Multiscale characteristics and cracking behavior in reservoir sandstone under dry-wet cycles: Insights from NMR, AE and SEM

The periodic water-level fluctuations in the hydro-fluctuation belt of the Three Gorges reservoir progressively degrade the mechanical integrity of reservoir rock mass, thereby promoting slope instability and related geohazards. To elucidate the multiscale damage evolution and cracking behavior of sandstone under hydromechanical loading, here we conducted a series of dry-wet cyclic experiments, synchronized with acoustic emission (AE), nuclear magnetic resonance (NMR), and scanning electron microscopy (SEM). The results show that the early warning points occur at 91.76%-98.53% of peak stress, whereas the critical fracture point consistently appears beyond 99% of peak stress. When the number of dry-wet cycle increases from 0 to 30, the proportion of tensile cracks increases from 31.05% to 48.93%, indicating a progressive transition from shear-dominated failure to a tensile-shear mixed mode. NMR results reveal an exponential increasement in porosity with the number of dry-wet cycles. The T₂ spectrum evolves from unimodal to bimodal, suggesting enhanced the pore-scale heterogeneity. This phenomenon is accompanied by a significant reorganization of the pore structure, with the proportion of micropores decreasing from 8.09% to 0.24% and that of macropores increasing from 50.78% to 65.30%. SEM observations corroborate the multiscale degradation mechanism, revealing that early-stage damage is characterized by dissolution pits and isolated microcracks on mineral surfaces. In contrast, progressive cycling substantially weakens interparticle cementation, generates abundant debris, and promotes the coalescence of microcracks into macroscopic fractures. This integrated AE-NMR-SEM framework links pore-scale evolution to macroscopic failure processes, providing a quantitative method for precursor identification and failure-mode transition assessment in reservoir rock mass.