Non-Exponential 1H and 2H NMR Relaxation and Self-Diffusion in Asphaltene-Maltene Solutions

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 (T1 and T2) 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.

Modeling Molecular Interactions with Wetting and Non-Wetting Rock Surfaces by Combining Electron Paramagnetic Resonance and NMR Relaxometry

“…In order to understand the role of wettability on the observed relaxation properties of fluids, it appears reasonable to address all related properties independently of each other. Comparative studies of water and oil have been carried out and are close to the logging situation; what has hardly been obtained was a finer distinction of the different components in crude oil, for instance, a distinction between saturated and aromatic moleculesthere are indeed hints that aromatic maltenes in crude oil show a different relaxation behavior if the content of asphaltenes, which are also predominantly aromatic in nature, is high. , …”

“…Comparative studies of water and oil have been carried out and are close to the logging situation; what has hardly been obtained was a finer distinction of the different components in crude oil, for instance, a distinction between saturated and aromatic molecules�there are indeed hints that aromatic maltenes in crude oil show a different relaxation behavior if the content of asphaltenes, which are also predominantly aromatic in nature, is high. 10,11 Another approach is the systematic variation of the surface properties, e.g., by changing the relaxivity with cleaning, 12 or by changing its wettability. The latter can be carried out by simulating the natural exposure of the reservoir rocks to crude oil, i.e., by saturating the initially water-wet rock with crude oil or certain solutions and determining how the internal surface becomes covered with insoluble residues, making the sample oilwet in consequence.…”