Relation and Correlation between NMR Relaxation Times, Diffusion Coefficients, and Viscosity of Heavy Crude Oils

We present a theory and experiments that relate the NMR longitudinal T1 and transverse T2 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 VO2+. For heavy crude oils with viscosity η above a certain threshold ηc, the effective 1D confinement causes a transition from the usual Stokes–Einstein relation for the translational diffusion coefficient D ∝ 1/η below ηc to a wetting behavior D ∼ Cte close to the asphaltene aggregates above ηc. The theory is compared successfully with the universal viscosity dependencies of relaxation times T1 and T2 observed over a large range of viscosities. The theory reproduces the relaxation features of the 2D correlation spectra T1–T2 and D–T2 for heavy crude oils when varying the asphaltene concentration. Thi...