High-temperature heat storage in geological media: high-resolution simulation of near-borehole processes

Subsurface energy storage of high-temperature heat in middle-deep formations below drinking water aquifers may offer a new possibility of hosting large amounts of energy in the subsurface. An adequate system and process understanding is required in order to predict and assess the complex and interacting induced effects. To this end, the near-field effects of high-temperature heat storage were studied through numerical simulations using a 100 m borehole heat exchanger placed at 500 m depth, accounting for the detailed geometry of U-pipes, grout, borehole and soil. Coupled thermohydraulic simulations showed that the simulated system may store up to 260 MWh of thermal energy using an injection temperature of 90 degrees C and that the temperature evolution with time is strongly dependent on the radial distance from the borehole and the soil permeability and is strongly sensitive to the thermal conductivity of the grout material. At low soil permeability, heat transfer is controlled by conduction and the amount of heat stored is mainly influenced by the soil thermal conductivity. A higher soil permeability leads to convection in the soil with a doubling of the stored heat. In this case, the amount of heat stored is not sensitive to the soil thermal conductivity but to the grout thermal conductivity.