Multiscale Mechanical Heterogeneity and Progressive Failure Mechanisms of Laminated Shale

Lamination-induced heterogeneity and directional failure complicate mechanical parameter selection for wellbore stability and stimulation in offshore and onshore shale reservoirs. Nanoindentation, directional compression, continuous scratch testing, well-log interpretation, and post-loading computed tomography (CT) were integrated to examine local stiffness, specimen-scale load bearing, and spatial heterogeneity. In the laminated limy shale sample, matrix and lamina Young’s modulus (E) values were 36.90–39.17 and 4.23–18.80 GPa, respectively. Shale uniaxial compressive strength (UCS) ranged from 39.01 to 220.18 MPa. Staged stress drops and sustained post-peak load bearing coexisted with main fractures, intersecting branches, and subparallel fracture surfaces. Mean mechanical specific energy (MSE) values were 205.05 and 315.86 MPa for the laminated and layered limy shale cores, with range-to-mean ratios of approximately 135% and 74%, respectively. The laminated core combined lower mean cutting resistance with larger relative fluctuations. Log-derived UCS varied from 143.87 to 144.65 MPa near the sampling interval, with pronounced strength transitions across a broader depth window. Matrix–lamina stiffness contrasts and interface constraints jointly control directional deformation and load sharing. Stress–strain, acoustic emission and CT observations support an interpretation in which successive local fracture redistributes load through remaining material and compressive fracture contacts, allowing abrupt stress drops to coexist with sustained post-peak deformation. Closely spaced variations in cutting resistance coexist with smoother variations in log-derived UCS, consistent with differences in measurement coverage and the use of empirical conversion relations. Directional UCS and continuous MSE consequently characterize complementary aspects of mechanical heterogeneity: specimen strength and spatial variations in cutting resistance. These findings support the selection of direction-dependent mechanical parameters for wellbore stability assessment and hydraulic stimulation design in unconventional shale reservoirs.