Time-Dependent Tensile Degradation and Associated Microstructural and Fracture Characteristics of Coal under ScCO ₂ –H ₂ O Conditions

In CO2-enhanced coalbed methane (CO2-ECBM) recovery, prolonged interaction between supercritical CO2 (ScCO2) and residual water within confined fractures may degrade the mechanical integrity of coal and influence reservoir stability. However, the time-dependent tensile degradation behavior and its underlying structural mechanisms under ScCO2–H2O conditions remain insufficiently understood. This study conducted ScCO2–H2O immersion experiments (0–28 days), integrating ultrasonic monitoring, Brazilian splitting tests, XRD analysis, SEM observation, fracture morphology quantification, and acoustic emission (AE) monitoring. Results show that the tensile strength decreases sharply from approximately 1.0 MPa to 0.3–0.6 MPa within the first 7 days, followed by a slower attenuation phase, stabilizing around 0.5 MPa by day 28. The ultrasonically derived damage variable exhibits nonlinear growth and tends toward stabilization at later stages, indicating progressive internal deterioration. XRD results reveal increases in aromatic layer size (La) and stacking height (Lc), accompanied by elevated disorder parameters, while the interlayer spacing (d002) remains relatively stable, suggesting an internal structural rearrangement and increasing heterogeneity rather than simple lattice expansion. Fracture surface complexity reaches a maximum at intermediate exposure (around 7 days) and decreases with prolonged soaking. AE (RA-AF) analysis demonstrates a systematic transition toward tensile-dominated microcracking after extended exposure. Macroscopic tensile attenuation is therefore governed by microstructural rearrangement, heterogeneity development, and matrix weakening. The results establish a nanomeso-macro coupling framework linking ScCO2–H2O-induced structural evolution to mechanical degradation, providing a structural basis for evaluating long-term geomechanical response during CO2-related subsurface operations.