Impact of subsurface heterogeneity and thermally induced buoyancy-driven flow on high-temperature aquifer thermal energy storage

• Numerical prediction of the thermal response of a high-temperature ATES. • Aquifer heterogeneity quantified from high resolution borehole data. • An indicator approach allowed the quantification of geological variability. • Uncertainty assessment of storage performance and induced subsurface temperatures. • Benefit from additional exploration boreholes to reduce uncertainty quantified. The seasonal mismatch between supply and demand of multiple low-carbon heating technologies creates the need for high-capacity seasonal heat storage. High-temperature aquifer thermal energy storage (HT-ATES) offers one means of facilitating this. However, at high storage temperatures, buoyancy-driven flow may transfer the heat within aquifers upwards and thus diminish HT-ATES’ storage performance and impact the subsurface temperature distribution induced by the storage operation. Heterogeneity of the storage aquifer, especially internal layering, may significantly alter the induced flow fields. The present study addresses the impact of geological heterogeneity on the m-to-hm scale on the extent of buoyancy-driven flow and the resulting uncertainty in predicted storage performance and thermal impacts. The work has a tripartite structure: In Part 1, a large, publicly available borehole database is geostatistically analyzed using an indicator approach. Multiple ensembles of heterogeneous material distribution realizations are generated and parametrized for a HT-ATES site by sequential indicator simulation, conditioned on borehole data. These material distributions form the basis for Parts 2 and 3. In Part 2, thermo-hydraulic-coupled HT-ATES simulations are performed for all realizations with transient, realistic pumping rates to derive the range of expected storage performance and temperature distributions in the subsurface. The results indicate that heterogeneity leads to significant uncertainty, particularly in scenarios that involve high-permeability layers in the vicinity of the warm well, because buoyancy-driven flow reduces the recovery of stored heat. In Part 3, the assumption of one additional exploration well as conditioning data is found to reduce the uncertainty in thermal recovery by a factor of approximately 5 and to significantly alleviate the uncertainty of the subsurface temperature predictions. Additional subsurface investigation using exploration wells thus yields valuable information for HT-ATES system planning and design.