PROBING THE STRUCTURE OF POROUS-MEDIA USING NMR SPIN ECHOES

The NMR pulsed field gradient spin-echo (PFGSE) amplitude M(k,t) is a direct measure of the diffusion propagator of the molecules of a fluid diffusing in the pores of a porous medium. For small values of k = ga mma g delta, where g is the gradient strength, delta is the duration o f the gradient pulse, and gamma the gyromagnetic ratio, the PFGSE ampl itude gives the diffusion coefficient D(t). The exact short-time diffu sion coefficient, D(t)/D-0 = 1 - (4 root D(0)tS)/(9V(p) root pi) - D(0 )tHS/(12V(p)) + ptS/6V(p), provides an important method For determinin g the surface to pore-volume ratio S/V-p. Here the mean surface curvat ure H = (1/R(1) + 1/R(2)). Combining early D(t) with the magnetization decay one obtains the surface relaxivity rho. The long-time effective diffusion constant derived from PFGSE gives information on the tortuo sity of the connected space. The diffusion coefficient measured by PFG SE equals that derived from electrical conductivity only when rho = 0. Exact solutions with partially absorbing boundary conditions for a pe riodic structure are used to illustrate the influence of rho on the di ffusion coefficient. M(k,t) can be well represented by a convolution o f the structure factor of the connected pore space with an appropriate Gaussian propagator. This ansatz provides a model-independent way of obtaining the structure fatter.