Carboxylic acid composition and acidity in crude oils and bitumens

Abstract

As the world’s demand for crude oil increases and the amount of conventional reserves decline, the proportion of high acidity oils being produced is increasing, but this acidity can cause corrosion problems during production and refining. Total Acid Number (TAN) values are often used to predict whether a crude oil may cause corrosion problems and thus affect the value of the oil, although the relationship between TAN and the organic acid composition of oils is not fully understood. This thesis investigate the types of acidic compounds that contribute to acidity in crude oils and the geochemical factors that influence the compositions and concentrations of these compound classes in a suite of oils and bitumens from a variety of different locations, including the North Sea, Venezuela, Canada and California. This work in this thesis includes the development of a modified ASTM D664 titrimetric assay method for measuring acid numbers on small samples of heavy crude oil, core extracts and also isolated maltene and asphaltene fractions. The TAN values in the crude oils and their fractions analysed ranges from 0.04 to 21.24 (mg KOH/g). The results show that in general, the maltene fraction contributes most to the acidity in crude oils, however in some samples a large proportion of the oil TAN is contributed by the asphaltenes, even though they are quantitatively a small percentage of the oil. The geological reasons for the occurrence of oils with these highly acidic asphaltenes are not currently known. The analysis of isolated carboxylic acid fractions from ten oils of different origins, including those from different source depositional environments, levels of biodegradation and thermal maturity, using a gas chromatographic method, showed that the concentrations of total carboxylic acids corresponded well with TAN and biodegradation, indicating that these acidic compounds may be a major control on the acidity in crude oils and that the concentration of these were in turn controlled by the extent of biodegradation. ii Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) was used to characterise a set of North Sea crude oils with TAN values ranging from 0.11 to 7.58 (mg KOH/g oil). This showed that the O2 compound class (assumed to be mainly carboxylic acid “COOH” species) appeared as the dominant compound class under the analytical conditions used, with a strong correlation (r2 = 0.989) of the O2/N ratio with the TAN values of the oils, indicating that these compounds may control the TAN in these samples. This observation also applied to their maltene and asphaltene fractions. The dominant acid species in the high acidity North Sea oils, maltenes and asphaltenes were three and four ringed naphthenic acids. As the TAN of the oils increased, the double bond equivalent (DBE) distributions shifted to higher values indicating that their molecular structures became more highly aromatic. Fourier Transform Infrared (FTIR) spectroscopy potentially offers a much more rapid analysis of acids in oils compared to the ASTM D664 method. This study included the development of a rapid method for the determination of TAN by FTIR spectroscopy of conventional and heavy crude oils using single bounce attenuated total reflectance (ATR) and multi bounce horizontal HATR accessories. Using multivariate data analysis software, a multivariate model that correlates infrared spectra with the TAN value was developed using carbonyl (C=O) absorption bands ranging from 1770 to1650 cm-1. It was found that the correlation of FTIR measured TAN versus ASTM D664 measurements, obtained by multi bounce HATR (r2 = 0.943) were better than correlations produced by single bounce ATR (r2 = 0.812). Based on these findings, the measurement of oil acidity and TAN using FTIR is simpler and faster and also allows the analysis of small sample sizes and avoids other problematic issues such as the fouling of electrodes that can be experienced using the ASTM D664 standard method