A novel description of heat transfer between fluid and rough-walled fractures in porous rock

裂隙多孔介质中的换热是地球系统中的一个重要过程。它是间歇泉、热液和火山系统等自然现象的驱动机制，也是落石和地震等地质灾害的驱动机制。此外，它也是地热系统等多种工业应用的基础。存在着广泛的方法，包括连续介质力学、多领域和对裂缝的明确定义。然而，关于传热学，目前的模型有两个主要缺陷：它们通常只考虑固液两相之间的热平衡，以及它们缺乏对传热过程中裂隙的适当表示。这两个障碍具有很强的关联性，因为高流速的裂缝往往会导致局部热不平衡，而岩石基质和裂缝流体之间的热传递却没有合适的描述。最近的实验室实验能够以迄今未知的精度研究这些过程，并能够与理论和数值模型进行深入比较。裂隙孔径、表面粗糙度和接触面积不仅对流体流动有显著影响，而且对换热也有很大影响。此外，温度影响流体性质，而应力又取决于温度和流体压力，从而影响裂缝表面的形态。因此，一个合适的传热模型需要包含水力和机械过程，从而产生一个完全耦合的热-液-力学模型。模型的开发将从实验室尺度的简化几何开始，以便与外部项目合作伙伴在厘米尺度上的实验结果进行良好的比较。该模型将推广到复杂的裂隙网络。然而，为了适应数百米范围的大规模应用，模型将使用统计方法进行升级，并在参数设置中进行更改，以纳入合适的参数，如裂缝密度。通过实例对比和应用，保证了所建模型的准确性。计划将开发的模型整合到广泛的科学数值流体流动解算器中。这种创新的方法可以应用到各种现有的建模方法中，而与所选的裂缝表示法无关。所提出的模型弥补了长期以来在考虑动态和静态参数的情况下对裂隙多孔介质中跨尺度热传递的一致描述的空白。首次详细研究了边界条件和约束对热传递的影响和相互作用。新模型将显著改进自然和工业应用中的换热计算。

Heat transfer in fractured porous media is an essential process in the earth system. It is driving mechanism in natural phenomena like geysers, hydrothermal and volcanic systems, as well as in geo-hazards like rockfalls and earthquakes. Further, it is the base for several industrial applications like geothermal systems.Fluid flow in fractured porous media is quite well understood. A broad range of approaches exists including continuum mechanics, multiple domains and an explicit definition of fractures. However, with respect to heat transfer recent models have two major drawbacks: They often only consider thermal equilibrium between the solid and fluid phase and they lack a suitable representation of fractures in the heat transfer process. Both obstacles are strongly connected, as fractures with high flow velocities often cause local thermal non-equilibrium but there is no suitable description for heat transfer between rock matrix and fracture fluid.In this project, a novel model will be developed to describe heat transfer in fractures including microscopic fracture surface morphology. Recent laboratory experiments permit a study of these processes in an accuracy unknown up until now and enable an in-depth comparison with theoretical and numerical models. Fracture aperture, surface roughness, and contact area significantly influence not only fluid flow but also heat transfer. Furthermore, temperature affects fluid properties and stress, which again depends on temperature and fluid pressure, affects the fracture surface morphology. Therefore, a suitable heat transfer model needs to incorporate hydraulic and mechanical processes, resulting in a fully coupled thermo-hydraulic-mechanical model.Model development will start with simplified geometries on laboratory scale to enable a good comparison with experimental results of external project partners at centimeter- scale. The model will be extended to complex fracture networks. However, to be suitable for large scale applications with an extent of hundreds of meters, the model will be upscaled using statistical methods and changed in its parameterization to incorporate suitable parameters, like fracture density. Comparison and application to field cases will ensure the accuracy of the developed model. Incorporation of the developed model into a broad selection of scientific numerical fluid flow solvers is planned. This innovative approach can be implemented into diverse existing modeling approaches, independent of the chosen fracture representation.The proposed model closes the long-standing gap of a consistent description of heat transfer across scales in fractured porous media considering dynamic as well as static parameters. For the first time influence and interaction of boundary conditions and constraints for heat transfer and transport can be studied in detail. Calculation of transferred heat in natural and industrial applications will significantly improve with the new model.