Thermogeological assessment of open-loop well-doublet schemes: a review and synthesis of analytical approaches

Use of well doublets for groundwater-sourced heating or cooling typically results in a “thermal plume” of cool or warm reinjected groundwater. Such a plume may be regarded either as a potential anthropogenic geothermal resource or as pollution, depending on downstream aquifer usage. A thermal plume may pose an external risk to downstream users and environmental receptors or an internal risk to the sustainability of the well doublet, due to the phenomenon of thermal feedback. A three-tier assessment of the risk of thermal feedback is proposed, based on: (1) consideration of well separation and yield; (2) analytical modelling of heat migration in a doublet to ascertain breakthrough time and post-breakthrough temperature evolution and (3) numerical modelling of complex scenarios.