Pulsed field gradient NMR study of the translational mobility in porous media: restricted diffusion, internal magnetic fields, flows and molecular exchange

The basic principles of the pulse field gradient NMR (PFG NMR) study of porous media are discussed in the article. It is shown that self-diffusion coefficient of the fluid molecules confined in the porous media dependence on the diffusion time includes direct information about restriction sizes within porous matrix. The question of the inducted in the porous media internal magnetic fields is discussed from the point of view of the required using of the high pulse field gradients in order to avoid experimental data distortion.
It is shown that information about porous media structure and fluid localization could be obtained by the study of the diffusion decay in the internal field gradients (without applying pulse field gradient). The experiment where one can study dependence of the stimulated echo amplitude on the time interval between first and second RF pulses ($\tau$-scanning) with different diffusion times is offered to get information about internal magnetic field gradient (IMFG) distribution. The offered approach of data analysis allows to evaluate IMFG values and width of their distribution. Particularly, is shown the in the partially saturated porous media the information about internal magnetic field gradients allows to conclude about fluid localization in the porous matrix.
One of the topical questions of the fluid filtration through the porous media problem is the question of the “stagnant” zone quantitative characterization. The possibilities of the PFG NMR technique are demonstrated in the paper: registration of the fluid molecules share in the “stagnant” zones, determination of the characteristics of the molecular exchange between “stagnant” zone molecules and molecules involved in the flow, etc.
PFG NMR possibilities to study exchange process between different fluid phases in the porous media are discussed. Particularly, it shown that the apparent population of the molecules in the different “phases” dependence on the diffusion time allows one to obtain information not only about average molecular life-time in the phase, but also information about distribution function of the life times.