Reversibility of Asphaltene Aggregation As Revealed by Magnetic Resonance Imaging in Situ

Abstract

Aggregation of asphaltenes followed by precipitation presents severe problems for existing technologies in the production, recovery, and processing of heavy oils. Better understanding of asphaltene behavior behind the processes of their precipitation and dissolution is vital to address this issue. While investigating the inhomogeneity of different oil systems, the reversibility of the asphaltene aggregation process initiated by flocculant in either asphaltene solution in toluene or crude heavy oil was revealed and investigated using magnetic resonance imaging methods. It was found that the inhomogeneous distribution of the flocculant initiates local spatial-selective asphaltene aggregation registered in a thin layer around the flocculant/oil sample interface. The local excess of flocculant concentration over the threshold of asphaltene precipitation onset is a driving force of this process. As the flocculant diffuses into the volume of the sample, a decrease of the asphaltene flocculated area is observed until it disappears when the equilibrium composition throughout the whole volume of the system is achieved. Depending on the overall flocculant concentration, the asphaltene aggregation may not be reversible and could be followed by subsequent precipitation of the asphaltene aggregates. The similarity of the phenomena observed for the model asphaltene solutions and crude heavy oil samples was established. Partial mechanical stirring of the multicomponent system comprising flocculant and oil or asphaltene solution does not prevent the formation of the local zones with increased concentration of asphaltene aggregates; those sizes evolve depending on the flocculant concentration. The results obtained in this work are consistent with the generally accepted concept of asphaltene precipitation reversibility depending on the system composition and are compatible with the observations obtained by other methods. The approach presented can provide deeper insight into the asphaltene precipitation reversibility issue and can facilitate the understanding of asphaltene behavior in heavy oils.

Fractionation and Characterization of Petroleum Asphaltene: Focus on Metalopetroleomics

“…NMR methods [23,24] have proven to be extremely useful for defining the average molecular parameters of asphaltene fractions. Through the application of 1H-NMR [89][90][91], 13C-NMR [89,[92][93][94], distorsionless enhancement by polarization transfer (DEPT) [95,96], solid-state (SS) NMR [94,97,98], and magnetic resonance imaging (MRI) [99][100][101], the hydrocarbon skeleton could be described in much detail. Although the chemical structure of asphaltene samples depends on geographical origin and even wells in the same crude oil production region, the deduced picture of asphaltenes from these studies is that of aromatic polycyclic clusters variably substituted with alkyl chains that may be quite long (up to C 10 -C 12 ), and connected by alkyl and heteroatom bridges.…”

NMR Spectroscopy Analysis of Asphaltenes

“…We have so far addressed the application of solution and solid-state high- and low-field NMR on the characterization of asphaltenes chemical and molecular structure, interactions, morphology of association and aggregation. Magnetic resonance imaging (MRI) has also been applied, ,, albeit a few to study asphaltenes for understanding the process of association, aggregation, and precipitation in bulk. This technique has been widely employed to many areas of research and development , because of its nondestructive visualization of various processes occurring in optically opaque systems.…”

“…In the subsequent year, Morozov et al have furthered the study in quest of understanding of asphaltene aggregation and precipitation . They have observed the asphaltene precipitation and dissolution process initiated by flocculant both in toluene–asphaltene solution and crude heavy oil are reversible.…”

“…Elapsed times are indicated in the figure. The appearance of a thin layer within the mixing zone and deposits are indicated by arrows (Reprinted with permission from ref . Copyright 2017 American Chemical Society).…”

“…NMR spectroscopy has extensively been used in both solution , and solid-state for characterization, structural elucidation, and analysis of thermal cracking products of asphaltenes . NMR imaging is employed also for studying asphaltenes …”