The Contrasting Behavior of Strongly and Weakly Interfacially Active Asphaltenes on the Rheology of Model Waxy Oils

Asphaltenes and waxes are two components of crude oil that cause flow assurance issues. Although the components coexist, few studies have considered the effect of asphaltenes on wax crystallization and gel-forming properties. Furthermore, the current understanding remains contradictory with both wax-alleviating and wax-aggravating behaviors observed. In this study, asphaltenes extracted from a heavy crude oil were fractionated into strongly and weakly interfacially active asphaltenes by partitioning at a water-oil interface. The two asphaltene fractions exhibited contrasting physicochemical properties, with the strongly interfacially active asphaltenes (IAA) being more polar due to their higher heteroatom content (particularly S and O) and forming larger aggregates in the solution compared to the weakly interfacially active asphaltenes (referred to as remaining asphaltenes, RA). The two asphaltene fractions lowered both the wax gelation temperature and wax appearance temperature; however, the effect was comparable. The unit cell lattice structure of the wax particle remained unchanged in the presence of asphaltenes, but the wax particles were found to be smaller with RA compared to IAA. However, the key finding of the study is how the two asphaltene fractions affected the yield strength of the gelled wax. For RA, the yield strength was lowered with an increasing asphaltene concentration, whereas for IAA, the overall effect was to increase the gel yield strength. Because the properties of the wax particles were largely unchanged by the two asphaltene fractions, the result suggests that the asphaltene-asphaltene interaction contributes to the overall yield strength. It was shown that the interaction between RA and RA is repulsive with negligible adhesion, whereas that between IAA and IAA is attractive with strong adhesion. These structure-breaker and structure-maker properties of the two asphaltenes confirm that the asphaltene-asphaltene interaction significantly contributes to modifying the yield strength of a waxy gel.