Study on Acoustic Emission <i>b</i> -Value Characteristics in Hydraulically Fractured Shale Fracture Propagation under High Confining Pressure Conditions

Investigating the propagation behavior of hydraulic fractures in shale reservoirs under high-stress conditions in deep formations is critically important. This study utilized full-diameter shale cores and conducted hydraulic fracturing experiments through a self-developed triaxial hydraulic fracturing experimental system for high-stress rocks. The research focused on the propagation laws of hydraulic fractures in shale under high axial stress conditions and the variation patterns of the acoustic emission (AE) <i>b</i>-value during fracture propagation. The results indicate that the hydraulic fracturing pressure curves under different confining pressures exhibit two patterns during the shale failure stage: single breakdown and secondary breakdown. Under low confining pressure, the fracturing fluid is prone to leak-off in the hydraulic fracture channel formed during a single breakdown event. In contrast, under high confining pressure, the fracturing fluid is less likely to leak-off in the hydraulic fracture channel formed during the first breakdown of the secondary failure process. The energy accumulated by the fracturing fluid in the fracture channel leads to refracturing of the shale. Thus, under high confining pressure, shale specimens are prone to forming complex fracture networks. As the confining pressure increases, the morphology of hydraulic fractures transitions from simple to complex, and the fracture propagation direction becomes less constrained by the maximum principal stress. Under a high confining pressure of 59 MPa and a stress difference coefficient of 0, shale specimens are more likely to form complex fracture networks during hydraulic fracturing, while the peak AE <i>b</i>-value is correspondingly lower. This study provides a qualitative understanding and scientific explanation of the propagation behavior of hydraulic fractures in shale under high confining pressure conditions.