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Transport of Heat in hEteRogeneous Media

Periodic Reporting for period 1 - THERM (Transport of Heat in hEteRogeneous Media)

Reporting period: 2020-09-01 to 2022-08-31

Nowadays global efforts to optimize methods for extracting geothermal energy are motivated by the necessity to produce CO2-neutral energy from renewable resources. To achieve a practically useful energy production capacity, the development of reservoirs is required at depth, where the rock temperature is sufficiently high for electric power generation. Such hydraulically stimulated reservoirs are called enhanced geothermal systems (EGS) and expected to efficiently extract stored geothermal energy while providing high flow rates of production wells. As heat is transported towards the production wells by advection, only the connected flowing fractures are involved in diffusive heat transfer from the rock towards the fluid. Understanding heat transport in fractured media is critically needed to develop and test new technologies for energy production and storage in geothermal reservoirs. The general objective of the THERM project is to develop a quantitative understanding of the complex coupled processes involved in heat transport and driven by fracture network discontinuities and properties of self-affine fracture surfaces themselves.
- 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.
The exploitation of the results produced by THERM, which are communicated in high level scientific journals and conferences, is carried out on two levels. Firstly, we currently discuss potential applications of the theoretical results, which represents the first modelling framework explicitly coupling fluid dynamics and heat transport, with research teams working on geothermal energy projects. Secondly, the produced field data, may become an international reference in the field of geothermics and hydrogeology. Field data will be made available to the scientific community through the H+ observatory database ( which is coordinated by the host and designed to integrate data of different nature.
Schematic illustration of EGS and the corresponding production temperature curves