Thermal response of heterolithic deposits in flooded coal mines: implication for heat storage potential

Heat transfer rates are critical to underground heat storage recovery potential and sustainability of thermal abstraction for heating and cooling buildings. A 17-day heat injection – abstraction experiment into a flooded, disused mine working was conducted at the UK Geoenergy Observatory in Glasgow. Analysis of the thermal response of different lithologies intersected by an injection borehole during and after a heat injection experiment is used to quantify the heat exchange between rock mass and circulating mine water. The monitoring data from Distributed Temperature Sensing (DTS) has been analysed and numerical models using COMSOL Multiphysics were developed to characterise the rates and controls on thermal processes during heat injection and recovery. The results suggest the key control of the borehole construction on the temperature change in the first 10 hour of heat injection. In the long term, the thermal response mainly depends on the thermal conductivity of the lithologies. The radial heat transfer reaches a steady charging rate of 23 W/m2 and 16 W/m2 in the sandstone and clay intervals, respectively, and a maximum of 14 W/m2 and 10 W/m2 at the start of recovery. This is accompanied by upward heat diffusion/convection from the mine working. This study demonstrates the ability of DTS to identify lithological heterogeneities at a high resolution, and the importance of considering the overburden structure and lithology for thermal storage applications.