Minewater Geothermal: Mine-groundwater interactions and the effects on Geothermal Resource feasibility

Flooded, abandoned coal mines represent a reliable, low-enthalpy geothermal resource, providing significant subsurface stores of water, which heat can be extracted from or rejected to. Understanding the hydraulic processes not just within the mine, but of the local groundwater regime is key for accurate characterisation and modelling of the heat extraction and storage potential of Minewater Geothermal (MWG) systems. Here, we investigate the interaction between the local groundwater regime and MWG system, their influences on each other, and how ambient flow is diverted, with the mine representing a preferential pathway. Using MODFLOW and Python, we have developed a 3D groundwater model, to assess the rate of groundwater flow through flooded coal mines.   

 

During active mining, continual pumping of water was necessary to maintain dry working conditions. Once closed, pumps were shut off and the mines were left to flood through groundwater rebound. The UK Mining Remediation Authority are responsible for the monitoring of regional coalfield groundwater resources, investigating the impact on individual MWG sites, however, requires an understanding of the local regime.

 

Post-mining hydrogeology is very uncertain, with increased fracturing and hydraulic conductivity of surrounding strata producing irregular flow paths into the open mine galleries, the extent of which at individual mine sites is not known. To deal with this level of hydrogeological uncertainty, a broad modelling approach has been taken. We have completed sensitivity analysis of a conceptual model to gain a first-order view of how the distribution of hydraulic conductivity values alters the amount of water flowing through the mines, and the determination of a scaling relationship. We have also carried out transient pumping tests to calibrate the conceptual model.

 

Here, we present the results of i) conceptual modelling, ii) sensitivity analysis and iii) transient pumping tests, aiming to assess how varying hydrogeology of mined strata influences the rates of groundwater flow through the mine. These initial findings suggest that local groundwater flow has a significant role in the hydraulic conditions of MWG schemes and should be a key consideration when selecting sites for new schemes. Significant flow through the mines could be positive or negative, depending the on the use. For extraction schemes, a regular supply of warm water would be beneficial to the longevity of the scheme. Whereas, for storage purposes, a strong flow through the mine would carry the warm water away from the site leaving it unusable. This becomes a complex issue for seasonal storage/extraction schemes, such as the ICHS project at Durham University.