Study of flow and hydrodynamic dispersion in a porous medium using pulsed-field-gradient magnetic resonance

This paper reports on the use of pulsed-field gradient (PFG) magnetic resonance (MR) techniques to obtain displacement and velocity spectra of steady-state, saturated flow through a column packed with glass beads. The displacement spectra obtained by PFG MR correspond to travel-distance probability-density functions (PDF) for initial conditions of a concentration impulse in a column with zero concentration, These spectra show strong dispersion-time dependence, and depart from Gaussian-shaped PDFs for short dispersion times. These data provide estimates of the dispersion-time dependence of transverse and longitudinal dispersion coefficients. The longitudinal dispersion coefficient reaches its long-time behaviour more slowly than the transverse coefficient; long-time values obtained from MR data agree well with those calculated using existing empirical correlations. A model based on three components of apparent velocities and dispersion coefficients is sufficient to describe the time dependence of displacement spectra for water flow through the bead column. The short-distance component arise because of convection-dispersion-diffusion processes within the narrow necks between particles. The long-distance component, on the other hand, represents a macroscopic convection-dispersion process. This study shows that PFG MR flow spectroscopy is a simple but potentially useful method for the investigation of flow and hydrodynamic dispersion in porous media, especially for time-dependent phenomena.