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. For doing so, several heat tracer tests have been carried on in asingle well between two connected fractures. We completed this experimental work with numerical modeling ofthermal transport in fractures embedded in an impermeable conductive matrix.The thermal tracer tests were achieved in a crystalline rock aquifer at the experimental site of Ploemeur(H+ observatory network). The experimental setup consists in injecting hot water in a fracture isolated by a doublestraddle packer in the borehole while pumping and monitoring the temperature in a fracture crossing the sameborehole at greater elevation. Several tracer tests were achieved at different pumping and injection rates. Thisexperimental set up allowed to estimate temperature breakthrough for different tracer test durations and hydraulicconfigurations from fully convergent to perfect dipole tracer tests. Thanks to those tests and numerical modelingof heat transport in fractures, we demonstrate that temperature recovery is highly dependent on flow rate andstreamlines shape. Thus, thermal storage rate is inversely proportional to flow and is maximized in perfect dipoleconfiguration. These thermal tracer tests and numerical modeling allow to define the most efficient configurationfor optimizing shallow geothermal storage in fractured rock
