科研之友
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摘要
Nuclear-magnetic-resonance (NMR) relaxation experimentation is an effective technique for nondestructively probing the dynamics of proton-bearing fluids in porous media. The frequency-dependent relaxation rate T-1(-1) can yield a wealth of information on the fluid dynamics within the pore provided data can be fit to a suitable spin diffusion model. A spin diffusion model yields the dipolar correlation function G(t) describing the relative translational motion of pairs of H-1 spins which then can be Fourier transformed to yield T-1(-1). G(t) for spins confined to a quasi-two-dimensional (Q2D) pore of thickness h is determined using theoretical and Monte Carlo techniques. G(t) shows a transition from three-to two-dimensional motion with the transition time proportional to h(2). T-1(-1) is found to be independent of frequency over the range 0.01-100 MHz provided h greater than or similar to 5 nm and increases with decreasing frequency and decreasing h for pores of thickness h < 3 nm. T-1(-1) increases linearly with the bulk water diffusion correlation time tau(b) allowing a simple and direct estimate of the bulk water diffusion coefficient from the high-frequency limit of T-1(-1) dispersion measurements in systems where the influence of paramagnetic impurities is negligible. Monte Carlo simulations of hydrated Q2D pores are executed for a range of surface-to-bulk desorption rates for a thin pore. G(t) is found to decorrelate when spins move from the surface to the bulk, display three-dimensional properties at intermediate times, and finally show a bulk-mediated surface diffusion (Levy) mechanism at longer times. The results may be used to interpret NMR relaxation rates in hydrated porous systems in which the paramagnetic impurity density is negligible.
- 出版日期2017-3-28
