Asphaltenes, the most complex and surface-active constituents of crude oil, readily aggregate, precipitate, and deposit onto solid interfaces, creating significant operational challenges across the petroleum value chain. Their deposition disrupts flow assurance in upstream production, reduces efficiency in downstream refining, and contributes to stability and sludge formation in marine fuels. Although the general principles governing asphaltene precipitation are understood, much less is known about how elevated temperatures and oxidative conditions common in thermal recovery, refining processes, and marine fuel handling affect deposition kinetics and the chemical composition of foulant layers. This PhD thesis investigates the stability, aggregation behaviour, and surface deposition of asphaltenes under these high-temperature and oxidising environments to establish mechanistic connections between bulk destabilisation and interfacial fouling.
Temperature-dependent aggregation and deposition behaviour were studied using dynamic light scattering (DLS) and a custom built high-temperature, high-pressure quartz crystal microbalance (HTHP QCM-D). Increasing temperature consistently decreased solubility and accelerated aggregation, with particle sizes increasing from submicron ranges to several micrometres, particularly in less aromatic solvent systems. QCM-D measurements showed that deposition increased monotonically with temperature irrespective of solvent aromaticity, driven primarily by reduced viscosity and enhanced diffusivity rather than solubility effects alone. Initial adsorption kinetics indicated the presence of large, fast-diffusing aggregates even in aromatic solvents. Characterisation of deposits extracted from the stainless-steel surfaces further showed that the increase in deposited mass with temperature are associated with changes in heteroatom content primarily oxygen-containing species suggesting a temperature and chemically selective deposition mechanism.
To further probe asphaltene deposit chemistry at increasing temperatures (50–150 °C) and under varying solvent aromaticity conditions, deposits were analysed using positive-ion APPI FT-ICR MS. Deposits formed in toluene were dominated by mono-oxygenated species and exhibited sharper mid-DBE enrichment, while mixed aromatic–aliphatic solvents produced broader DBE distributions characteristic of more bulk-like aggregation. With increasing temperature, deposits shifted toward higher O/C and lower H/C ratios, indicating temperature-enhanced selective adsorption of polyfunctional oxygenated species. These findings show that both temperature and solvent aromaticity govern which chemical fractions preferentially deposit on solid surfaces.
Finally, the role of oxidation was studied, where asphaltenes were oxidised using an ozone/oxygen mixture, which increased the oxygen content from 2.36 wt.% to 6.84 wt.% and introduced oxygen-containing functional groups that increase polarity and strengthen intermolecular interactions. Oxidised asphaltenes (OA) displayed significantly reduced stability compared with whole asphaltenes (WA), with the precipitation onset shifting from ~50 vol% to ~40 vol% heptane. Optical microscopy confirmed faster and more extensive aggregate growth in OA. At the solid–liquid interface, OA exhibited enhanced surface activity, forming thicker, rougher, and more viscoelastic layers compared to the more rigid monolayers formed by WA. Additionally, testing a commercial inhibitor dodecylbenzene sulfonic acid, DBSA showed that while it effectively stabilised whole asphaltenes, significantly higher concentrations were required to mitigate the deposition of oxidised species.
Together, these findings demonstrate that temperature and oxidation influence asphaltene behaviour through complementary but distinct mechanisms. Temperature promotes accelerated aggregation and selective deposition, while oxidation enhances polarity, strengthens intermolecular attraction, and amplifies deposition propensity. By integrating high-temperature adsorption measurements with detailed chemical analysis of bulk and deposited material, this thesis establishes a mechanistic framework that clarifies how asphaltenes destabilise and foul surfaces under realistic thermal and oxidative conditions. The outcomes provide a basis for designing more effective dispersants and predictive tools for managing asphaltene deposition in heavy-oil systems.
