Threshold gradients and stress-induced fractal flow in abandoned-mine heat recovery shaped by rough fracture dynamics

• An innovative fractal-THM coupling model encodes fracture microstructures to link microevolution with Darcy-scale balances, solving traditional models’ neglect of fracture microstructure evolution. • Numerical tests identify a clear sensitivity hierarchy: fracture multiplicity dominates flow resistance/thermal output, followed by tortuosity, with roughness secondary, offering clear targets for system optimization. • The model is fully validated via field data and analytical solutions, with predictions of pore pressure, stress, and temperature matching measured/analytical values well. • It quantifies reservoir parameter evolution and fracture parameters’ effects on efficiency, providing actionable guidance for heat recovery system design and operation. Closed underground mines are increasingly recognized as low-to-medium enthalpy thermal reservoirs. From a fluid-mechanics perspective, flooded panels and stress-disturbed fractured strata comprise a high-connectivity porous-fracture network in which temperature-dependent density and viscosity, buoyancy, and injection-driven pressure gradients jointly govern flow regimes and heat advection. Mining-induced stress redistribution dynamically remodels rough fracture geometry, altering permeability, hydraulic tortuosity, and threshold pressure gradients. To capture these effects, a fractal-multiphysics method is employed that encodes fracture multiplicity, tortuosity, and surface roughness into transport properties and embeds them within coupled thermal–hydraulic–mechanical governing equations solved by finite elements. This fluid-behavior-centric formulation links microstructural evolution to Darcy-scale momentum and energy balances without explicit pore-scale meshing, enabling analysis of mobility changes, and buoyancy–forced-flow competition under realistic operating schedules. Numerical assessments under representative closed-mine conditions indicate that fracture multiplicity exerts the dominant control on effective flow resistance and thermal output, followed by tortuosity, with surface roughness playing a secondary role within the examined ranges. The results provide an integrated basis for evaluating carrier mobility, predicting thermal recovery, and managing stress and pressure risks in abandoned-mine heat recovery systems.