This study presents a laboratory-scale investigation of Fractured Thermal Energy Storage (FTES) as a solution for long-term heat storage in low-permeability geological formations. While conventional Aquifer Thermal Energy Storage (ATES) relies on high-permeability media, FTES uses hydraulic fractures to enable fluid circulation in crystalline rocks such as granite, expanding thermal storage to previously inaccessible environments.

At the GeoEnergy Lab (EPFL), experiments are conducted on 25 ×25 ×25 cm blocks of Lanhelin granite. A controlled hydraulic fracture is generated, and heated water is injected through a closed-loop system. Surface and fluid temperatures are monitored using thermistors and thermocouples, while an insulated enclosure minimises external temperature effects. The system is fully automated to enable multi-cycle testing.

Results show asymmetrical heating above and below the fracture, attributed to residual well effects, as well as a consistently steeper heating slope compared to cooling. The thermal response is sensitive to flowrate but surprisingly stable across varying injection temperatures. Repeated cycles demonstrate strong reproducibility.

Future work includes testing multi-fracture systems and developing a simplified energy balance model. These r