The Effect of Asphaltenes on Crystallisation, Deposition and Solid-state Structure of Waxy Fluids

Asphaltenes and waxes are two components of crude oil that cause flow assurance issues. Although the components coexist, studies on the effect of asphaltenes on wax formation are contradictory and understanding is inconclusive. Such a contradiction has been attributed to the complex chemistry of asphaltenes and how this influences their interaction with paraffin waxes at a molecular level.
To better understand the effect of different structural, chemical and physical features of asphaltenes, a novel asphaltene fractionation method known as “extended-SARA” (E-SARA) was used to separate the whole asphaltene (WA) in the crude oils into strongly and weakly interfacially active asphaltenes by partitioning at an oil-water interface. These subfractions are commonly termed interfacially active asphaltenes (IAA) and remaining asphaltenes (RA) and have been shown to exhibit differing physicochemical properties. Therefore, this PhD thesis investigates the effect of those contrasting physicochemical properties on the wax crystallisation kinetics and gel-forming properties. Then, the study is extended to examine how asphaltenes (WA) impact the wax deposition behavior and modify the wax deposit properties under static and dynamic flow regimes.
The presence of RA or IAA asphaltenes, studied using polythermal crystallisation and turbidometric detection, was found to reduce the crystallisability of octacosane (C28) – a model waxy compound– by increasing the solubility temperature and broadening the metastable zone width. The study reveals a two-step crystallisation inhibition mechanism, where asphaltenes suppress initial molecular aggregation and later inhibit crystal growth. This effect was more pronounced with smaller asphaltene clusters at lower concentrations, which have greater crystallisation inhibitory power.
Further analysis of asphaltenes extracted from heavy crude oil shows that both RA and IAA lowered the wax gelation and appearance temperatures. RA decreased the gel yield strength with increasing concentration, while IAA increased it due to differing asphaltene-asphaltene interactions. RA interactions are repulsive with negligible adhesion, while IAA interactions are attractive with strong adhesion, highlighting their structure-breaking and structure-making properties.
In the wax deposition studies, the addition of asphaltenes during thermally-driven (static) deposition was found to reduce the rate of wax deposition and thickness of wax deposits formed by lowering the wax appearance temperature (WAT), requiring greater subcooling. Under shear-driven regime, hydrodynamics dominate, leaving the deposition rate and thickness unaffected by asphaltenes. However, asphaltenes increased the average molecular weight and wax content of the deposits. The overall yield strength of deposits remains balanced due to increased wax content countering the reduced wax gel strength.
These findings provide new insights into the complex mechanisms by which asphaltenes modify wax crystallisation, gel formation, and deposition, enhancing the understanding of flow assurance in crude oil production.