Influence of Mine Geometry and Working Type on Geothermal Exploitation and Thermal Storage

Abstract Thermal plumes developed during two heat injection experiments in different flooded mine working levels at the UK Geoenergy Observatory in Glasgow recorded thermal breakthrough times (for 0.1 °C change) of 4.8 and 2.3 days under similar test conditions. Synthesis of the mine abandonment plan, borehole, and outcrop data was used to characterise differences in the mine geometry and working type. New insights on how backfilled, collapsed, and open voids affected groundwater flow and heat transport were obtained from the hydrogeological, wellhead, and downhole direct temperature sensor data measured during and after pumping of the open loop borehole doublets. Over 17 days, heat injection into the Glasgow Upper mine workings resulted in the gradual development of a thermal plume, within an area interpreted to have a greater percentage of backfill and more tortuous flow paths. After 3.9 days, temperatures at the abstraction borehole started to increase at ≈ 0.03 °C/day, with a ≈ 2.3 °C differential maintained 10 days after heat injection and pumping stopped. In the five-day Glasgow Main heat injection experiment, temperature increases after 1.4 days were ≈ 0.14 °C/d, which was interpreted to result from preferential flow and heat transport pathways with less surface area for heat transport through conduction to the rock mass, and quicker transit time to the abstraction borehole. This provides real-world, at-scale quantification for geothermal exploitation and mine thermal energy storage, and demonstrates how mine working type, which influences flow and heat transport, can affect system performance.