Coupled THM Modeling to Predict Surface Deformation in Legacy Coal Mine Heat Geobatteries

Multiple clean energy sources are being adopted to collectively support the energy transition and decarbonization efforts. Legacy coal mines in shallow subsurface layers are potential heat geobatteries that can be repurposed to capture and store waste heat from nearby sources (e.g., supercomputing clusters and large-scale data centres). This stored heat can partially meet the heating needs of local residential and business units during winter months.

Repeated cycles of hot water injection and extraction in the coal mines cause thermo–hydro–mechanical (THM) changes, resulting in subsurface rock strains that are transferred to the ground surface as uplift/subsidence (deformation). Identifying magnitudes, patterns and key variables influencing surface deformation requires building representative coupled THM models.

Galleries to Calories (G2C) is a pilot project testing the geobattery concept in the Scottish legacy mines of the Midlothian Coalfield, southeast of Edinburgh. The subsurface is characterized by thin coal seams hosted within heterogeneous layered strata, posing challenges for reliable coupled THM models to predict deformation.

A fully coupled THM model was initially built using COMSOL Multiphysics and OpenGeoSys for basic model setup assessment and results comparison. The final model was validated using historical ground surface uplift data resulting from water rebound in the coal seams driven by regional groundwater flow. The validated model was then used to quantify and predict surface deformation caused by seasonal heat injection and extraction. The key variables influencing surface deformation were identified and ranked based on sensitivity analyses that accounted for parametric, structural, and regional groundwater flow uncertainties.

The study outcomes provide guidelines for defining technical design and operational constraints that ensure system stability and limit surface deformation, thereby reducing risks to existing buildings and infrastructure above the project area.