Hydraulic fracturing technology is widely used in the oil and gas extraction industry. However, the mechanical mechanisms of fracture extension and penetration under fracturing conditions are not clear so far. On this basis, indoor tests on the evolution of hydraulic fracture extension and activation of bedding planer facets in rock specimens were conducted to analyze and explore the mechanical mechanisms statistically. The experimental results show that the observed micro-cracking in the rock specimen sections at the microscopic scale is not due to macroscopic fluid pressure, as compressive stresses are found at the micro-crack opening in the tests. In contrast, the micro-cracks are generated by the concentration of tensile stresses before extensional fracture, implying that bedding plane activation is caused by crack extension along the bedding plane. Furthermore, statistical analysis of micro-crack lengths shows that the rocks exhibit anisotropy in fracture toughness and that the crack propagation resistance perpendicular to the bedding plane is as much as two times greater than that parallel to the bedding plane.

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