Drilling into mines for heat: geological synthesis of the UK Geoenergy Observatory in Glasgow and implications for mine water heat resources

Thermal energy from groundwater in abandoned, flooded, coal mines has the potential to make a significant contribution to decarbonisation of heat and Net-Zero carbon emissions. In Glasgow, UK, a subsurface observatory has been constructed for mine water heat and heat storage research. We synthesise geological and mine water resource findings from a four-year period of borehole planning, drilling, logging and testing. The heterogenous bedrock is typical of the Scottish Coal Measures Group, whereas superficial deposits are more sand- and gravel-dominated than prognosed. Mine water boreholes encountered workings in the Glasgow Upper, Glasgow Ell and Glasgow Main coal seams, proving water-filled voids, mine waste, fractured rock mass and intact coal pillars with high yields on initial hydrogeological testing. Whilst the depth and extent of mine workings delineated on mine abandonment plans proved accurate, metre-scale variability was expected and proved in the boreholes. A mine water reservoir classification established from the observatory boreholes highlights the resource potential in areas of total extraction, stowage, and stoop and room workings. Since their spatial extent is more extensive across the UK than shafts or roadways, increasing the mine water energy evidence base and reducing exploration risk in these types of legacy workings is important.