Application of NMR Solvent Relaxation and SAXS to Asphaltenes Solutions Characterization

Abstract

Asphaltenic crude oil is usually described as a colloidal dispersion of sub‐micronic asphaltene particle in maltene. The understanding of macroscopic properties (solubility, viscosity…) often requires asphaltene particle size measurements. Traditionally, methods like for instance small angle (x‐rays or neutrons) scattering are used. The objective of this work is to test an alternative and simple method through 1H NMR solvent measurements. Asphaltenes fractions were isolated by different methods (solvent precipitation, ultra‐centrifugation…) and dispersed in various solvents at different concentrations. Asphaltene particle size were obtained from small angle x‐ray scattering (SAXS) experiments. Specific solvent relaxation rate R sp (difference between relaxation rate of solution and pure solvent), determined from Carr‐Purcell‐Meiboom‐Gill (CPMG) pulses sequences, varies linearly with asphaltene content up to concentrations of few percents. Using the fast exchange model between bulk and surface molecules, we try to correlate specific relaxation rate with particle size considerations. We show that simple geometric models are not able to completely explain experimental data; other considerations and refined modelizations must be taking into account.