We present a theory and experiments that relate the NMR longitudinal T 1 and transverse T 2 relaxation times to the viscosity η for heavy crude oils with different asphaltene concentrations. The nuclear magnetic relaxation equations are based on a one-dimensional (1D) hydrocarbon translational diffusion in a transient porous network of slowly rotating asphaltene macroaggregates containing paramagnetic species VO 2+ . For heavy crude oils with viscosity η above a certain threshold η c , the effective 1D confinement causes a transition from the usual Stokes−Einstein relation for the translational diffusion coefficient D ∝ 1/η below η c to a wetting behavior D ∼ C te close to the asphaltene aggregates above η c . The theory is compared successfully with the universal viscosity dependencies of relaxation times T 1 and T 2 observed over a large range of viscosities. The theory reproduces the relaxation features of the 2D correlation spectra T 1 −T 2 and D−T 2 for heavy crude oils when varying the asphaltene concentration. This foundation is important because these measurements can be performed down-hole, thus giving a valuable tool for investigating in situ the molecular dynamics of petroleum fluids.

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.

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