NMR T dispersion of crude oils from 10 kHz to 20 MHz.

In this article T dispersion measurements on a set of crude oils that span a viscosity range of 0.7 cP up to 2·10 cP are reported. Larmor frequencies were varied from 10 kHz up to 20 MHz. The relaxation dispersion measurements are interpreted in terms of the threshold viscosity model, which allows model applicability tests presented in literature to be extended to Larmor frequencies below 1 MHz. It is shown that the measurements can be equally well described by assuming a proton-proton interaction process as by assuming proton-electronic spin interactions. A power-law frequency dependence of the threshold viscosity parameter is observed for both types of spin interactions. Modifications to the description of rotational diffusion in the threshold viscosity model are suggested that may account for the observed frequency dependence in the threshold viscosity parameter. Without modifications, the threshold viscosity model is shown to have a limited application range of η ≥ 40 cP and ω/2π ≥ 700 kHz, in which model parameters have a physically justifiable order of magnitude for the set of crude oils studied. Outside this range, the threshold viscosity model can still be applied as a parametrization of T dispersion effects for the complete set of experiments presented in this study. This explicit viscosity and Larmor frequency dependent parametrization may be used as first-order approximation to T dispersion in crude oils for which only oil viscosity is known and may be applied to polarization level and pulse sequence simulations in low field NMR relaxometry studies up to 20 MHz for a wide range of crude oil viscosities, and in particular for better interpretation of NMR flow measurements on crude oils.