Heat and Solute Transport in Fracture Networks

基岩含水层中的工业和废物场所产生的污染物通常由于离散裂缝和裂缝互连提供的通道而迅速而广泛地迁移。相反，在地下水流速足够高的情况下（即在注入或提取条件下），热输运的速率要慢得多，但也受裂缝路径的控制。运移过程包括单个裂缝内通道的弯曲度、基质孔隙度的可变性、裂缝互连的可变性、通过优先通道的横向运移、水力梯度的时间效应以及裂缝交叉处发生的弥散效应。在拟议的研究中，我们将重点关注破裂岩石中的两个尺度，一个是局部尺度（即邻近注入或回采井），另一个是裂缝网络中更大的运输距离（>150 m）。局部尺度研究的目的是检查离散裂缝中热量传递的关键过程，以评估“短路”的可能性，裂缝中的流体与围岩之间的热交换效率明显低下，以及热分散，热交换过程中另一种潜在的低效率。局部尺度的研究将通过示踪实验进行，在具有多种裂缝特征的晶体岩石中，在具有良好特征的地点进行钻孔之间的示踪实验。研究的这一部分将由两名硕士生和一名博士生进行。大规模的研究将包括传输数据集的汇编，并使用离散裂缝网络模型（由于大规模的计算限制，在可能的情况下）对这些数据集进行模拟，目的是解决裂缝路径的减少和屏障的影响将如何影响这些尺度上的传输预测。还将寻求更有效（即易于处理）的方法来模拟这种规模的运移，使用替代的拉格朗日（随流体运动的参考系）方法和平均技术来捕捉大块岩石中裂缝和岩石之间溶质交换的过程。一项博士研究将进行大规模研究。硕士生和博士生每年将有一名本科生暑期学生支持他们的研究。所有学生都将接受水文地质学的现场和实验室技术培训，以及协助分析现场和实验室数据的分析和数值模拟。这项研究的结果将为裂缝中污染物和热量运移的基本过程提供评估，并将为水文地质学家和工程师在风险管理和废物设施修复，深层地下碳捕获管理以及地热能和地下储热的设计和应用提供实际应用。

Contaminants emanating from industrial and waste sites in bedrock aquifers typically migrate rapidly and widely due to the pathways provided by discrete fractures and fracture interconnections. Conversely, heat transport occurs at a much slower rate but is also governed by fracture pathways provided groundwater velocity is high enough (i.e. under injection or withdrawal conditions). Transport processes include the tortuosity of pathways within individual fractures, the variability of matrix porosity, the variability of fracture interconnections, lateral migration through preferential pathways, temporal effects in the hydraulic gradients, and the dispersive effects that occur at fracture intersections. In the proposed study we will focus on two scales of transport in fractured rock, one at the local-scale (i.e. adjacent to an injection or withdrawal well) and the other at much large transport distances (>150 m) in fracture networks. The intent with the local scale studies is to examine the key processes of heat transport in discrete fractures to evaluate the potential for "short-circuiting", a significant inefficiency in heat exchange between the fluid in the fractured and the surrounding rock, and thermal dispersion, another potential inefficiency in the heat exchange process. The local scale studies will be conducted via tracer experiments conducted between boreholes in a well-characterized site in crystalline rock pervaded by multiple fracture features. This component of the study will be conducted by two Masters students and one PhD student. The large-scale study will involve the compilation of transport data sets and the simulation of these using a discrete fracture network model (where possible due to computational limits at large scale), with the intent to resolve how the diminishment of fracture pathways and the impact of barriers will influence prediction of transport at these scales. More efficient (i.e. tractable) means by which to simulate transport at this scale will also be sought using alternative Lagrangian (frame of reference which moves with the fluid) approaches, and averaging techniques which capture the process of solute exchange between the fractures and the rock in large blocks. One PhD study will conduct the large-scale study. The Masters and PhD students will be supported in their research by an undergraduate summer student in each year. All students will be trained in the field and laboratory techniques used in hydrogeology, and in the analytical and numerical modeling that assist in the analysis of the field and lab data. The outcome of this research will provide an assessment of the fundamental processes governing contaminant and heat migration in fractures, and will have practical application by hydrogeologists and engineers in the risk management and remediation of waste facilities, the management of carbon capture in the deep subsurface, and the design and application of geothermal energy and subsurface heat storage.