Time based fractionation of asphaltenes

Abstract

The precipitation of asphaltenes has been established as a time-dependent process. Asphaltenes precipitated first are expected to be the most unstable asphaltenes and cause severe problems in oil productions. It has also been observed that the aggregation rate of asphaltenes is controlled by asphaltene concentration. In this study, we utilized time and solubility based fractionation to investigate the polydispersity of asphaltenes and differences in the aggregation tendency for asphaltenes precipitated at different asphaltene concentrations. Asphaltenes extracted from two different crude oils (A1 and Kl) were separately mixed with toluene to generate model mixtures with two different asphaltene concentrations (3 wt% and 8 Wt% asphaltenes in toluene). The solutions were fractionated by adding heptane as a precipitant at fixed concentrations. Destabilized asphaltenes were then collected at different times until equilibrium was reached. Microscopy and small-angle X-ray scattering (SAXS) results showed that the aggregation rate of fractionated asphaltenes strongly depends on time and asphaltene concentration. The fraction that precipitated earlier is more unstable and form larger nanoaggregates than the fraction precipitated later. Moreover, the results of asphaltene concentration suggest that asphaltenes precipitated from a higher asphaltene-content solution have more variation in properties, which influences their aggregation behaviour, compared to solutions with a lower asphaltene-content.