International audienceGeothermal energy is a renewable energy source particularly attractive due to associated low greenhouse gasemission rates. Crystalline rocks are in general considered of poor interest for geothermal applications atshallow depths (< 100m), because of the low permeability of the medium. In some cases, fractures may enhancepermeability, but thermal energy storage at these shallow depths is still remaining very challenging because of thelow storativity of the medium. Within this framework, the purpose of this study is to test the possibility of efficientthermal energy storage in shallow fractured rocks with a single well semi open loop heat exchanger (standingcolumn well). For doing so, several heat tracer tests have been achieved along a borehole between two connectedfractures.The heat tracer tests have been achieved at the experimental site of Ploemeur (H+ observatory network).The tracer tests consist in monitoring the temperature in the upper fracture while injecting hot water in the deeperone thanks to a field boiler. For such an experimental setup, the main difficulty to interpret the data comes fromthe requirement for separating the temperature advective signal of the tracer test (temperature recovery) from theheat increase due to injection of hot water through the borehole which induces heat losses all along the injectiontube in the water column. For doing so, in addition to a double straddle packer used for isolating the injectionchamber, the particularity of the experimental set up is the use of fiber optic distributed temperature sensing(FO-DTS); an innovative technology which allows spatial and temporal monitoring of the temperature all alongthe well. Thanks to this tool, we were able to estimate heat increases coming from diffusion along the injectiontube which is found much lower than localized temperature increases resulting from tracer test recovery. Withlocal temperatures probes, separating both effects would not have been feasible. We also show through signalprocessing how diffusive and advective effects may be differentiated. This allowed us to estimate temperaturerecovery for different heat tracer durations and setups. In particular we show that temperature recovery is highlydependent on hydraulic configuration such as perfect dipole or fully convergent heat tracer test
