Fracture connectivity-controlled permeability evolution in fractured carbonate reservoirs under cyclic thermal loading

Fractured carbonate reservoirs commonly exhibit permeability governed by fracture-network architecture rather than matrix pore space alone, yet predicting permeability evolution during cyclic thermal stimulation remains challenging because fracture aperture enlargement and connectivity development contribute differently over time. This study investigates coupled thermo-mechanical fracture-network and permeability evolution in fractured limestone subjected to cyclic thermal loading between 25 and 180 °C. A fractured core plug was evaluated through stepwise thermal cycling at 0, 1, 2, 5, and 10 cycles. Post-cycle nitrogen permeability measurements were integrated with high-resolution micro-computed tomography and graph-based fracture-network analysis. Mean fracture aperture increased from 0.15 to 0.39 mm, the largest connected fracture cluster expanded from 18% to 84% of fracture voxels, Connectivity Index rose from 0.21 to 0.91, and permeability increased from 28 to 58 mD. A coupled aperture-connectivity permeability model is introduced to quantify the joint effects of mean aperture, Connectivity Index, and their interaction on permeability evolution. Model comparison shows that aperture-only and cubic-law-type models are most suitable during early cycles, whereas connectivity-based and coupled models better describe nonlinear permeability enhancement after fracture coalescence. Graph-topological descriptors, anisotropic connectivity, and a continuum-damage-inspired variable further characterize fracture-network evolution. The results indicate an apparent experimental percolation transition at CI ≈ 0.6-0.65, beyond which permeability becomes increasingly connectivity dominated. The proposed three-stage framework distinguishes aperture-dominated, transitional connectivity-growth, and connectivity-dominated regimes to guide cyclic thermal-stimulation optimization while limiting excessive fracture growth, preferential channeling, and thermal short-circuiting.