HR: 0800h
AN: NS31B-0378    [Abstracts]
TI: A Laboratory Study of NMR Relaxation Times and Pore Coupling in Heterogeneous Porous Media
AU: * Grunewald, E
EM: elliotg@stanford.edu
AF: Department of Geophysics, Stanford University, 397 Panama Mall, Stanford, CA 94305, United States
AU: Knight, R
EM: rknight@stanford.edu
AF: Department of Geophysics, Stanford University, 397 Panama Mall, Stanford, CA 94305, United States
AB: Nuclear magnetic resonance (NMR) relaxation measurements are utilized in geophysical applications to estimate internal pore geometry, but these estimates often fail in highly heterogeneous materials. Traditional interpretation of NMR relaxation data assumes a model of isolated pores in which each proton samples only one pore type, and the distribution of relaxation times is directly scaled to estimate a pore-size distribution. This model breaks down, however, for systems with strongly-coupled heterogeneous pores in which protons readily diffuse between multiple pores prior to relaxing. In this latter case, the meaning of the relaxation time distribution is not well understood. We have explored the link between NMR relaxation times and pore geometry in heterogeneous porous media though a series of laboratory experiments in which we have varied the degree of pore coupling. For these experiments, we analyzed water-saturated silica gels with a well-defined pore structure comprised of submicron intragranular pores (150 Å) and much larger intergranular pores (~30 μm). NMR measurements for pure samples exhibit strong pore coupling, and relaxation time distributions show a single broad peak resulting from diffusional averaging of the two pore types. When the surface relaxivity of the gels was increased by adsorbing paramagnetic Fe(III) to the pore surfaces, we found that pore coupling was reduced, yielding relaxation time distributions with two distinct peaks corresponding to the true pore size distribution. We attribute this decrease in pore coupling to accelerated surface relaxation and a subsequent reduction in the distance protons can diffuse before relaxing. One-dimensional analytical models supplement interpretation of our laboratory experiments and provide insight into the processes driving relaxation in each system. This study allows us to identify the conditions under which pore coupling affects the NMR measurement and provides a basis for further experiments to improve the interpretation of relaxation times in heterogeneous geologic materials.
DE: 1835 Hydrogeophysics
DE: 1859 Rocks: physical properties
DE: 1894 Instruments and techniques: modeling
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting