Mine water geothermal energy presents a unique opportunity for former mining regions to become hubs for renewable energy production and storage, particularly as part of 5th generation energy networks. This supports sustainable energy goals and also revitalizes former mining areas. Northwestern Europe, with its coal mining history, offers an ideal environment for these systems. This abstract highlights the Walloon region's efforts in Belgium to harness this untapped resource to support renewable energy and regional development. A key milestone of these efforts is the 2019 study assessing mine water regional potential for geothermal applications. The methodology for regional assessment and the modeling approaches applied at the local scale is presented.

First, the innovative methodology developed to assess the geothermal potential of abandoned mines at a regional scale for thermal energy production and storage is introduced. Second, the modeling approaches applied during feasibility studies to assess the actual local potential are discussed. A detailed modeling framework, designed to evaluate the energy production and storage capacity of mine water systems in relation to surface demand, is also explained. By considering local factors such as subsurface characteristics and energy requirements, this work offers a new perspective on how former mines can be effectively repurposed for renewable energy projects.

The study's key finding is that, under conservative assumptions, Wallonia has the potential to develop several geothermal projects like the pioneering mine water geothermal project in Heerlen, the Netherlands. Based on this, the Walloon Administration launched three feasibility studies for mine geothermal projects in the most important mining districts — Couchant de Mons, Charleroi, and Liège. Depending on local conditions, the studies showed that either demand or the subsurface can limit the development of geothermal projects using mine water. The study also highlights the value of various modeling approaches and the challenges of collecting, digitizing, georeferencing mine maps and related data to build usable 3D models incorporating mine galleries, shafts and panels.

The implications of this work are relevant for the future of renewable energy in former mining regions. The findings have led to a call to implement a pilot geothermal project in the Liège mining basin, which could serve as a model for similar initiatives across Wallonia or Europe. This research highlights the potential for abandoned mines to play a central role in the energy transition, providing both a sustainable energy source and storage solution while addressing environmental and economic challenges in post-mining areas.

Geothermal use of flooded mines has become increasingly established in recent years. In addition to research projects, there are dozens of applications exploiting warm mine water for heating purposes. One of the recently implemented projects is the use of the former Dannenbaum colliery in Bochum, Germany, where mine water is to be utilised for heating and cooling applications. As the hydrodynamic and geochemical changes in mine water associated with geothermal applications are largely unknown to date, the abandoned Dannenbaum colliery was selected as a case study for analogue modelling in the “Agricola Model Mine” in Pretoria, South Africa.

Analogue modelling is used to simulate hydrodynamic and hydrochemical processes on a model scale. Configured to represent the conditions at the Dannenbaum colliery, the 4 × 6 metre large, analogue “Agricola Model Mine” was implemented for a mine water geothermal experiment. For this purpose, conditions such as water temperature, density stratification and pumping rates were set up to match those at the colliery, with warm mine water being pumped and re-injected into the flooded mine. Continuous monitoring of parameters (temperature, electrical conductivity, tracer concentration) throughout the mine water body and the use of tracers have allowed changes in the system to be recorded.

As in most flooded underground mines, the analogue model initially contained stratification between different water bodies, similar to the one at the Danneborg colliery. However, the reinjection of the lower warm mineralised water into the upper cold freshwater body caused the density stratification to collapse. This was accompanied by a change in the water composition throughout the entire flooded model mine. At the discharge point itself, the water quality initially deteriorated due to the breakdown of the stratification. In addition, the water temperature changed as a result of the mixing in the entire model mine, forming a mixed water body.

It is expected that these phenomena observed in the analogue model will also occur at the Dannenbaum colliery and the nearby mine water discharge point at the Friedlicher Nachbar colliery. Yet, detailed studies will have to be done in the coming months and years to investigate these effects. Experiments in the analogue model indicate that a change in the water chemistry is likely. These early findings will allow consideration of measures to be taken on site. In addition, attention should be paid to the collapse of density stratification in future geothermal mining water applications.