Studying the effects of asphaltene oxidation on wax crystallization and rheological properties in waxy crude oils

Abstract

Oxidation in crude oil production is unavoidable. Oxidized asphaltenes, interacting with waxes during crystallization, profoundly alter wax precipitation and gel behavior in crude oils. This research evaluated the impact of asphaltene oxidation on the wax precipitation characteristics and rheological properties of waxy crude oils during the processes of crude oils extraction and transportation. A comprehensive structural characterization of the asphaltenes was conducted, revealing an increase in aromaticity and a reduction in the length and abundance of aliphatic chains following oxidation, accompanied by the formation of larger asphaltene aggregates. Subsequently, an investigation into the wax precipitation behaviors of waxy model oils showed that asphaltenes were involved in the crystallization processes of wax molecules, which had a minor impact on wax precipitation characteristics (crystallization and crystallization enthalpy). However, it had a significant influence on the rheological properties (pour point, flow curve and gel strength) of the systems. And it was also observed that the degree of influence was dependent on the structure and composition of the given asphaltenes and waxes. Asphaltenes with abundant aliphatic chains (before oxidation) tended to assume favorable positions during the crystallization process, which interfered to some extent with the normal growth of wax crystals and resulting in a transition from a “strong gel” to a “weak gel” network structure of wax crystals. Conversely, the oxidized asphaltenes with a high aromaticity exhibited a greater interference with wax crystal growth. This interference not only more effectively hindered the establishment of the wax crystal network but also prompted the formation of an “extremely fragile gel structure”.