Geothermal heat extraction from abandoned mines

Water from abandoned, flooded mines can form an excellent low-enthalpy, renewable long-term heat source, provided it is managed properly. Its sustainability, however, is only as good as its proper management. The poor understanding of the condition of the mine, post-closure makes the investment in these projects risky compared to other alternatives. Our modelling allows us to explore uncertainties and reduce a variety of project risks.

By combining numerical and analytical methods with digitised legacy mine data, we developed a tool to estimate the variations in the abstraction water temperature over the lifetime of a project. We couple the heat transfer approximation method originally proposed by Rodriguez and Diaz (2009) to that of flow in a pipe network as described by Todini and Pilati (1987). We refine the original heat transfer approximation by accounting for a flow regime specific heat transfer coefficient between the rock mass and the water, as prescribed by Loredo et al. (2017). We also develop a novel weighting function to account for the interference between adjacent mine galleries.

This method is applied to investigate the scenario in which multiple users will extract heat from the same mine water block. We investigate the interference resulting from heat extraction at multiple locations, using a mine system from the North East of England as a study case. The results of this study provide constraints on the maximum mine water extraction rates and proximity of the different users. The poorly constrained connectivity (through mine shafts, connecting roadways or porous flow) between mine workings from different coal seams is shown to be one of the most significant uncertainties in assessing the feasibility of a mine system as a sustainable heat source.

References:

• Loredo C, Banks D, Roqueñí N. Evaluation of analytical models for heat transfer in mine tunnels. Geothermics 2017; 69; 153-164.
• Rodriguez R and Díaz M. Analysis of the utilization of mine galleries as geothermal heat exchangers by means a semi-empirical prediction method. Renewable Energy 2009; 34(7), 1716-1725.
• Todini E and Pilati S. A gradient method for the analysis of pipe networks. Computer app. In water supply 1987; 1-20, v1.