Bultreys Tom, Ellman Sharon, Schlepütz Christian M, Boone Matthieu N, Pakkaner Gülce Kalyoncu, Wang Shan, Borji Mostafa, Van Offenwert Stefanie, Moazami Goudarzi Niloofar, Goethals Wannes, Winardhi Chandra Widyananda, Cnudde Veerle
Ghent University Centre for X-ray Tomography (UGCT), Ghent University, Ghent 9000, Belgium.
Department of Geology, Ghent University, Ghent 9000, Belgium.
Proc Natl Acad Sci U S A. 2024 Mar 19;121(12):e2316723121. doi: 10.1073/pnas.2316723121. Epub 2024 Mar 13.
Many environmental and industrial processes depend on how fluids displace each other in porous materials. However, the flow dynamics that govern this process are still poorly understood, hampered by the lack of methods to measure flows in optically opaque, microscopic geometries. We introduce a 4D microvelocimetry method based on high-resolution X-ray computed tomography with fast imaging rates (up to 4 Hz). We use this to measure flow fields during unsteady-state drainage, injecting a viscous fluid into rock and filter samples. This provides experimental insight into the nonequilibrium energy dynamics of this process. We show that fluid displacements convert surface energy into kinetic energy. The latter corresponds to velocity perturbations in the pore-scale flow field behind the invading fluid front, reaching local velocities more than 40 times faster than the constant pump rate. The characteristic length scale of these perturbations exceeds the characteristic pore size by more than an order of magnitude. These flow field observations suggest that nonlocal dynamic effects may be long-ranged even at low capillary numbers, impacting the local viscous-capillary force balance and the representative elementary volume. Furthermore, the velocity perturbations can enhance unsaturated dispersive mixing and colloid transport and yet, are not accounted for in current models. Overall, this work shows that 4D X-ray velocimetry opens the way to solve long-standing fundamental questions regarding flow and transport in porous materials, underlying models of, e.g., groundwater pollution remediation and subsurface storage of CO and hydrogen.
许多环境和工业过程都取决于流体在多孔材料中如何相互驱替。然而，由于缺乏在光学不透明的微观几何结构中测量流动的方法，控制这一过程的流动动力学仍然鲜为人知。我们引入了一种基于高分辨率X射线计算机断层扫描且具有快速成像速率（高达4赫兹）的四维微测速方法。我们用此方法测量非稳态排水过程中的流场，将粘性流体注入岩石和过滤器样品中。这为该过程的非平衡能量动力学提供了实验性见解。我们表明，流体驱替将表面能转化为动能。后者对应于侵入流体前沿后方孔隙尺度流场中的速度扰动，其局部速度比恒定泵速快40倍以上。这些扰动的特征长度尺度比特征孔径大一个多数量级。这些流场观测结果表明，即使在低毛细管数下，非局部动态效应也可能具有长程性，影响局部粘性-毛细力平衡和代表性单元体积。此外，速度扰动可以增强非饱和分散混合和胶体传输，然而，当前模型并未考虑这些因素。总体而言，这项工作表明，四维X射线测速法为解决有关多孔材料中流动和传输的长期基本问题开辟了道路，这些问题是例如地下水污染修复以及二氧化碳和氢气地下储存等模型的基础。
