NMR Studies of Fluid Diffusion in Confining Geometries
- PDF / 541,395 Bytes
- 11 Pages / 420.48 x 639 pts Page_size
- 109 Downloads / 215 Views
NMR Studies of FluidDiffusion in ConfiningGeometries MICHAEL JEROSCH-HEROLD*and HANS THOMANN* and A. H. THOMPSON** *Exxon Corporate Research,Annandale, NJ 08829. **Exxon Production Research Company, Houston,TX 77252.
Diffusion dynamics for fluids in sandstone rocks were studied by CPMG and pulsed field gradient (PFG) NMR measurements. Stretched exponential decays of the transverse magnetization were observed while varying the interpulse spacing, T, in the CPMG experiment which sets the time for diffusion. A crossover from free to restricted diffusion is evident in the dependence of both T 2 and the stretch exponent, P2 , on the diffusion time T. The T2 (cr)data is fit to a model which interpolates between free and restricted diffusion and has only one free adjustable length scale despite the fact that the sandstone rocks are characterized by a pore size distribution. This NMR derived length scale correlates with a characteristic length derived from Hg injection experiments at the percolation threshold. We compare these results from the CPMG experiment with stimulated echo PFG-NMR diffusion measurements. This provides a complementary method of probing the diffusion dynamics since the longitudinal magnetization is insensitive to the dephasing effects of diffusion in internal field gradients.
NMR is well established as a versatile tool in the study of porous media for probing the pore space geometry and the dynamics of fluids confined within the void space 1 . For porous media such as rocks the NMR signal observed is in most cases from the IH nuclei in a water or hydrocarbon molecule. The proton on the fluid molecules acts as a local probe as the molecule diffuses or flows through the pore space. Protons in the grain material do not contribute to the NMR signal under the standard experimental conditions used for liquids. Many types of NMR experiments have been applied to porous media studies. The distribution of fluids in porous media has been visualized using NMR imaging techniques. Pulsed field gradient techniques have been used to study fluid diffusion and flow. The dynamics of fluids in confining geometries was first observed in NMR relaxation studies. The macroscopic magnetization for the ensemble of nuclear spins is a vector which is usually described by a longitudinal component parallel to an applied time-independent magnetic field and a transverse component orthogonal to this field. Following a pulsed excitation, the decay of the NMR magnetization is characterized by two time constants. One time constant, known as the spin-lattice relaxation time, or simply TI, describes the time scale for the re-alignment of the longitudinal magnetization. The relaxation time, T 2, describes the
Mat. Res. Soc. Symp. Proc. Vol. 290. @1993 Materials Research Society
162
time scale for the decay of the phase coherence of the transverse magnetization. The phase coherence is established by the excitation pulse. The initial relaxation studies of fluids in confining geometries used models 2 in which the source of the longitudi
Data Loading...
