Dipole and Convergent Single-Well Thermal Tracer Tests for Characterizing the Effect of Flow Configuration on Thermal Recovery

Abstract

Experimental characterization of thermal transport in fractured media through thermal tracer tests is crucial for environmental and industrial applications such as the prediction of geothermal system efficiency. However, such experiments have been poorly achieved in fractured rock due to the low permeability and complexity of these media. We have thus little knowledge about the effect of flow configuration on thermal recovery during thermal tracer tests in such systems. We present here the experimental set up and results of several single-well thermal tracer tests for different flow configurations, from fully convergent to perfect dipole, achieved in a fractured crystalline rock aquifer at the experimental site of Plœmeur (H+ observatory network). The monitoring of temperature using Fiber-Optic Distributed Temperature Sensing (FO-DTS) associated with appropriate data processing allowed to properly highlight the heat inflow in the borehole and to estimate temperature breakthroughs for the different tests. Results show that thermal recovery is mainly controlled by advection processes in convergent flow configuration while in perfect dipole flow field, thermal exchanges with the rock matrix are more important, inducing lower thermal recovery.

Continuous Dissolved Gas Tracing of Fracture‐Matrix Exchanges

“…Following the observations of breakthrough curve tailing, two different conceptual models for the fracture geometry are tested: the parallel plate model and the channel model (Figure S5). The analytical solutions described by De La Bernardie (2018) and De La Bernardie et al (2018, 2019) based on Laplace transforms (Text S2) are used. These Laplace transform solutions are inverted using the algorithms from Hoog et al (1982) and Hollenbeck (1998).…”

Heat Tracing in a Fractured Aquifer with Injection of Hot and Cold Water

“…Equation (3) can be used to express the thermal power, individually for the injection and pumping points, by dividing the thermal energy change by the corresponding injection and observation time, respectively. For fractured rocks, the thermal recovery rate r thermal ( t ) expresses the relation between the injection power P in and the instantaneous recovery power P ( t ) (Equation 4) (de la Bernardie et al 2019): where P ( t ) is the instantaneous recovery power (W); P in is the injection power (W); Q inflow is the water injection flow rate (m 3 s −1 ); Q outflow is the water extraction flow rate (m 3 s −1 ); T injection is the temperature of the injected water (°C); and is the temperature difference (Equation 1) (°C or K).…”

“…Consequently, solutions based only on the advection–dispersion equation can have limited prediction capabilities for transport in fractured rocks (e.g., Bodin et al 2003 ). Recent studies using stronger diffusing tracers such as dissolved gases (Hoffmann et al 2020 ) or heat (Read et al 2013 ; Klepikova et al 2016a ; de la Bernardie et al 2018 ; de la Bernardie et al 2019 ; Hoffmann et al 2021 ) have shown a potential to better constrain fractured aquifer conceptualizations with matrix diffusion information. Although reliable temperature tracer experiments have been performed in porous alluvial aquifers (Wagner et al 2014 ; Wildemeersch et al 2014 ; Klepikova et al 2016b ; Sarris et al 2018 ) and fractured porous media (Read et al 2013 ; Klepikova et al 2016a ; de la Bernardie et al 2018 ; de la Bernardie et al 2019 ; Hoffmann et al 2021 ), temperature information is still rarely analyzed by hydrogeologists, because dilution is high (Kurylyk and Irvine 2019 ) and the signal may be difficult to detect.…”

“…Recent studies using stronger diffusing tracers such as dissolved gases (Hoffmann et al 2020 ) or heat (Read et al 2013 ; Klepikova et al 2016a ; de la Bernardie et al 2018 ; de la Bernardie et al 2019 ; Hoffmann et al 2021 ) have shown a potential to better constrain fractured aquifer conceptualizations with matrix diffusion information. Although reliable temperature tracer experiments have been performed in porous alluvial aquifers (Wagner et al 2014 ; Wildemeersch et al 2014 ; Klepikova et al 2016b ; Sarris et al 2018 ) and fractured porous media (Read et al 2013 ; Klepikova et al 2016a ; de la Bernardie et al 2018 ; de la Bernardie et al 2019 ; Hoffmann et al 2021 ), temperature information is still rarely analyzed by hydrogeologists, because dilution is high (Kurylyk and Irvine 2019 ) and the signal may be difficult to detect. Nevertheless, to understand and quantify the local matrix diffusion or mobile–immobile water interactions, the use of temperature information collected with high‐resolution sensors is very promising (Anderson 2005 ; Pehme et al 2014 ; Irvine et al 2015 ; Kurylyk and Irvine 2019 ).…”

“…Here, this is applied for a weathered/fractured granite aquifer at the meter scale, bringing new insight regarding hydraulic and thermal characterization. For this purpose, the performed temperature tracer experiments are characterized by their energy recovery rate (e.g., de la Bernardie et al 2019 ) and interpreted using numerical modeling that considers multiple discrete fractures and density‐viscosity dependent flow and transport (Graf and Therrien 2005 , 2007 ; Graf and Simmons 2009 ; Hoffmann et al 2019 ). To our current knowledge, although there are analytical solutions dealing with a synthetic case of a cold water injection (Ascencio et al 2014 ), this is the first time that cold water is used as an injected tracer in the field for hydrogeological aquifer characterization.…”