Yufeng He

Shale oil reservoirs are distinguished by their extremely low permeability and the presence of numerous natural bedding interfaces. As a typical layered sedimentary rock, shale requires hydraulic fracturing for efficient recovery, making it crucial to investigate the influence of these bedding properties on the hydraulic fracturing process. To address gaps in existing research regarding the influence of multiple bedding interface properties, this study utilizes artificially laminated specimens in conjunction with triaxial hydraulic fracturing experiments and CT imaging techniques. This approach enables a systematic investigation into the effects of bedding interface properties and horizontal principal-stress difference on hydraulic fracturing behavior. The findings reveal that the shear strength of the bedding interfaces is governed jointly by their cohesion, their friction angle, and the prevailing normal-stress condition. Under the tested conditions, specimens containing the relatively stronger bedding interfaces generally exhibited higher breakdown pressures, indicating greater resistance to hydraulic fracture initiation. Notably, specimens containing the intermediate-strength bedding interfaces exhibited greater hydraulic input work before fracture breakdown, primarily owing to the longer injection duration under these conditions. Among the representative specimens examined, the highest reconstructed fracture-network complexity was observed for the intermediate-strength bedding interface at a horizontal principal-stress difference of 6 MPa. Overall, hydraulic fracture propagation was strongly governed by the imposed principal-stress regime, whereas variations in bedding-interface properties influenced fracture deflection and the stress-dependent shear resistance of the interfaces.

A comprehensive understanding of hydraulic fracturing behavior and its impact on regional stress distribution under varying principal stress conditions is essential for preventing dynamic disasters. In this study, true triaxial hydraulic fracturing experiments were conducted using roof sandstone from the Mengcun coal mine. The 3D structure of the hydraulic fractures was reconstructed using CT scanning and numerical simulation to elucidate the effect of intricate geostress conditions on hydraulic fracture propagation. The results indicate that the difference in maximum principal stress plays a crucial role in initiating and propagating hydraulic fractures. Specifically, a greater difference in maximum principal stress increases the likelihood of hydraulic fracture deflection. As this stress difference rises, the angle of hydraulic fracture deflection increases. Additionally, the presence of a hydraulic fracture alters the characteristics of the stress field, leading to stress concentration at the hydraulic fracture tip and stress unloading on both sides. Although the effects of injection rate and rock lithology were not considered in this study, this study remains valuable for optimizing hydraulic fracturing parameters in coal seam roofs.

Understanding the rules of hydraulic fracturing behavior under varying principal stress conditions and its impact on the distortion of the regional stress field is essential for enhancing the effectiveness of hydraulic fracturing in relieving pressure from coal seam rooftops. This paper intends to conduct experiments on top slab sandstone under different stress scenarios. By employing high-resolution CT three-dimensional reconstruction along with numerical simulations, we uncovered the impact of complex geostress conditions on hydraulic fracturing modes and its effect on the regional stress field. The results indicate that the difference in maximum principal stress plays a crucial role in initiating and propagating hydraulic fractures. Specifically, a greater difference in maximum principal stress increases the likelihood of hydraulic fracture deflection. As this stress difference rises, the angle of hydraulic fracture deflection increases. Additionally, the presence of a hydraulic fracture alters the characteristics of the stress field, leading to stress concentration at the hydraulic fracture tip and stress unloading on both sides. These findings provide a theoretical foundation for optimizing deep coal seam roof engineering fracturing parameters.