Characterisation of Waxy Gas-Condensates by High Temperature Capillary Gas Chromatography and Oxidative Degradation

David J. Heath

PhD – March 1995

Department of Environmental Sciences, University of Plymouth

in collaboration with

British Gas plc, U.K.

High molecular weight (HMW) hydrocarbons (defined herein as C35+ compounds) are difficult to characterise by conventional analytical methods. Very few studies have reported precise and reproducible quantification of such compounds in fossil fuels. Nonetheless, such components have important effects on the physical and biological fate of fossil fuels in the geosphere. For example, the phase behaviour of waxy gas condensates is significantly affected by the varying proportions of HMW compounds. Similarly HMW compounds are amongst the most resistant petroleum components to biodegradation. The current study reports the development of reproducible quantitative high temperature capillary gas chromatography (HTCGC) methods for studying both these aspects of the chemistry of HMW hydrocarbons. In addition those hydrocarbons which remain unresolved when analysed by gas chromatography (so called unresolved complex mixtures UCMs) are also studied. UCMs may account for a large portion of the hydrocarbons in many fossil fuels yet very little is known about their composition. Knowledge of these compounds may be important in enhancing the prediction of phase behaviour. Oxidative degradation and GC-MS is used to elucidate the types of structures present within the UCM.

The concentrations of C35+ hydrocarbons in two unusually waxy gas condensates from high temperature wells in the North Sea were determined by HTCGC. The whole C35+ fraction comprised about 20% of the total hydrocarbons and consisted of compounds with carbon numbers extending up to and beyond C80. By paying particular attention to sample dissolution and injection, good reproducibility and precision were obtained. For example, for authentic n-C20 to n-C60 alkanes a relative standard deviation of under 5% for manual injection, linear response factors (1.01 C20 to 0.99 C60), and a linear calibration for 5 ng to 25 ng on-column were found. Limits of detection are reported for the first time for HMW n-alkanes. The limits were found to be as low as 0.8 ng for C20 to 1.87 ng for C60. Tristearin is proposed as a suitable HTCGC internal standard for quantification since the FID response factor (1.1) was close to that of the HMW n-alkanes and response was linear. Importantly, when co-injected with the two waxy North Sea condensates, tristearin was adequately separated from the closest eluting alkanes, n-C59n-C60 under normal operating conditions. Qualitative characterisation of the HMW compounds in the waxy gas condensates and in synthetic wax blends (polywax 1000) using HTCGC-EI MS and HTCGC-CI MS produced molecular ions or pseudo molecular ions for n-alkanes up to n-C60. The spectra of some HMW compounds contained fragment ions characteristic of branched compounds but detailed characterisation was very limited.

This study has also shown, for the first time, the significance of the unresolved complex mixture in gas condensates. UCM hydrocarbons accounted for over 20% of the total hydrocarbons in a waxy North Sea condensate. The condensate was first distilled and the distillate UCMs isolated. These were found to be between 64 to 97% unresolved after molecular sieving (5Å) and urea adduction. The UCMs were oxidised using CrO3/AcOH which produced 5-12% CO2, and 55-83% dichloromethane-soluble products. Thus 65-94% of the original UCMs were accounted for as oxidation products. The remainder were thought to be water soluble acids which could not be determined in the presence of the AcOH reagent. Of the recovered oxidised products, 27-81% were resolved and these comprised mainly n-monocarboxylic acids (19-48%). The average chain length was found to be C12 indicating the average length of alkyl groups. Branched acids, ketones, ketoacids, n-dicarboxylic acids, branched dicarboxylic acids, lactones, isoprenoid acids, alkylcyclohexane carboxylic acids and toluic acids accounted for the majority of the remaining resolved products. The distillate UCMs all showed variations in amounts of products but not in composition. Retro-structural analysis suggested that the UCM in the gas condensate was mainly aliphatic and branched. The number of isomers of simple branched alkanes over the UCM molecular weight range (determined by cryoscopy) was calculated to be over 15000. Overall, oxidation provided structural information for about half of the UCM.

HTCGC was also used to measure the biodegradability of HMW alkanes in a waxy Indonesian oil. Traditional alkane isolation techniques (TLC and CC) discriminated against HMW compounds above C40 whereas adsorption onto alumina in a warm cyclohexane slurry provided an aliphatic fraction still rich in HMW compounds and suitable as a biodegradation substrate. A waxy Indonesian oil was subjected to 136 day biodegradation by Pseudomonas fluorescens. Extraction efficiencies of over 90% (RSD <5%) were obtained for n-alkanes up to C60 using continuous liquid-liquid extraction. Over 80% of the oil aliphatic fraction was degraded within 14 days. After 136 days only 14% of the original aliphatic fraction remained, yet surprisingly no decreases in the concentrations of compounds above C45 were observed. However, the use of a rapid screening biodegradation method proved conclusively that Pseudomonas fluorescens was capable of utilising n-alkanes up to C60 once the bacteria had acclimated to the HMW alkanes. This is the first report of bacterial utilisation of an n-alkane as large as C60.

Copyright 1995 D.J. Heath. All rights reserved.

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