Temperature-Triggered Rearrangement of Asphaltene Aggregates as Revealed by Pulsed-Field Gradient NMR

Abstract

The tendency of asphaltenes for aggregation followed by precipitation and deposition plays a crucial role in the petroleum industry since these processes present severe problems during the production, recovery, and processing of crude oils and fossil hydrocarbon feedstocks. The dynamics of oil asphaltene aggregates dissolved in chloroform at different concentrations varied in a wide range that was investigated at temperatures from 0 to 55 °C using the Pulsed-Field Gradient NMR technique. The components attributed to nanoaggregates and macroaggregates were successfully resolved, which allowed us to measure their diffusion coefficients. The diffusion coefficients for all types of aggregates grow as the asphaltene concentration decreases, whereas the partial weight of the aggregates increases with the increase of asphaltene concentration. The difference in diffusion behavior of the aggregates of different types was registered when passing the critical concentration range 10–20 g/L. The nano- and macroaggregates behave independently when the asphaltene concentration is higher than 20 g/L (concentrated regime), while below 20 g/L (semidiluted regime) the components related to the different types of aggregates cannot be properly resolved. It was found that regardless of the asphaltene concentration, the diffusion coefficients for nano- and macroaggregates demonstrate similar temperature behavior giving the straight lines in the Arrhenius coordinates which change their slopes when passing the temperature range 20–30 °C. The phenomenon evidences the thermally induced cleavage of noncovalent bonds with subsequent rearrangement of asphaltene aggregates that is observed for all concentration regimes covering the existence of asphaltene aggregates of all types. The data obtained are well consistent with the modern concept of asphaltene aggregate structure and fairly agree with the data obtained earlier. We believe these results will contribute essentially to a better understanding of the fundamental behavior of asphaltenes and their aggregates, providing a deep insight into aggregate transformation triggered by the temperature.

Impact of Asphaltenes on the Adsorption Behavior of Petroleum Vanadyl Porphyrins: Kinetic and Thermodynamic Aspects

“…The point is that asphaltene nanoaggregates could undergo rearrangement upon adsorption, such as a change in composition and/or aggregation number. Such an opinion is supported by a number of works, in which rearrangement of dissolved asphaltene nanoaggregates or release of petroporphyrins from them upon change of solution concentration 8 and temperature 9,62 was observed. If the nanoaggregate structure is condition-sensitive, then it could be anticipated that the composition of adsorbed nanoaggregates would differ from the one in the bulk of the solution.…”

Influence of High-Molecular n-Alkane Associates on Rheological Behavior of the Crude Oil Residue

“…Figure shows even more enlarged images of oil containing two large resin and asphaltene particles. It is known that asphaltene molecules tend to self-aggregate and can form both asphaltene nanoaggregates and clusters of asphaltene nanoaggregates. − According to the size of spherical formations seen in Figure , asphaltenes together with resins can form much larger conglomerates than previously thought. Apparently, paraffin hydrocarbons play a certain role in this process.…”

“…Due to the diversity of asphaltenes’ structures and the tendency to interact through various intermolecular forces (hydrogen bonds, acid–base, donor–acceptor, dipole–dipole, π-complex, exchange, etc. ), asphaltenes in good solvents and in crude oil can be observed both in the molecular state (dispersed asphaltenes) and in the form of supramolecular structures (aggregated asphaltenes). − It is known that asphaltenes tend to self-aggregation and can form asphaltene nanoaggregates of 6–10 molecules suspended in oil . Further association leads to the formation of clusters of asphaltene nanoaggregates. − The clusters consist of 8–10 nanoaggregates, the size of which is 2–5 nm. ,, The “continental” model of asphaltene structure assumes the formation of stacking structuresaggregates in the form of packs of several molecules with a plane-parallel arrangement of aromatic systems, held together with π–π and hydrogen bonds .…”

Viscosity Modification of Heavy Crude Oil by Using a Chitosan-Based Cationic Surfactant

“…Previously, various methods have been used to detect the asphaltenes aggregation and precipitation, for instance, high-Q ultrasonics, small-angle neutron scattering (SANS), X-ray diffraction (XRD), , NMR, − DC conductivity, , and centrifugation. , The formation of nanoaggregates starts with the conglomeration of <10 molecules at 50–150 mg/L concentration, which is called the critical nanoaggregation concentration (CNAC). Whereas, the concentration of asphaltenes increases to 1.2 to 2 g/L, this concentration is called the critical cluster concentration (CCC), with an average size of 6 nm.…”