Analysis of Naphthenic Acids and Derivatization Agents Using Two-Dimensional Gas Chromatography and Mass Spectrometry: Impact on Flow Assurance Predictions

Abstract

In this work, a sensitivity study was conducted on naphthenic acid derivatization agents. Four silylation chemistries and one methylation chemical were initially evaluated on 10 model naphthenic acids using gas chromatography. An experimental design procedure was setup to look at a number of effects, including contact time, catalyst presence, and reagent concentration. Overall, the silylation agents resulted in higher derivatization yields compared to the methylation agent. Moreover, the silylation agents did not show major evidence of selective derivatization as a function of the naphthenic acid structure. A silylation agent [(N,O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA)] was then used to test commercial naphthenic acid mixtures with two-dimensional gas chromatography coupled to time-of-flight mass spectrometry using the electron-impact source (GC×GC−TOFMS). The results enabled identification of many different naphthenic acid species. To test the derivatization agents on realistic samples, naphthenic acid extracts obtained from two crude oils of flow assurance significance were separated with a liquid-phase extraction procedure. The naphthenic acids were then treated with a silylation agent (BSTFA) and a methylation agent (BF3/methanol). The derivatized naphthenic acids together with the non-derivatized naphthenic acids from both crude oils were further examined using medium-resolution time-of-flight mass spectrometry with an electrospray source (TOFMS). Differences were observed in the TOFMS spectra for the naphthenic acid extracts. Extracts that did not contain ARN naphthenic acid species did not show major differences between non-derivatized and derivatized spectra in the negative mode. Extracts that contained ARN did show differences between derivatized and non-derivatized samples in the negative mode. Use of BSTFA resulted in enhanced signals for ARN, particularly the second ionization. Use of BF3/methanol resulted in a poor ARN response compared to the non-derivatized spectra. ARN species were also observed in the positive mode after treatment with both BSTFA and BF3/methanol, but signals were very poor. Moreover, use of BF3/methanol resulted in poor solubility of the naphthenic acid extracts from the crude oil containing ARN species. No solubility issues were observed with the use of BSTFA. Overall, the results point to the shortcomings of the application of methylation chemicals as derivatization agents, particularly for naphthenic acids extracted from crude oil samples containing high-molecular-weight acids of flow assurance significance (e.g., ARN species).