Correlation of Transverse and Rotational Diffusion Coefficient:  A Probe of Chemical Composition in Hydrocarbon Oils

Abstract

Measurements of relaxation time and diffusion coefficient by nuclear magnetic resonance are well-established techniques to study molecular motions in fluids. Diffusion measurements sense the translational diffusion coefficients of the molecules, whereas relaxation times measured at low magnetic fields probe predominantly the rotational diffusion of the molecules. Many complex fluids are composed of a mixture of molecules with a wide distribution of sizes and chemical properties. This results in correspondingly wide distributions of measured diffusion coefficients and relaxation times. To first order, these distributions are determined by the distribution of molecular sizes. Here we show that additional information can be obtained on the chemical composition by measuring two-dimensional diffusion-relaxation distribution functions, a quantity that depends also on the shape and chemical interactions of molecules. We illustrate this with experimental results of diffusion-relaxation distribution functions on a series of hydrocarbon mixtures. For oils without significant amounts of asphaltenes, the diffusion-relaxation distribution functions follow a power-law behavior with an exponent that depends on the relative abundance of saturates and aromatics. Oils with asphaltene deviate from this trend, as asphaltene molecules act as relaxation contrast agent for other molecules without affecting their diffusion coefficient significantly. In waxy oils below the wax appearance temperature a gel forms. This is reflected in the measured diffusion-relaxation distribution functions, where the restrictions due to the gel network reduce the diffusion coefficients without affecting the relaxation rates significantly.

Probing Dynamics and Interaction of Maltenes with Asphaltene Aggregates in Crude Oils by Multiscale NMR

“…The 1D D-projection of 2D D-T 2 maps presented in Figure 5a shows that the diffusion distributions were almost not changed with changing of asphaltene concentration as consistent with previously report from Mutina et al 21 . We also show that the diffusion coefficient does not vary much by high field 2D NMR DOSY in section 3.4.…”

“…One sees that these fits reproduce quite well the essential features of the experimental data of Figure 6a as well as the Figure 6 of ref. 21 in presence of asphaltene. This is particularly true for a gyration radius R agg~2 nm corresponding to the value found in reservoir 27 .…”

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

“…Related studies were carried out for modeling the interaction of aromatics with a graphene surface [15][16][17][18], where graphene can be considered as an idealized approximation of asphaltene. In an experimental low-magnetic field investigation comparing the 1 H relaxation properties of saturated-rich and aromatics-rich oils with those of asphaltene-rich oils [6], a tendency was found towards larger T 1 /T 2 ratios in crude oils containing asphaltene, but with no obvious signature of aromatic maltenes. These studies suggest that the interaction of aromatics with the polycyclic asphaltene molecules is stronger than that of alkanes, leading to preferential orientations and possibly extended lifetimes of the maltene in the vicinity of the asphaltene aggregate.…”

“…A detailed investigation of the dependence of 1 H relaxation and diffusion properties on molecular sizes has been discussed for linear alkanes [4,5]. In the more complicated situation of most crude oils, with many thousands of components, two-dimensional techniques that correlate two of the mentioned three parameters were employed as a means to assess oil composition [6]. In the presence of asphaltenes and resins, the relationship becomes more difficult to resolve since the interaction of soluble components (maltenes) with asphaltenes affects their relaxation, and possibly also their diffusion properties.…”

Sensitivity and resolution of two-dimensional NMR diffusion-relaxation measurements

“…successfully applied to fluid mixtures with multiple aqueous, hydrocarbon and lipid phases and inside porous media [2,3,4,5,6,7,8,9,10]. Using instruments with inside-out geometries that are lowered into boreholes, it is possible to apply this technique to characterize the fluids in earth formations, many kilometers underground [11,12].…”