Properties of Asphaltenes with emphasis on NMR self-diffusion as the investigating technique

Doktorsavhandling, 2003

Asphaltenes are high-molecular weight, highly aromatic molecules that have a strong tendency to aggregate. Asphaltenes are found in oil and may, if they form particles, cause problems during oil production and handling. The objective of this thesis is to provide a better understanding of asphaltene chemistry, aggregation and flocculation behaviour, and to add to the knowledge of how to prevent asphaltenes from forming particles. The approach used is partly methodological in that the NMR self-diffusion technique, also known as pulsed gradient spin echo (PGSE) NMR, was demonstrated to be valuable for studying the average diffusion and structure of asphaltenes. Studies of the diffusion of asphaltenes in mixed solvent / non-solvent systems indicated that a certain threshold concentration of non-solvent had to be reached before flocculation of asphaltenes occurred. Above the threshold concentration, studies of the initial flocculation using a UV-VIS spectrophotometer in the kinetic mode provided information about the kinetic stability of the asphaltene model systems. With the aim to obtain less complex asphaltene mixtures that could be easier to characterize, Cold Lake asphaltenes were fractionated into sub-fractions by a liquid-liquid extraction procedure. The initial flocculation rate of the asphaltenes from various sub-fractions was studied and placed in relation to their properties, such as aromaticity and average diffusion. It was shown that the sub-fraction containing the slowest diffusing and most aromatic species also contained the asphaltenes that most easily flocculated when alkane was added. Hence, it seemed that one asphaltene sub-fraction was especially troublesome in terms of instability. Furthermore, studies by X-ray photoelectron scattering (XPS) indicated that the sub-fraction with the smallest and least aromatic asphaltenes also contained carboxylic functional groups, while the sub-fraction that contained the most troublesome asphaltenes did not. Chemical interactions between asphaltenes and naphthenic acids were studied by PGSE NMR. The diffusion was component resolved using the program component resolved spectroscopy, CORE. Near infra red (NIR) spectroscopy was used to complement the PGSE NMR measurements. The results indicated that interactions between the asphaltenes and naphthenic acids were related to the type of crude from which the asphaltenes were separated. Moreover, studies were made to compare the efficiency of amphiphilic molecules on asphaltene sub-fractions. Amphiphilic molecules with acidic functional groups adsorbed preferentially to the troublesome asphaltene sub-fraction of Cold-Lake asphaltenes, and were also shown to be the best stabilisers for the asphaltenes.