Study on shear deformation and activation mechanisms of micro-faults in shale formation—Based on a similar physical model for simulating the fracture process of multi-scale fault

The study of shear mechanical response and deformation mechanisms of micro-faults is of significant importance for guiding the safety and efficiency during the CO₂ storage process. In this study, physical simulation experiments of the small deformation were employed. Combined with acoustic emission and digital image correlation methods, the influence of geometric properties on the macro mechanical behavior, damage patterns, and crack propagation of micro-faults were clarified. The shear mechanical mechanisms and tip damage zone characteristics of micro-faults were revealed and finally, the model's comparability was verified. The results indicate that: (1) The experimental model was suitable for simulating shear deformation of micro-faults in shale. It exhibited nonlinear elastic-plastic-frictional characteristics and replicated shear and tensile fracture mechanisms. (2) Faults geometric features controlled the mechanical stability of fractured rock masses. When faults were aligned at 0° or 90° to the shear direction, filled with minerals, and of smaller scale, small-scale shear cracks predominantly developed, leading to stronger overall stability. In contrast, when faults intersected obliquely with the shear direction, large-scale tensile cracks became dominant, making them more prone to shear instability. (3) The fracture types of the experimental model were highly similar to the Riedel shear model, with prevalent Riedel shear (R) and shears parallel to the PDZ (Y). Fault geometric features and stress states controlled the differences in mechanical mechanisms of shear zone formation. (4) Variations in stress-strain differences at the tips of faults with different orientations lead to the formation of complex tip damage zones. The en-echelon fractures formed by the connection of R-shear and Y-shear at different stages exhibit the highest degree of development.