Numerical simulation of water flow in three dimensional heterogeneous porous media observed in a magnetic resonance imaging experiment
Numerical simulation of water flow in three dimensional heterogeneous porous media observed in a magnetic resonance imaging experiment
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磁共振成像实验中观察到的三维非均质多孔介质中水流的数值模拟
DOI：
10.1029/2007wr006213
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发表时间：
2008
影响因子：
5.4
通讯作者：
A. Webb
中科院分区：
文献类型：
--
作者：
H. Yoon;Changyong Zhang;C. Werth;A. Valocchi;A. Webb
利用磁共振成像(MRI)获得了掺入顺磁示踪剂的水(即1H)流经三维(3D)流动池的序列图像，该流动池充满了空间相关的1cm3块的非均匀分布，每个块包含五个不同的砂粒中的一个。从每个体素(0.1875×0.1875×0.225 cm~3)的磁共振信号强度分布中获得示踪剂浓度穿透曲线(BTC)。体素尺度BTC的平均值分别为0.25×0.25 cm~2、1×1 cm~2和整个流动单元截面，均沿主流方向以0.25 cm的增量递增，并与数值模拟结果进行了比较。不同类型砂土的水力传导性(K)和分散度值不同，计算了平均到达时间和第二中心矩的均方根误差(RMSE)值，以及在三种平均尺度上模拟和测量的BTC之间的RMSE值。孔隙率比测量值高5%的K的测量值和颗粒尺寸数量级的纵向分散度值，产生了充分捕捉整个流动池横截面上平均的实验BTC的模拟BTC，以及具有更均匀K场特征的流动池区域中较小尺度(即1×1 cm~2和0.25×0.25 cm~2)的BTC。然而，在以K对比度较大为特征的流动池区域，模型模拟在较小尺度上与BTC的偏差更大，那里发生了流动旁路。这可能是由于实验和数值导电场之间的微小差异所致。比较了低K区的机械弥散和扩散，发现分子扩散对低K区的机械弥散同样重要，而横向弥散对预测的改善作用不大。这些结果表明，即使在最受控制的条件下，预测细尺度(相对于渗透率变化)的水流也是非常具有挑战性的。这可能对反应传输的建模有很大的影响，在反应传输中，反应物的停留时间和混合可能会受到水流路径的很大影响。
Magnetic resonance imaging (MRI) was used to obtain sequential images of water (i.e., 1H) doped with a paramagnetic tracer as it flowed through a three‐dimensional (3D) flowcell packed with a spatially correlated heterogeneous distribution of 1 cm3 blocks, each containing one of five different sand fractions. Tracer concentration breakthrough curves (BTCs) were obtained from MRI signal intensity profiles at each voxel (0.1875 × 0.1875 × 0.225 cm3). Voxel scale BTCs were averaged over 0.25 × 0.25 cm2, 1 × 1 cm2, and entire flowcell cross‐sections, all at 0.25 cm increments along the main flow direction, and compared with numerical simulations. Hydraulic conductivity (K) and dispersivity values for each of the five sand types were varied, and root‐mean squared error (RMSE) values for mean arrival times and second central moments, and RMSE values between simulated and measured BTCs, at each of the three averaging scales were calculated. Measured values of K with a porosity 5% higher than measured values, and longitudinal dispersivity values on the order of the grain size, yielded simulated BTCs that adequately captured experimental BTCs averaged over the entire flowcell cross‐section, and BTCs at smaller scales (i.e., 1 × 1 cm2 and 0.25 × 0.25 cm2) in regions of the flow cell characterized by more uniform K fields. However, model simulations deviated more from BTCs at smaller scales in regions of the flowcell characterized by greater K contrast, where flow‐bypassing occurred. This may be due to small discrepancies between the experimental and numerical conductivity fields. Comparison of mechanical dispersion and diffusion in low K zones indicates that molecular diffusion is equally important to mechanical dispersion in these zones and transverse dispersion did not have much effect on improving prediction. These results illustrate that predicting water flow at fine scales (relative to permeability variations) is very challenging, even under the most controlled conditions. This may have large implications for modeling reactive transport, where reactant residence time and mixing can be greatly impacted by water flowpaths.