On the influence of wetting behaviour on relaxation of adsorbed liquids - A combined NMR, EPR and DNP study of aged rocks.

The influence of wettability modification in natural rocks has been studied by means of NMR relaxation dispersion of water and several organic liquids, employing H, F and H relaxometry. Berea, Bentheimer and chalk were aged with a bitumen solution, altering the samples from water-wet to mixed-wet. Relaxation measurements were supported by EPR and DNP experiments which are sensitive to bulk radicals and interfacial radicals, respectively. The effect of ageing on relaxation was found to be twofold: first, the change from water-wet to oil-wet affects the dynamics of molecules differently, in particular their immediate interaction with the surface, which is reflected in their relaxation times; second, the bitumen cover includes paramagnetic impurities which act as additional relaxation sinks to all molecules. EPR was used to confirm the amount of deposited material and the total radical content of the rock samples, whereas DNP revealed a small but significant signal enhancement due to the surface-bound bitumen containing stable radicals. The DNP enhancement is dominated by the Solid Effect despite the low viscosity of the interacting fluids.