Ionic strength-dependent pre-asymptoic diffusion coefficient distribution in porous media - Determination through the pulsed field gradient technique

1H pulsed-field gradient nuclear magnetic resonance was used to study early-time water diffusivity in porous media. The method has also been proven useful to characterize porous media from the standpoint of time-dependent diffusion. In porous media such as zeolite, biological cells or reservoir rocks, water molecular diffusion is time-as well as ionic strength-dependent. Diffusion phenomena are relevant transport mechanisms in low-permeability gas reservoirs and consequently in characterization efforts. Model systems are used here to unveil effects of ionic strength and pore size on the self- and effective-diffusion coefficient distributions. For this purpose, diffusion in aqueous phase in beadpacks made up of different glass bead sizes is analyzed. Ion size, ionic strength and hydration play an important role on the resulting diffusion coefficient. Ion-ion interaction leads to an increase of the diffusion coefficient by distorting water structure (hydrogen bond). While for the diffusion coefficient distribution derived using the regularized Inverse Laplace Transform method, ionic strength has little effect on the dominant peak position. With the increase in pore size, the dominant peak shifts to larger diffusion coefficient.