Geothermal Potential of Closed Underground Mines: Numerical study of the Con Mine (Northwest Territories, Canada)Online pre-recorded
Flooded mines constitute groundwater reservoirs that can be exploited with geothermal heat pumps systems. Modelling such a reservoir is a challenging task because it requires solving the equations of groundwater flow and heat transport within the mine voids having a complex geometry and the surrounding medium, whose hydraulic parameters may have been affected by mining. In this study, we present a tridimensional numerical model developed with COMSOL Multiphysics with the objective of estimating the geothermal heat pump and underground energy storage potential of the Con Mine located near Yellowknife, in the Northwest Territories. This software used the Finite element method to simulate the transient 3D temperature fields within the water and in the rock mass. The shafts and tunnels of the mine are represented with 1D elements embedded in a tridimensional matrix. The thermal and petrophysical properties were evaluated at the mine site and in the laboratory with outcrops survey and core analysis. The numerical model was calibrated to reproduce hydraulic head and temperature measured in one of the shafts while groundwater was pumped out of the mine. Permeability of stopes, shafts and the host rock have been adjusted to fit the pumping data. Then, long-term temperature of the water under different cases of geothermal heat pump operation was simulated for 25 years. Therefore, it was possible to determine the number of different buildings that could theoretically be heated with the mine water considering the change of building loads affected by the climate. Heat injection in the mine during summer and heat extraction during winter was considered to represent both cooling and heating loads. The total power extracted depends on the depth and flow rate of the pump to be installed in the mine shaft. For a flow rate of 0.06 m3/s, the total power is 3.4 versus 46 MW when the pump depth is 0,3 versus 1 km. The number of buildings to be heated and their surface increase with the power available. We also simulated heat production with solar panels in summer as a mean of energy storage. The advantage to store heat in the ground is to increase the number of buildings to be heated but overall, it may be easier to increase the flow rate or place the pump at greater deep. The model indicates a favourable geothermal potential for the Con Mine. The next step to develop this resource is to design a district heat pump system to exploit the mine water.