The impact of different mining techniques on the geothermal potential of abandoned coal mines

Abandoned and flooded coal mines have a huge geothermal potential. By circulating mine water and extracting its heat, it can provide a renewable, low-carbon low-enthalpy heat source for domestic and industrial purposes. But capital costs from drilling into those mine workings are significant, and investigating the geothermal potential of a mine system prior to drilling are essential for the success of any mine water geothermal energy (MWGE) system. Numerical modelling provides a quick and low-cost methodology to assess the feasibility of a planned MWGE system, for example to determine optimal mine water abstraction and re-injection sites.

Past coal mining was typically done using two different mining techniques. 1) The room-and-pillar (also referred to as pillar-and-stall) method was used to mine coal through digging tunnels (galleries and roadways), leaving pillars of coal untouched to prevent collapse of the coal seam. 2) The long-wall mining technique used machines to extract the entire coal seam, and allowed collapse in a controlled manner, thereby creating a porous layer of rubble referred to as ‘goaf’. Often both techniques were used within a single mine system. These two techniques result in very different remnant mine geometries, and it is important to address these differences in any MWGE modelling attempt.

We have developed a computationally fast and flexible modelling tool GEMSToolbox to assess the feasibility of mine workings as MWGE system by combining numerical and (semi-)analytical methods. The tool accounts for both room-and-pillar and long-wall mining techniques. In this study, we investigate the geothermal effects of both techniques for mines that are entirely constructed through one technique only, and for mines that combine the two techniques. Both techniques suffer from significant uncertainties in the effective model parameters: e.g. mine galleries may have collapsed since the mine closure, while for goaf, the effective porosity and hydraulic transmissivity are poorly constrained. Furthermore, in mine systems where both techniques were applied, water preferentially flows through the galleries, which potentially makes heat extraction from the goaf areas less efficient. The results of this study are applied to a real-world mine block in the North East of England.