Dipolar NMR relaxation of adsorbates on surfaces of controlled wettability

Abstract

In reservoir rocks, the term “ageing” refers to extended exposition to crude oil; a typically water-wet sandstone will then gradually become oil-wet as a consequence of the deposition of insoluble fractions of oil onto the surface grains. Rocks have been aged artificially by subjecting them to a bitumen solution at elevated temperature in order to achieve comparable surface properties for three different types of rock: Bentheimer, Berea Buff and Liège Chalk. Using saturated and aromatic model compounds as proxies for crude oil, the nuclear magnetic resonance (NMR) relaxation dispersion in native and aged rocks was compared and correlated to the properties of paramagnetic impurities in these rock types. Perfluorated liquids were found to follow the same trend as deuterated and naturally occurring oil components, suggesting they can be used as suitable tracers for wettability studies since the ¹⁹F nucleus is absent in natural sources. By combining electron paramagnetic resonance (EPR) and dynamic nuclear polarization (DNP) it becomes possible to identify and quantify the origin of the dominating relaxation processes between native and aged rocks, providing an alternative approach to assess wettability in natural rocks.