Direct characterisation of transport and mixing in unsaturated porous media

多孔介质中的多相流动和水动力分散是许多地下和工程应用的关键过程，如渗透带污染、土壤修复和油气回收。清洁水和能源被确定为全球挑战领域，为了确保可持续获取，我们敦促加强对基本过程的理解，提高我们的预测能力。多孔介质中的流体动力弥散受时空变化的速度场控制，由饱和多孔介质的平流-弥散方程描述。然而，在非饱和多孔介质（存在两种或两种以上不混相流体）的情况下，菲克式平流-色散方程不再有效。在非饱和多孔介质中，不仅速度场随饱和拓扑的变化而变化，而且速度的空间变化也会发生数量级的变化，即使在均匀介质中，一些区域也会出现水动力停滞。在停滞区和流动区，输运时间尺度可能会有数量级的差异，这使得浓度曲线非常偏斜。这种特性被称为非菲克行为，它随滞流饱和度和流动动力学的变化而显著变化。现有的非菲克理论（如MIM理论）已被用来对外部测量的浓度曲线进行逆建模。令人满意的“逆建模”结果通常被认为是这些理论有效性的指示，尽管这种看法在最近的二维微模型实验中受到挑战。在这个项目中，我们阐明了我们理解中的主要误解和差距，并假设停滞饱和的概念在现有理论中被错误地采用。滞流饱和度是一个两相流动变量，取决于流体的拓扑结构。应将其视为两相流与非菲克输运模型以水动力一致方式耦合的关键“变量”，这在文献中是缺失的。为了解决理解上的差距和误解，我们在两个工作包（WP）中设想了一个新的实验和建模。在WP1中，我们将首次使用快速经过时间（4D） x射线微层析成像技术直接可视化示踪剂在油湿和水湿非饱和多孔介质中的分散和混合。所有实验都将使用内部开发的孔隙尺度模型进行计算设计，实验装置将在亨利-莫斯利x射线成像设备中进行测试。4D实验预计将在钻石光源（DLS）中进行，由钻石曼彻斯特合作组织提供便利。在WP2中，将对4D x射线图像进行分析，提取不同流动条件下的浓度场、饱和度场。输运性质包括停滞饱和度、色散系数和传质率将从获得的图像中量化。最后，对现有模型的有效性进行检验，建立新的理论框架。该项目将受益于与世界知名科学家的合作，包括普林斯顿大学的M. Celia教授（MC）、斯图加特大学的H. Steeb教授（HS）和魏茨曼科学研究所的Brian Berkowitz教授（BB），以及PI团队的两名博士生的部分支持。MC将带来他在多孔介质多相流和输运及其在大规模环境工程问题中的应用方面的关键知识。BB在非均匀多孔介质中非菲克输运的数学分析方面有着著名的记录。HS是复杂多孔介质过程的微ct成像专家，并通过提供实验设备支持WP1。此外，DLS光束线科学家Nghia Vo博士将在数据采集和重建方面支持该项目。

Multiphase flow and hydrodynamic dispersion in porous media are the key processes in many subsurface and engineering applications, such as vadose zone contamination, soil remediation, and hydrocarbon recovery. To secure sustainable access to clean water and energy, which are identified as the global challenge areas, we urge to enhance our understanding of fundamental processes and improve our predictive capability. Hydrodynamic dispersion in porous media is controlled by the spatially and temporally variable velocity field, described by the advection-dispersion equation for saturated porous media. However, in the case of an unsaturated porous medium (where two or more immiscible fluids are present), the Fickian advection-dispersion equation is not anymore valid. In unsaturated porous media, not only the velocity field varies with the change of saturation topology, but also the spatial variation of velocity can change by orders of magnitude, such that even in homogeneous media some regions become hydrodynamically stagnant. Transport time scales in the stagnant and flowing regions can be different by orders of magnitude, which makes the concentration profiles very skewed. This feature is referred to as the non-Fickian behaviour that varies dramatically with the stagnant saturation and flow dynamics. The existing non-Fickian theories (e.g. the MIM theory) have been used to inversely model the externally-measured concentration profiles. The satisfactory "inverse modelling" results are generally regarded as an indication of the validity of these theories, although this perception has been challenged in the recent 2D micromodel experiments.In this project we articulate the major misconceptions and gaps in our understanding and we hypothesize that the concept of stagnant saturation has been incorrectly employed in the existing theories. The stagnant saturation is a two-phase flow variable, which depends on fluids topology. It should be regarded as the key "variable" to couple two-phase flow and the non-Fickian transport modelling in a hydrodynamically-consistent way, which is absent in the literature. To address the gaps and misconceptions in understanding, we have envisaged a novel experiments and modelling in two work packages (WP). In WP1, we will deliver the first direct visualisation of dispersion and mixing of a tracer in oil-wet and water-wet unsaturated porous media using the fast elapsed-time (4D) X-ray micro-tomography. All experiments will be computationally designed using the in-house developed pore-scale models and the experimental setup will be tested in Henry-Moseley X-ray Imaging Facilities. The 4D experiments are envisaged to take place in the Diamond Light Source (DLS), facilitated by the Diamond Manchester Collaboration. In WP2, the 4D X-ray images will be analysed to extract the concentration field, saturation field at different flow conditions. Transport properties including stagnant saturation, dispersion coefficient, and mass transfer rate will be quantified from the obtained images. Finally, the validity of the existing established models will be tested and a new theoretical framework will be developed. The project will benefit from the collaborations with world-renowned scientists, Prof. M. Celia (MC) from Princeton University, Prof. H. Steeb (HS) from Stuttgart University, and Prof. Brian Berkowitz (BB) from Weizmann Institute of Science and the partial support of two PhD students of the PI's team. MC will bring his key knowledge in multiphase flow and transport in porous media and their applications in large-scale problems of environmental engineering. BB has a renowned track record in mathematical analysis of non-Fickian transport in heterogeneous porous media. HS is an expert in microCT imaging of complex porous media processes and supports the WP1 by offering experimental equipment. Also, the DLS beamline scientist, Dr Nghia Vo, will support the project in data acquisition and reconstruction.