Probing porous media with time-dependent gas diffusion NMR

We have developed and tested pulsed-field-gradient NMR techniques to measure time-dependent noble gas diffusion as a probe of the microstructure of heterogeneous porous media (the lung, foams, reservoir rock, etc.). We measured the time-dependent (i.e., restricted) diffusion of xenon gas in model porous media-randomly packed glass beads-with results that are consistent with numerical calculations (see Fig. 1). Free gas diffusion coefficients were measured for times much smaller than that required for xenon atoms to diffuse across pores in the restricted medium. For longer diffusion times, the measured diffusion coefficient decreased as the xenon atoms increasingly interacted with boundary restrictions. The initial decrease of the diffusion coefficient was found to be proportional to the ratio of the surface area to pore volume of the bead pack, while the asymptotic value of the restricted diffusion coefficient provided a good measure of the tortuosity of the porous system. (The tortuosity is related to the fluid permeability and porosity of the medium).


Recently, we began NMR studies of xenon gas infused in Fontainebleau sandstone, a typical oil and natural-gas-bearing reservoir rock with a microstructure of great interest to the petroleum industry. Fig. 2 shows a one-dimensional magnetization profile (or image) indicating a uniform distribution of xenon gas through the rock pore structure. We have also measured the ¹²⁹Xe spin polarization lifetime (T₁) within the rock to be greater than 5 seconds, and the coherence time (T₂*) to be ~ 5 ms at 4.7 tesla.



Note: These investigations were performed in close collaboration with Dr. Samuel Patz at the MR Division of the Brigham and Women's Hospital, and Drs. Martin Hurlimann and Lawrence Schwartz of Schlumberger-Doll.

References:

Probing porous media with gas diffusion NMR.

Pulsed-field-gradient measurements of time-dependent gas diffusion.