Coupled Imaging and THM-Damage Modeling of Supercritical CO₂ Fracturing in Coal

Supercritical CO₂ (scCO₂) fracturing offers a water-free and environmentally favorable approach in enhancing coal seam permeability while concurrently enabling carbon storage. In this study, we explore the mechanics of fracture initiation, propagation, and evolution of network morphology for scCO₂ injection in large cylindrical coal specimens (100 mm diameter × 200 mm height) constrained by direct and detailed observation. Triaxial fracturing tests compared the performance of supercritical (sc) CO₂, liquid (L-)CO₂, and gaseous CO₂ undergoing phase transition against water-based hydraulic fracturing. scCO₂ demonstrated the most minimal fracture initiation pressures and yielded complex initiation pressures that were reduced by approximately 4.6% in comparison to water. It exhibited a high degree of interconnected fracture networks in contrast to those observed in aqueous fracturing, a conclusion that was substantiated by X-ray CT imaging and acoustic emission analysis. Fully coupled thermo-hydro-mechanical-damage (THM-D) modeling reproduced key experimental fracture geometries and captured the evolution of three-dimensional damage zones, highlighting the influence of thermophysical properties and temperature–pressure coupling on fracture propagation. This methodological system of “experimental-AE/CT monitoring-THM-D modeling” demonstrate the technical potential of scCO₂ as an effective fracturing medium in enhancing coalbed methane recovery and promoting CO₂ sequestration, and establish a validated modeling framework for predicting field-scale performance.