Hydraulic Fracture Vertical Propagation Mechanism in Interlayered Brittle Shale Formations: An Experimental Investigation

To investigate the vertical propagation mechanism of hydraulic fractures in interlayered brittle shale formations in the Qingyi member of the Southern Songliao Basin in Northeast China, an experimental model for simulating the shale within thin sandstone interlayers is designed. This model reflects the relative difference of the mechanical properties and brittleness characteristics between shale and sandstone. When combining a series of true triaxial hydraulic fracturing experiments with fracture area measurement, three-dimensional fracture reconstruction, and acoustic emission (AE) monitoring, the effects of the key geological and engineering factors on the fracture vertical propagation behaviors in interlayered shale formations are quantitatively studied. The experimental results showed four types of hydraulic fracture propagation patterns in interlayered shale formation: arresting pattern, deflecting pattern, penetrating pattern and composite pattern. The interlayer dip angle is negatively correlated with the penetrating ability of the hydraulic fracture, while the vertical in situ stress difference and interface cementation strength are positively correlated with the penetrating ability of the hydraulic fracture. The brittleness characteristics of the interlayer have a considerable effect on the propagation behavior of the hydraulic fracture. The interlayer with weak brittleness inhibits the hydraulic fractures' ability to penetrate the interfaces and interlayers owing to the plastic deformation of the interlayer, thereby consuming considerably more elastic energy that should have been applied to fracture propagation. Under a higher injection rate and viscosity, the infiltration of the fracturing fluid in the interface is less, the hydraulic energy accumulated at the fracture tip is more concentrated, and the hydraulic fracture's ability to penetrate through the interface into the interlayer is strengthened. The results of this study can provide a deeper understanding of fracture geometry and the fracture intersection mechanism in the continental interlayered shale formation, providing a more accurate guidance for fracturing parameter optimization.

Automated summary

This study investigates the vertical propagation mechanism of hydraulic fractures in interlayered brittle shale formations. Experimental modeling and quantitative analysis are conducted to study the effects of geological and engineering factors on fracture propagation behavior. The results show that interlayer dip angle, vertical in situ stress difference, and interface cementation strength are important factors affecting fracture propagation. The brittleness characteristics of the interlayer also have a significant impact. The findings provide a deeper understanding of fracture geometry and the fracture intersection mechanism, offering accurate guidance for fracturing parameter optimization.