Influence of bedding orientation on shale damage evolution: A combined in-situ micro-CT and digital volume correlation investigation

Abstract

The bedding structure of shale significantly influences its mechanical anisotropy. However, the meso-scale anisotropic control mechanism of bedding on shale damage evolution remains insufficiently understood. This study employs in-situ uniaxial compression CT scanning experiments, combined with grayscale thresholding and deep learning-based image segmentation, to achieve high-precision 3D reconstructions of shale pore-fracture networks. Additionally, by integrating Digital Volume Correlation (DVC) with image analysis, a cross-scale quantitative characterization and synergistic evaluation are conducted, bridging the evolution of microstructural damage with macroscopic full-field deformation in bedded shale. The results reveal that: (1) The dominant geometric factors influencing the complexity of the shale pore-fracture network during loading vary with bedding orientation: number and spatial distribution dominate for 0° shale, volume and area for 30°/60° shale, and coupled geometric parameters for 90° shale. (2) Displacement and strain fields exhibit distinct characteristics related to the bedding angle: 0° shale shows quasi-uniform deformation dominated by axial compaction; 30° and 60° shales form significant strain concentration bands along bedding planes due to shear slip effects; 90° shale is driven by radial tension, leading to tensile strain localization parallel to the bedding direction. (3) The strain accommodation mechanism in shale transitions with the bedding angle: it shifts from being dominated by matrix compaction and diffuse micro-damage at low angles to being primarily controlled by fracture propagation along bedding planes at high angles. In high-angle bedded shale, pre-existing pores and fractures tend to preferentially act as nucleation sites for damage initiation and strain localization.