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

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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.