Heat and Solute Transport in Fracture Networks

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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.