MRI Centre, Department of Physics, University of New Brunswick, Fredericton, Canada.
J Contam Hydrol. 2011 Jul 1;125(1-4):47-56. doi: 10.1016/j.jconhyd.2011.04.008. Epub 2011 May 8.
This work is focused on measuring the concentration distribution of a conservative tracer in a homogeneous synthetic porous material and in heterogeneous natural sandstone using MRI techniques, and on the use of spatially resolved porosity data to define spatially variable diffusion coefficients in heterogeneous media. The measurements are made by employing SPRITE, a fast MRI method that yields quantitative, spatially-resolved tracer concentrations in porous media. Diffusion experiments involving the migration of H(2)O into D(2)O-saturated porous media are conducted. One-dimensional spatial distributions of H(2)O-tracer concentrations acquired from experiments with the homogeneous synthetic calcium silicate are fitted with the one-dimensional analytical solution of Fick's second law to confirm that the experimental method provides results that are consistent with expectations for Fickian diffusion in porous media. The MRI-measured concentration profiles match well with the solution for Fick's second law and provide a pore-water diffusion coefficient of 1.75×10(-9)m(2)s(-1). The experimental approach was then extended to evaluate diffusion in a heterogeneous natural sandstone in three dimensions. The relatively high hydraulic conductivity of the sandstone, and the contrast in fluid density between the H(2)O tracer and the D(2)O pore fluid, lead to solute transport by a combination of diffusion and density-driven advection. The MRI measurements of spatially distributed tracer concentration, combined with numerical simulations allow for the identification of the respective influences of advection and diffusion. The experimental data are interpreted with the aid of MIN3P-D - a multicomponent reactive transport code that includes the coupled processes of diffusion and density-driven advection. The model defines local diffusion coefficients as a function of spatially resolved porosity measurements. The D(e) values calculated for the heterogeneous sandstone and used to simulate diffusive and advective transport range from 5.4×10(-12) to 1.0×10(-10)m(2)s(-1). These methods have broad applicability to studies of contaminant migration in geological materials.
这项工作专注于使用 MRI 技术测量单相合成多孔材料和非均质天然砂岩中保守示踪剂的浓度分布，并利用空间分辨孔隙率数据定义非均质介质中的空间变化扩散系数。通过使用 SPRITE（一种快速 MRI 方法）进行测量，该方法可在多孔介质中获得定量的、空间分辨的示踪剂浓度。进行了涉及 H(2)O 迁移到 D(2)O 饱和多孔介质的扩散实验。对单相合成硅酸钙实验中获得的 H(2)O 示踪剂浓度的一维空间分布进行拟合，与 Fick 第二定律的一维解析解进行拟合，以确认实验方法提供的结果与多孔介质中 Fickian 扩散的预期结果一致。MRI 测量的浓度分布与 Fick 第二定律的解吻合良好，并提供了 1.75×10(-9)m(2)s(-1)的孔隙水扩散系数。然后，该实验方法扩展到评估三相非均质天然砂岩中的扩散。砂岩具有较高的水力传导率，以及 H(2)O 示踪剂与 D(2)O 孔隙流体之间的流体密度对比，导致溶质传输由扩散和密度驱动的对流的组合。空间分布示踪剂浓度的 MRI 测量值与数值模拟相结合，可确定对流和扩散各自的影响。借助 MIN3P-D（一种包含扩散和密度驱动对流耦合过程的多组分反应性传输代码）对实验数据进行解释。该模型将局部扩散系数定义为空间分辨孔隙率测量值的函数。为模拟扩散和对流传输而计算的非均质砂岩的 D(e)值范围为 5.4×10(-12) 至 1.0×10(-10)m(2)s(-1)。这些方法广泛适用于地质材料中污染物迁移的研究。
