Solute transport in unsaturated porous media is of interest in many engineering and environmental applications. The interplay between small-scale, local forces and the porous microstructure exerts a strong control on the transport of fluids and solutes at the larger, macroscopic scales. Heterogeneity in pore geometry is intrinsic to natural materials across a large range of scales. This multiscale nature, and the intricate links between two-phase flow and solute transport, remain far from well understood, by and large. Here, we use high-resolution direct simulation to quantify solute mixing and dispersion behavior within correlated porous media during drainage under an unfavorable viscosity ratio. Through analysis of flow and transport at multiple realizations, we find that increasing spatial correlations in pore sizes increase the size of the required Representative Elementary Volume (REV). We show that increasing the correlation length enhances solute dispersivity through its impact on the spatial distribution of low-velocity (diffusion-dominated) and high-velocity (advection-dominated) regions. Fluid saturation is shown to directly affect diffusive mass flux among high- and low-velocity zones. Another indirect effect of correlated heterogeneity on solute transport is through its control of the drainage patterns via repeated alteration in the connectivity of flowing pathways. Our findings improve quantitative understanding of solute mixing and dispersion under two-phase conditions, highly relevant to some of our most urgent environmental problems.

中文翻译：

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具有空间相关紊乱的不饱和多孔介质中的溶质输运


不饱和多孔介质中的溶质迁移引起了许多工程和环境应用的兴趣。小尺度、局部力和多孔微观结构之间的相互作用对更大、宏观尺度上的流体和溶质的传输产生强有力的控制。孔隙几何形状的异质性是天然材料在大范围内所固有的。总的来说，这种多尺度性质以及两相流和溶质输运之间的复杂联系仍然远未得到充分理解。在这里，我们使用高分辨率直接模拟来量化在不利的粘度比下排水过程中相关多孔介质内的溶质混合和分散行为。通过对多种实现的流动和传输的分析，我们发现增加孔径的空间相关性会增加所需的代表性基本体积（REV）的大小。我们表明，增加相关长度可以通过影响低速（扩散主导）和高速（平流主导）区域的空间分布来增强溶质分散性。流体饱和度被证明直接影响高速区和低速区之间的扩散质量通量。相关异质性对溶质运输的另一个间接影响是通过反复改变流动路径的连通性来控制排水模式。我们的研究结果提高了对两相条件下溶质混合和分散的定量理解，这与我们一些最紧迫的环境问题高度相关。