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

Abstract

Asphaltenes cause many problems in crude oil transportation, refinery and production. A clear understanding of asphaltene behavior in crude oils is thus crucial for improving crude oil production. Here, we used noninvasive multiscale NMR techniques such as low field NMR 2D D-T 2 , T 1 -T 2 , nuclear magnetic relaxation dispersion (NMRD) techniques as well as high field 2D NMR DOSY for probing the dynamics and interaction of maltenes with asphaltene aggregates in crude oil. We prepared different asphaltene concentrations ranging between 0% wt (pure maltenes) and 9% wt (native crude oil) by using a dilution procedure that maintains the asphaltene aggregates structure initially present in native crude oil. We showed by multiscale NMR techniques that changing asphaltene concentration induces a drastic modification of hydrocarbon dynamics. In native crude oil, we observe anomalous relation between the diffusion coefficient (D) and the transverse relaxation time (T 2 ) that we can reproduce theoretically by a quasi 1D diffusion of hydrocarbons in between slowly rotating asphaltene macroaggregates. The relation D α T 2 usually observed in crude oils without asphaltenes was progressively appearing when asphaltene concentration decreased. Our 2D D-T 2 results at 2.5 and 23 MHz show that the diffusion coefficient varies by a factor 4 while the T 2 changes by more than an order of magnitude when the asphaltene concentrations vary. These observations prove that the hydrocarbon dynamics changes drastically at nanoscale and vary much less at large length scales (µm). Moreover, the 2D D-T 2 also shows two different dynamics for short and long chain hydrocarbons in crude oil. This is confirmed by high field 2D NMR DOSY. We found also that the ratio T 1 /T 2 in 2D T 1 -T 2 distribution was progressively reduced when asphaltene concentration decrease with an upward bent away in native crude oil at short T 2 that we succeeded to interpret with the quasi 1D hydrocarbon dynamics. We observed that the NMRD profiles are completely different for crude oil with and without asphaltenes. In native crude oil, NMRD techniques clearly evidence a 2D translational diffusion of short hydrocarbon chains at proximity of the asphaltene nanoaggregates. In crude oil without asphaltene, the NMRD profile is explained in term of a rotational reorientation of hydrocarbon chains. Most of our proposed multiscale NMR techniques could be made down-hole and then can give an invaluable tool for a clear understanding of asphaltene behavior in crude oil.

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

“…The continuous lines in Fig. 23b represent the best fits of the NMRD data obtained [110] for the different samples with a theoretical model proposed in Refs. [112,136] using a relaxation model (Eqs.…”

“…23b shows the remarkable features of the log-average h1/T 1 i, for native crude oil (containing 9% wt asphaltene), and samples containing 4.55% wt asphaltene (obtained by dilution of the native crude oil with maltene) or 0% asphaltene (i.e. pure maltene) [110]. The continuous lines in Fig.…”

“…Instead of studying a large number of crude oils of different asphaltene concentrations, it is easier to use only a single native crude oil of high asphaltene concentration and to derive from this sample some others with lower asphaltene concentrations by dilution with its own maltene using a known protocol of the petroleum industry (see Section 2.1 of Ref. [110]). The other components in crude oils such as ''saturated", ''aromatic" and ''resin" components of the SARA analysis [107] are soluble in nalkane solvents and defined as maltenes.…”

“…In particular, the anomalous nuclear spin relaxation features found for heavy crude oil include the following relaxation features: (i) T 2 is shorter than T 1 ; (ii) T 2 / 1= ffiffiffi g p and is almost independent of the Larmor frequency x 0 /2p; (iii) above a certain viscosity, T 1 is almost independent of viscosity but depends strongly on the Larmor frequency T 1 / ffiffiffiffiffiffi ffi x 0 p . (iv) two-dimensional spincorrelation plots T 1 -T 2 and D-T 2 have been also used for measuring the molecular dynamics over different length-and time-scales [110]. For heavy crude oils, the presence of asphaltenes creates some anomalous features in the observed two-dimensional correlation plots displayed in Figs.…”

“…NMRD studies have also been useful in investigating the structure and dynamics of crude oils in bulk (taken largely from Vorapalawut et al [110]). Fig.…”