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

Abstract

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.Supplementary material Borehole reports and other datasets are available at https://ukgeos.ac.uk/data-downloads (mixture of over 20 DOI datasets and reports/data packs published openly on https://nora.nerc.ac.uk, all material is deposited in the National Geoscience Data Centre)

Role of Subsurface Geo-Energy Pilot and Demonstration Sites in Delivering Net Zero

“…The UK Geoenergy Observatories (UKGEOS), Glasgow Observatory was established between 2016 and 2020 as an at-scale infrastructure for mine water heat and heat storage research in a representative urban setting (Monaghan et al, 2017a;Monaghan et al, 2019;Monaghan et al, 2021). It comprises 12 boreholes, four research compounds, surface monitoring equipment and open data.…”

A Review of the Performance of Minewater Heating and Cooling Systems

“…At the recently drilled UKGEOS minewater research site in Glasgow (4.2011° W, 55.8383° N [109]), where "mine plans were examined by experienced geologists prior to drilling, 3 of the 6 target mine workings encountered a void" upon drilling [110]. These were, however, largely pillar and room workings and it was recognized at the outset that the georeferencing error on the mine plans was greater than the dimension of the room/void.…”

“…One can probably assume therefore that the drilling outcome partly represents the statistical ratio of voids to pillars, possibly combined with subsequent extraction of pillars. Monaghan et al [109] thus reported that inaccuracy was experienced at the "metre scale". Mine plans were examined by experienced geologists prior to drilling and targets chosen with great care, the rate of success of encountering voids as anticipated was good, but by no means perfect.…”

The Value of a Hole in Coal: Assessment of Seasonal Thermal Energy Storage and Recovery in Flooded Coal Mines

“…The prospectivity of the subsurface voids depends on the hydraulic conditions in place, which could be potentially affected by anthropogenic material filling, clay-rich coal breccia sedimentation and roof collapse (Andrews et al, 2020). The latter could result in fracture networks around extraction panels, as evidenced in the exploration results of the Glasgow Geoenergy Observatory (Monaghan et al, 2021), that could increase the "effective thermal conductivity" of the host rock, i.e., rock mass surrounding the mine voids, and improve the subsurface heat transport (Chiasson et al, 2000). Additional adverse effects that could constrain the exploitation of the heat resource include: high water pumping costs and corrosion of the installations (Walls et al, 2021), iron oxyhydroxide precipitation (Banks et al, 2019), potential surface alterations due to mine-water level changes (Todd et al, 2019) and hypersaturation of calcite (Jagert et al, 2018).…”

“…Additional adverse effects that could constrain the exploitation of the heat resource include: high water pumping costs and corrosion of the installations (Walls et al, 2021), iron oxyhydroxide precipitation (Banks et al, 2019), potential surface alterations due to mine-water level changes (Todd et al, 2019) and hypersaturation of calcite (Jagert et al, 2018). On a large scale, the regional groundwater flow could result in an unwanted dispersal of heat if the location of injection and extraction boreholes is not properly optimized (Raymond and Therrien, 2014); particularly, in flooded panels with a strong hydraulic connection with shallow aquifers or surrounded by thermally conductive lithologies, as is the case of the Glasgow Main Coal (Monaghan et al, 2021).…”