Monitoring responses to mine water geothermal use in a highly characterised and instrumented groundwater system

Mine water geothermal has great potential to provide low carbon heating, cooling and energy storage. Some successful examples have shown that a flooded mine is a reliable, low carbon heat source and could contribute to a new green energy future for many European post-mining regions. To date, however, this potential has been hindered by scientific and technical challenges that have resulted in delay, cost overrun, or even abandonment of some mine water geothermal projects. Key sources of uncertainty that present challenges for developers, operators and regulators are groundwater flow behaviour and temperature distribution in abandoned mines under abstraction/reinjection cycles, the long-term sustainability of the geothermal system, and its interactions and impacts in the surrounding environment. 

The UK Geoenergy Observatory (UKGEOS) in Glasgow, Scotland, is an at-scale research facility with exceptional levels of hydrogeological and thermal characterisation and downhole instrumentation designed to monitor and quantify subsurface change and provide data to address challenges and risks associated with mine water geothermal systems design and operation. The Observatory includes four mine water boreholes connected in an open loop configuration with pumps for abstraction/reinjection, a heat pump-chiller and three different heat exchangers to enable testing of multiple modes of heat pump operation (heating and cooling) and component performance. A further two boreholes intercepting mine workings are equipped with downhole electrical resistivity tomography (ERT) and hybrid fibre-optic cables for distributed temperature sensing (Passive and Active DTS). Together with five environmental monitoring boreholes, a seismic monitoring borehole and ten hydrogeological downhole data loggers for continuous pressure, temperature, and electrical conductivity monitoring, the dedicated Observatory, which is not connected to any customers, is well equipped to examine the interaction and impacts of geothermal energy systems.

In this work we present a comprehensive set of initial hydrogeological and thermal observations collected during the construction and commissioning stages of the Observatory, including long term baseline monitoring, results of initial well pumping and heat abstraction/reinjection tests. These observations include evidence for the general groundwater flow circulation in the system, groundwater level response to recharge events, different transmissivities in different mined zones, and limited connectivity between mine workings at different depths, the surrounding aquifers and the River Clyde. We have integrated hydrogeological, thermal, and other information to develop an initial conceptual hydrogeological model of the system. Using the conceptual model and field data we have developed flow and heat numerical models to evaluate alternative scenarios of heating and cooling. Modelling results indicate variable flow paths and response times for thermal breakthrough for different geothermal operational configurations. Academic and commercial researchers are encouraged to get in touch to discuss using the Observatory’s unique capability for future mine water geothermal energy investigations, including investigating the behaviour, sustainability and impacts of groundwater flow and temperature under geothermal abstraction/reinjection cycles.