Thermal energy storage in old flooded mines: how to tackle the hydrogeological issues

Groundwater in flooded abandoned mines could be used for geothermal purposes using heat pumps and an open loop involving pumping and re-injection. Hydraulic conductivity values of the mined rock zones have been artificially increased. However, long-term efficiency and the possible impacts of geothermal doublets must be studied involving a series of hydrogeological challenges. Hot water would be pumped from the deep parts of the mine works, and cold water re-injected in a shallower gallery or in shallow fractured rocks, with a seasonal flow inversion for building cooling during the hot season. Indeed, a ‘short-cut’ groundwater flow is to be avoided between the deep and the shallow parts of the mine. The true geometry of the interconnected network of open galleries and shafts can be highly complex and must be conceptualized realistically to ensure that the model is feasible but also reliable. This model must involve groundwater flow and heat transport, with temperature-dependent density and viscosity, in a complex 3D heterogeneous domain made of highly fractured rocks and partially collapsed exploitation zones, galleries, and shafts. Such a model is nevertheless widely recommended to design and optimize the short-, mid-, and long-term efficiency of the geothermal system as well as to assess possible environmental impacts. An example of simulations on a synthetic case will be used for illustration and preparation work before further application on a real case study.