Probing Asphaltene Aggregation in Native Crude Oils with Low-Field NMR

Abstract

We show that low-field proton nuclear magnetic resonance (NMR) relaxation and diffusion experiments can be used to study asphaltene aggregation directly in crude oils. Relaxation was found to be multiexponential, reflecting the composition of a complex fluid. Remarkably, the relaxation data for samples with different asphaltene concentrations can be collapsed onto each other by a simple rescaling of the time dimension with a concentration-dependent factor xi, whereas the observed diffusion behavior is unaffected by asphaltene concentration. We interpret this finding in terms of a theoretical model that explains the enhanced relaxation by the transitory entanglement of solvent hydrocarbons within asphaltene clusters and their subsequent slowed motion and diffusion within the cluster. We relate the measured scaling parameters xi to cluster sizes, which we find to be on the order of 2.2-4.4 nm for an effective sphere diameter. These sizes are in agreement with the typical values reported in the literature as well as with the small-angle X-ray scattering (SAXS) experiments performed on our samples.

Aggregate structure analysis of Colombian heavy crude oil-derived asphaltenes using small angle X-ray scattering

“…In these spectra two regions can easily be distinguished: a first one corresponding to small q-values (q < 3 x 10 -2 Å -1 ) in which the scattered intensity is a fastly decreasing function of q (or power-law scattering), and a second one at large q-values (q > 3 x 10 -2 Å -1 ) where the scattered intensity shows a plateau (or Guinier region) followed by a slight decrease of the intensity at higher q. Similar scattering profiles have been obtained previously in small angle scattering studies for some concentrated Asphaltene solutions and crudes (Espinat et al, 1998;Roux, Broseta, & Deme, 2001;Sheu, 2006;Gawrys, Blankenship, & Kilpatrick, 2006;Zielinski, Saha, Freed, Hürlimann, & Liu, 2010).…”

Fluorine tracers for the identification of molecular interaction with porous asphaltene aggregates in crude oil

“…An attempt to fit the data by the formalism detailed in [22] was unsuccessful. A comparably strong power-law dependence in crude oils has been reported in [8,24] and was interpreted by a model involving the tumbling of finite-sized asphaltene aggregates. However, qualitatively comparable results were found for a number of bitumen samples, one of which is shown in Fig.…”

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

“…In addition, the self-aggregated structures of asphaltene molecules in crude oil or asphaltene-solvent solutions interfere with the motions of the maltenes, slowing them down so that the reorientations of the maltenes no longer fulfill the extreme narrowing limit, and ratios larger than one and a frequency dependence of are observed [ 27 ]. Investigations of maltenes’ relaxation in the presence of asphaltenes show faster relaxation with higher asphaltene content and a stronger increase in the transverse than the longitudinal relaxation rate [ 13 , 19 ]. The latter observation indicates significant importance of the decreased maltene mobility due to contact and entanglement with the asphaltene structures, as the proximity to free electrons influences the longitudinal and transverse relaxation equally.…”

“…NMR relaxometry and diffusometry methods are used to access the microscopic and macroscopic maltene motion. Since the non-monoexponential relaxation of maltenes in crude oils persists in the absence of asphaltenes, its origin is located in the large number of different maltenes present in crude oils [ 19 ]. On the other hand, the broadening of the distribution for lower frequencies in asphaltenic crude oils indicates maltene populations in different relaxation environments as an additional cause for their non-monoexponential relaxation in the presence of asphaltenes.…”