Thermal impact on shallow groundwater systems by heat loss from hot wells: the impact of operational conditions and subsurface heterogeneity

Geothermal heat production and aquifer thermal energy storage have significant potential to contribute to the energy transition. However, due to higher temperature inside the wells used, it is known that this leads to heat loss through conduction to the surrounding cooler, shallower groundwater systems. Therefore it is important to be able to anticipates such impacts to allow effective monitoring and prevention or mitigation measures when needed. However the thermal impact on groundwater systems is expected to strongly depend on local conditions. Therefore, this study focused on the impact of operational conditions (e.g. effective well temperatures and intermittency) and aquifer conditions (e.g. permeabilities and heterogeneity) on the resulting heat transport processes into the aquifer by conduction and density driven flow. To evaluate the degree and variation of impact that may occur under field conditions, the heat loss to a shallow groundwater system was simulated using a 2D axisymmetric numerical MODFLOW 6 model for a wide range of conditions considering both the impact of conduction and density-driven flow. The results of this study indicate that the total thermal impact and its distribution (up to >10 m from the hot well in 10 years) in shallow groundwater systems is strongly impacted by the induced density driven flow in the relatively permeable layers of the groundwater system. Conduction is dominant in transfer of heat from the hot well in the low permeability confining layers and for mitigating temperature differences in the groundwater system induced by buoyancy flow. Overall, this study highlights the importance of considering local conditions in assessing thermal impact by heat losses from hot well casings, to allow distinguishing these thermal impacts from those induced by leakage and to allow efficient thermal groundwater impact monitoring.