- First, we have investigated numerically the influence of the statistical properties of the aperture field and upscaled hydraulic behavior on heat transport in rough rock fractures with realistic fracture geometries. By means of 3-D flow modelling based on the lubrification approximation (i.e. local cubic law), we investigate how the statistical parameters controlling spatial aperture variations in individual fractures control the heat exchange at the fluid/rock interface and heat transport by flow. Ensemble statistics over fracture realizations provide insights into the main hydraulic and geometrical parameters controlling the hydraulic and thermal behaviour of rough fractures. Our main finding is that under the conditions studied here, thermal behaviour of rough-walled rock fractures only depends on the hydraulic properties. The practical implication of our finding is that thermal exchanges at the scale of a single fracture is controlled by the effective hydraulic transmissivity. Provided that thermal properties of the host rock are known, this implies that (1) geothermal efficiency can be computed at field sites using hydraulic characterization alone, and predicted using well-known low-dimensional hydraulic parameterizations in terms of effective hydraulic properties and (2) heat tracer tests are reliable for inferring effective hydraulic transmissivity. The results of this study were published in Advances in Water Resources journal [Klepikova et al., 2021].
- Second, we proposed thermal dilution experiments monitored with Fiber-Optic Distributed Temperature Sensing (FO-DTS) for characterizing fractured media heterogeneity. The method is based on the physical injection of fluid with a contrasting temperature in order to introduce thermal anomaly along the FO cables deployed in open boreholes. First, we show that thermal dilution experiments are an effective approach to infer hydraulic connections between boreholes even in a low-permeability crystalline formation. Second, the method enables to obtain in-situ estimates of rock thermal conductivity with high spatial resolution. Compared to labor-intensive and time-consuming laboratory measurements or thermal conductivity, thermal dilution experiments are more suitable for effective thermal conductivity determination. Such investigations are critical to improve our understanding of flow and heat transport processes in Enhanced Geothermal Systems (EGS). The related manuscript was recently accepted for a publication in Journal of Hydrology.
