The multiscale characteristics and cracking behavior in reservoir sandstone subjected to dry-wet cycles: Insights from NMR, AE and SEM based analysis

The periodic water-level fluctuations in the hydro-fluctuation belt of the Three Gorges reservoir diminish the mechanical properties of the reservoir rock mass, leading to slope instability and associated geological disasters. To investigate the multiscale characteristics 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). The results show that the early warning points are primarily identified using the variance of AE parameters. According to the evolution of RA (rise time/amplitude) and AF (AE count/duration time), the tensile microcracks become pronounced as the number of dry-wet cycles increases. When the number of dry-wet cycle increases from 0 to 30, the proportion of tensile cracks increases substantially from 31.05% to 48.93%, signifying a transition in the failure mode from shear-dominated to a tensile-shear mixed one. The porosity of the sandstone increases exponentially with the number of dry-wet cycles. The T ₂ spectrum evolves from unimodal to bimodal, thereby enhancing the structural heterogeneity of the pore network. 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%. At a low number of cycles, the damage is characterized by the formation of dissolution pits and isolated microcracks on mineral surfaces. In contrast, the interparticle cementation is significantly weakened, resulting in the generation of large amounts of debris. In addition, the microcracks further propagate and coalesce into macroscopic fractures.