The structure and function of asphaltenes : an organic geochemical and ultrasound study

Abstract

A multidisciplinary approach is used in order to gain a deeper understanding of petroleum geochemistry and specifically the aggregation properties of the asphaltenes. Such knowledge is essential to solving problems caused by asphaltene deposition in both the downstream and upstream sectors of the oil industry. In this thesis a thorough organic geochemical characterisation of a set of asphaltenes originating from a variety of non-degraded to biodegraded crude oils from locations distributed around the world including Nigeria, United Kingdom, Middle East, North America and Serbia have been performed. Several geochemical techniques including Iatroscan analysis and thin layer chromatography were used to describe the crude oils. Fourier transform infrared (FTIR), ultrasonic spectrometry and ruthenium ion catalysed oxidation (RICO) analysis in combination with gas chromatography-mass spectrometry (GC-MS) were used for the molecular characterisation of the asphaltenes. The crude oils have been characterised to elucidate their level of degradation, source, depositional environment and thermal maturity by a variety of biomarker and non-biomarker parameters. The results reveal differences in the extent of biodegradation within the samples, ranging from level (1) to (6) using the Peters and Moldowan biodegradation scale (Peters et al., 2005a). The samples are sourced from either terrigenous organic matter deposited under oxic-suboxic conditions, marine organic matter deposited under anoxic conditions or a mixed contribution of terrestrial and marine sources. The processes to determine asphaltene concentrations in the oils were tested using Iatroscan and subsequently compared to gravimetric analysis. Results from both techniques show variations in asphaltene content in the whole oil. Following RICO analysis, acids are generated and released covalently bound hopanoic acids that reveal the existence of carbon number shift in the hopanes. The chemically bound biomarkers are consistent with low maturity distributions amongst all oils. Analysis of the FTIR spectra from the asphaltenes show that crude oil asphaltenes consist of aliphatic moieties bound to aromatic structures. Although, aliphatic moieties iv are dominant, carboxyl groups and other oxygenated functionalities are significant components present. Carboxyl groups were detected in most of the asphaltenes analysed in this work from around the world, but were not detected in heavily biodegraded samples. High-precision ultrasound resonance spectroscopy was used to elucidate the mechanism of asphaltene aggregation in toluene solution. A new method namely the attenuation of sound for detecting asphaltene aggregation was developed. This work has identified that asphaltenes exhibit a critical nanoaggregate concentration (CNAC) in toluene at approximately ~0.03 – 0.16 g/L, depending upon the nature and source of the asphaltene. The CNAC of asphaltenes from the non-biodegraded oils is ~0.03 – 0.10 g/L and from a biodegraded oil is ~0.16 g/L. The results confirm previous aggregation measurements in that asphaltenes can self-associate in solution to give rise to, more or less, extended aggregates. In general, CNAC is controlled by the effect of biodegradation and attenuation data has considerable potential to corroborate previous finds regarding CNAC.