Zehang Su, Yongfa Zhang, Anfa Long, Shengfeng Wu, 娄毓彦, T Zhang
The in situ stress field in shale formations exhibits significant spatial variation due to geological structures. The mechanisms influenced by different in situ stress variations during the hydraulic fracturing process are not yet fully understood, particularly under the interaction of axial compressive stress and injection conditions, where the evolution of fractures remains unclear. To address these research gaps, a series of hydraulic fracturing experiments in conjunction with acoustic emissions (AEs) was conducted on Changning shale to investigate the impact of different differential stresses (5, 15, and 25 MPa) on fracture propagation and failure modes. The AE techniques, fluorescent tracer observation, three-dimensional laser scanning, and three independent parameters [standard deviation (SD), θs, and Rs] were used to quantitatively assess the induced fracture morphology. The findings show that when axial preloading stress was below 30% of the sample’s uniaxial compressive strength, few macroscopic fractures were induced, although microcrack formation significantly increased. Higher preloading levels led to a reduction in AE activity and a corresponding decrease in breakdown pressure. With increased stress differences, preloading inhibited the formation of tensile microcracks and promoted shear and mixed-mode microcracks. Under high preloading conditions, fractures propagated in a vertical, winglike pattern, with smoother and more uniform surfaces as the stress difference increased. A synoptical diagram is provided to illustrate the relationship between microcrack damage and fracture propagation under varying preloading pressure conditions. These findings contribute to a deeper understanding of shale hydraulic fracturing mechanisms and offer practical guidance for optimizing field hydraulic fracturing designs.