Accurate characterization and fractal analysis of pore–fracture evolution in deep coal under temperature effect

The mechanical behavior of deep coal is governed by in-situ temperature, stress, and pore pressure. However, laboratory experiments often exaggerate thermal weakening, whereas in-situ core measurements under geologically constrained formation temperatures show minimal deterioration, underscoring the need to clarify coal’s true thermo-mechanical response under realistic geological conditions. This study integrates NMR analysis and SEM observations to characterize the thermal evolution of the pore–fracture system in deep coal, with fractal theory providing quantitative insight into the underlying mechanisms. Results reveal that, along the thermal path from ambient to in-situ and supra-in-situ conditions, coal undergoes progressive compaction, microcracking, fracture propagation, and structural reorganization, accompanied by a transition of pore water from bound to free states. Porosity exhibits a non-monotonic trend—slight reduction up to in-situ conditions, followed by significant increase at elevated temperatures. 3D NMRI visualizes the transition of pore-fractures from closed to open and interconnected states. The rise–fall trend of fractal dimension further indicates that thermoelastic compaction dominates below in-situ temperature, while thermal damage governs expansion and connectivity above in-situ temperature. This work establishes a visual and quantitative evidence chain for temperature-induced pore–fracture and fluid evolution in deep coal, revealing non-monotonic thermal responses and underlying mechanisms. The findings advance understanding of in-situ multi-field, multiphase coupling and provide theoretical support for deep coal mining and unconventional geoenergy recovery.