic Analysis of Heavy Ends Hydrocarbons. Part 3. Coupled Pyrolysis-GC Modeling

“…The large amount of species of the reduced free-radical pyrolysis model developed and explained in the first paper of this series 1 ). Similarly, the GC model developed in (COMSOL-MATLAB) and explained in detail in the second paper of this series 2,3 for predicting the zone's variances while every component is migrating and partitioning between the stationary and the gas phases was computing-time-demanding, and therefore an analytical and high performance GC model was required to be developed.…”