A scenario-based approach for characterising subsurface uncertainties in shallow mine geothermal flow systems.

Repurposing abandoned coal mines for geothermal energy production presents challenges due to subsurface uncertainties of both natural and geotechnical origin. This work introduces a conceptual scenario-based approach to describe and manage these uncertainties with an ensemble of geological/geotechnical scenarios based on real data. Model scenarios were created by varying mine void connectivity, mined boundary geometry, and room-and-pillar dimensions using Glasgow UK Geoenergy Observatory data. Inter- and intra-mine flow and heat pathways are modelled as distinct hydraulic facies: open water-filled voids, variable backfill material or goaf (collapsed roof rock) properties, and host rock lithology. Each hydraulic facies' petrophysical properties were explicitly incorporated into 3D grids based on conceptual sketches.

The aim of the scenario approach is to evaluate a range of possible dynamic responses for open-loop production of ambient mine water with cold water re-injection, enabling evaluation of prospects and risks.

We implemented the openDARTS flow thermal-hydraulic simulator, a scalable parallel modelling framework that utilises Operator-Based Linearisation (OBL) to handle a multi-physics flow. Our analysis demonstrates the impact of heterogeneity uncertainty in confining layers for robust long-term temperature predictions. These layers either provide thermal insulation or enhance conductive-convective heat transfer at the mine void – host rock boundary. Multiple simulated scenarios revealed valuable insights into how production performance in minewater systems performs under different operational strategies.