Interpretation of nuclear magnetic resonance (NMR) logs enables estimation of oil viscosity from the spectrum of spin-spin relaxation times. By now, several empirical and correlation equations have been proposed for this purpose. However, the range of resultant oil viscosities is extremely broad, suggesting that the proposed equations do not include factors that determine the mechanism of viscosity formation. The article focuses on the search for such factor. For a number of high-viscosity oil samples the temperature dependence of viscosity was estimated and Arrhenius dependence parameters, such as viscosity prefactor η0 and activation energy, were calculated. Spin-spin relaxation measurements characterized by T2 spin-spin relaxation time spectra were conducted for the same samples. A rheological and widely used energy models were considered to relate T2 relaxation time spectrum to dynamic viscosity. For a rheological model with viscous elements connected in series, the relationship between viscosity and average spin-spin relaxation time, calculated from the area under relaxation decay, is demonstrated. For energy model of viscosity formation an interpretation based on averaging of energy barriers is proposed. A generalization of the energy model was proposed to result in power-law relationship between the average logarithmic spin-spin relaxation time and measured viscosity. Validity of rheological, energy, and expanded models for viscosity estimation from spin-spin relaxation time spectrum characteristics was analyzed. It is demonstrated that viscosity estimated based on average logarithmic time of spin-spin relaxation should be corrected for η0 prefactor. This correction significantly reduces the range of correlation coefficient variations in viscosity estimates based on spectrum of spin-spin relaxation times.
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