A numerical approach to combined open gallery and porous flow in flooded mine workings

Mine water geothermal heating (MWGH) is increasingly recognised as a scalable, low-carbon heat source in former coalfields. Project viability, however, hinges on the hydraulics and heat exchange behaviour within complex networks, but also on realistic evaluation of heat pump performance. In this paper, we present an efficient numerical framework (GEMSToolbox) that couples steady-state pipe network hydraulics with semi-analytical heat exchange in both mine galleries and mined-out goaf zones, extended to include heat pump performance and pumping energy. This approach provides, for the first time, a full system evaluation tool dedicated to MWGH. A semi-analytical approach that efficiently represents 2-D porous flow in goaf zones enables rapid assessment of thermal evolution. We validate porous flow hydraulics and heat exchange against analytical benchmarks, and further compare model outputs with the well established software OpenGeoSys. This approach allows calculations that are an order of magnitude faster than conventional software tools, with temperature variation at the abstraction point of less than 4% over a 30-year period. A case study quantifies sensitivity to flow rate, injection and abstraction locations, porous zone properties, and pumping performance on a real mine system. The results highlight that the abstraction temperature is primarily controlled by flow rate and injection location, while pump efficiency strongly influences the system seasonal performance factor (SPF). This study reveals the value of GEMSToolbox in enabling large scale probabilistic site assessment which is not feasible with conventional software tools due to runtime constraints.