Abstract
The distribution of NMR relaxation times and diffusion coefficients in crude oils results from the vast number of different chemical species. In addition, the presence of asphaltenes provides different relaxation environments for the maltenes, generated by steric hindrance in the asphaltene aggregates and possibly by the spatial distribution of radicals. Since the dynamics of the maltenes is further modified by the interactions between maltenes and asphaltenes, these interactions—either through steric hindrances or promoted by aromatic-aromatic interactions—are of particular interest. Here, we aim at investigating the interaction between individual protonic and deuterated maltene species of different molecular size and aromaticity and the asphaltene macroaggregates by comparing the maltenes’ NMR relaxation ( and ) and translational diffusion (D) properties in the absence and presence of the asphaltene in model solutions. The ratio of the average transverse and longitudinal relaxation rates, describing the non-exponential relaxation of the maltenes in the presence of the asphaltene, and its variation with respect to the asphaltene-free solutions are discussed. The relaxation experiments reveal an apparent slowing down of the maltenes’ dynamics in the presence of asphaltenes, which differs between the individual maltenes. While for single-chained alkylbenzenes, a plateau of the relaxation rate ratio was found for long aliphatic chains, no impact of the maltenes’ aromaticity on the maltene–asphaltene interaction was unambiguously found. In contrast, the reduced diffusion coefficients of the maltenes in presence of the asphaltenes differ little and are attributed to the overall increased viscosity.
Highlights
Crude oils are complex fluids consisting of thousands of chemical species of different structures and sizes, which can be fractionated into saturates, aromatics, resins, and asphaltenes (SARA) based on their solubility in different solvents
The mean relaxation rate constants enable the construction of the R2/R1 ratio, equivalent to the widely used T1/T2 ratio, known from borehole analysis to evaluate the crude oil regardless of the detailed form of the relaxation time constant distributions
The mean relaxation rate constant is chosen, since the relaxation rate constant is, unlike the relaxation time constant, directly proportional to the spectral density function (see Equations (3)–(5)) and the mean relaxation rate constant is proportional to the mean spectral density function
Summary
Crude oils are complex fluids consisting of thousands of chemical species of different structures and sizes, which can be fractionated into saturates, aromatics, resins, and asphaltenes (SARA) based on their solubility in different solvents. The asphaltene molecules themselves consist of polycyclic aromatic hydrocarbons (PAH) with alkyl side chains and contain heteroatoms like O, N, and S. Their tendency to self-aggregate distinguishes them from the other oil components and results, after the formation of nanoaggregates and clusters, in asphaltene macroaggregates, which are colloidally dispersed in the crude oil or a solvent until they precipitate as black, friable solids up to dense solid deposits [1]
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