EXPERIMENTAL-MEASUREMENT OF DISPERSION PROCESSES AT SHORT TIMES USINGA PULSED-FIELD GRADIENT NMR TECHNIQUE

Dispersion at short times is studied using a PFG-NMR (pulsed field gra dient NMR) technique inside a fixed bed of nonconsolidated spherical b eads saturated with water flowing at a constant velocity. This allows measurement of the probability distribution of the displacement of wat er molecules along the magnetic field gradient during a preset measure ment time Delta: the mean displacement of the water molecules is varie d between 0.1 and 7.3 times the bead diameter by varying Delta between 20 and 100 ms and the bead diameter between 800 and 81 mu m. At short times, the displacement of the molecules is small enough so that the local displacement is proportional to the local velocity component alo ng the magnetic field gradient. At mean displacements larger than 5 be ad diameters, the displacement distribution is Gaussian and centered a bout the mean displacement; the width of the distribution corresponds to the macroscopic dispersion coefficient as measured by other techniq ues. At intermediate displacements, this distribution displays two pea ks corresponding to a combination of the two processes. The main featu res of this transition can be reproduced by a simple Monte-Carlo simul ation modeling the porous medium as a set of finite length tubes with random orientations. (C) 1997 American Institute of Physics.