Former mines could in future make an important contribution to the heating and cooling of residential and commercial buildings. Fraunhofer IEG, Bochum/Germany has now successfully completed four boreholes in a former, now flooded hard coal mine in Bochum as part of the EU PUSH-IT project. Using its mobile drilling rig BO.REX, the team reached a tunnel just a few metres wide at a depth of around 120 metres in each case: Precision drilling is therefore also possible cost-effectively using drilling rigs designed for near-surface geothermal energy. The boreholes demonstrate that flooded mines can be tapped using conventional geothermal drilling technology and can thus be utilised as a heat source or heat storage for homes, data centres and other commercial premises.
“The subsurface is a resource that has so far been overlooked in heat transformation. The project has shown that mine water can be accessed using conventional drilling techniques, even in mines that are no longer accessible. Instead of relying on the complex directional drilling techniques used in the oil and gas industry, we can also gain access to flooded mines using medium-sized rigs,” explains Florian Hahn, head of the “Post-mining utilisation” competence centre at Fraunhofer IEG. Even though there are natural geological formations that contain enough water for heating applications, mines generally offer higher flow rates. Furthermore, they are often located in urban areas – precisely where the heat is needed for heating or as process heat. Mine water reservoirs are also ideally suited to the long-term storage of heat, for example from data centres, in line with the principle of ‘heating in winter, cooling in summer’.
“We can now reliably access even narrow mine cavities with a mobile system,” says Jonas Güldenhaupt, Head of Drilling Operations at Fraunhofer IEG. “The ability to make flooded mines accessible again opens up new avenues for project developers and businesses to tap into mine water for their heating or cooling needs. We combine our knowledge of the subsurface with our own drilling and measurement technology to solve complex subsurface challenges.”
The high precision achieved with a medium-sized rig is particularly remarkable. At a depth of around 120 metres, mining cavities just a few metres wide were successfully accessed. Fraunhofer IEG planned four boreholes and carried them out using the mobile drilling rig BO.REX (Bochum Research and Exploration Drilling Rig). Despite a pulling force of 40 tonnes and a mast height of over 12 metres, the drilling rig is only 2.5 metres wide, thereby allowing for precise operation even in small open spaces between buildings. Specially adapted counterweights on the drill string, detailed knowledge of the subsurface and precise advance planning helped to guide the drill bit deep down into the flooded mine area.
Until now, disused mines have primarily been regarded as a long-term challenge for the mining industry. The drilling operations demonstrate that disused mines could form part of the energy infrastructure for the heating transition in future. The possibility of tapping into flooded mines via boreholes could be an important option for municipal utilities, local authorities, mine owners and project developers for the supply of heating and cooling.
Old mines fill with water once mining operations have ceased. Mine water can store a great deal of thermal energy: flooded mines can thus act as a large ‘geobattery’. In the PUSH-IT project, researchers are investigating how a mine can be developed and utilised as a storage facility for surplus heat with minimal effort. The boreholes provide access to the mine water body; through these, the water can be extracted and, after use, discharged elsewhere.
Stefan Klein, project leader of the Bochum-based PUSH-IT project, and his team have installed state-of-the-art measuring sensors in the boreholes and investigated the mining cavities using an underwater drone. The images show typical clear and turbid layers of water as well as the rock walls on either side, and confirm that the boreholes have penetrated the correct cavities as planned. All the results of the investigations are to be used for further applied research at the site. The boreholes also form the basis for the forthcoming storage test. This is intended to demonstrate the thermal storage capacity that the site can achieve. Only then will it be possible to assess whether and how the mine can serve in future as a heat source or heat storage facility for local users such as data centres or buildings in the south of Bochum, and whether investments in surface infrastructure are economically viable.
