Probing Maltene–Asphaltene Interaction in Crude Oil by Means of NMR Relaxation

Abstract

The distribution of NMR relaxation times and diffusion coefficients has been employed for modeling the composition of typical crude oils both in bulk and in the reservoir rock. Although the relation of relaxation times and diffusion coefficients with viscosity and molecular size in a typical maltene composition has been established for bulk oils, it is known that the presence of asphaltene affects the relaxation times in a complex way not sufficiently understood so far. In particular, it is expected that the interaction of aromatic or aliphatic maltene molecules, respectively, with asphaltene aggregates leads to a different influence on the relaxation time. In this study, we have applied, for the first time, fluorine containing tracer molecules to natural oils of different asphaltene content and investigated the tracers’ relaxation time ratios T 1/T 2 and the magnetic field dependence of longitudinal relaxation times, T 1(ω). This strategy has the advantage of specifically determining the behavior of individual tracer molecules, where molecular weight and aromaticity have been considered as variables. One main finding of this study is the remarkably strong change of T 1/T 2 between perfluorinated alkanes and aromatics in the presence of asphaltenes, whereas this ratio remains nearly constant for asphaltene-free oils. The results are interpreted in terms of selective maltene–asphaltene interaction on the basis of frequency dependent relaxation results. The strong contrast of relaxation times allows for a simplified quantification of either asphaltene concentration or aromaticity of maltenes in natural oils.

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

“…For all substances, the 19 F relaxation times ratios T 1 /T 2 were measured to be between 1.17 and 1.25, close to the expected value of 1.0 for fast tumbling molecules in a low-viscosity liquid [23]. Fig.…”

Multiscale nuclear magnetic relaxation dispersion of complex liquids in bulk and confinement

“…Given that the surface diffusion of the hydrocarbons close to the asphaltene nano-aggregates is 5.3 times smaller than the bulk diffusion, as shown by 2D NMR DOSY experiments [110], and given a molecular size d oil = 0.655 nm for the oil (octane), this latter time s S gives an average radius of exploration for 2D hydrocarbon diffusion, r 2D ¼ The very long residence time found (s S ) s m ) proves there is efficient interaction between maltene and asphaltene, and agrees with the theoretical model of anisotropic 2D diffusion of hydrocarbons in proximity to the asphaltene nano-aggregates having very slow rotation on such a time scale [136]. A complementary study of fluorine-containing tracer molecules in natural oils of different asphaltene content was proposed by Stapf et al for proving the selective maltene-asphaltene interaction by analysing frequencydependent relaxation results [139]. For the crude oil largely without asphaltene (i.e.…”

Quantifying Crude Oil Contamination in Sand and Soil by EPR Spectroscopy

“…In quantitative studies, the EPR signal is found to correlate with the asphaltene content. The NMR relaxation properties of liquid components in oil, the so-called maltenes, were likewise found to correlate with the asphaltene content [43,44], suggesting that the latter contain the largest amount of unpaired electrons in crude oil, and therefore act as a relaxation agent in analogy to contrast agents in aqueous solution. Current literature concludes that free, delocalized electrons and vanadyl ions (VO 2+ ) constitute the major contributions to the EPR spectrum, though their relative proportion varies greatly.…”