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Bedding planes and property contrast between layers plays an important role in preventing hydraulic fracture height extension in shale reservoirs. However, the intrinsic causes of fracture containment have not been elucidated, and few studies have considered the effects of mixed factors on fracture height extension. Based on the continuum-discontinuum element method, a 3D hydraulic-mechanical coupling model was established to explore the influence mechanism of bedding planes, stress and modulus difference between layers, and mixing factors on fracture height extension in a transversely isotropic shale reservoir. Shear failure usually occurs preferentially when the fracture approaches the bedding plane, and the essential reason for fracture containment is that the shear slip on the bedding plane blunts the fracture tip. The stronger stress barrier formed by the higher minimum principal stress in adjacent formation effectively enables the fracture to terminate at the interlayer interface. The lower modulus in adjacent formation is more prone to rock initiation, but the larger elastic deformation absorbs the fracture tip stress and impedes height propagation. The comprehensive effect of bedding planes and property contrast between layers on fracture propagation is more complex than that of a single factor. When different factors have positive effects on fracture containment, the fracture is easier to terminate at the bedding plane. Otherwise, the dominant factor determines whether the fracture can be captured. No matter what the conditions, the pressure relief in the bedding plane can slow down the propagation rate. The findings can offer theoretical support for predicting fracture height during field fracturing in shale reservoirs. [ABSTRACT FROM AUTHOR]

Shale reservoirs are usually buried deep, and contain lots of complex natural fractures (NFs) under the influence of tectonic stress and faults. Previous studies usually focus on small-scale reservoir models and there is also a lack of research that completely focuses on NF characteristics. Using the continuous-discontinuous element method (CDEM), a Hydro-Mechanical (HM) coupling model was constructed to explore the propagation mechanism of hydraulic fracture (HF) and the effects of NF characteristics on HF propagation. The results show that as the HF approaches the NFs, the NFs preferentially tend to undergo shear failure, resulting in a decelerated HF propagation rate and an increased injection pressure. After water enters the NFs, the HF rapidly propagates, so the fracture area swiftly increases and the HF has a more tortuous fracture morphology. The NF orientation (α) and the internal friction angle (φ) exhibit a pronounced and distinct effect on HF propagation. Specifically, when α is 30°∼60°, the HF propagation is more tortuous and complex, so it is easier to form a larger transverse reconstruction range. When the NF density (η) is no less than 0.12, the HF tends to open the NFs and form a more tortuous morphology, but a higher η does not necessarily imply a higher fracture complexity. A longer NF (l > 12 m) benefits to improve the longitudinal reservoir reconstruction scope, but has little impact on the overall reconstruction effect. In the presence of stress shadow effect, it is improbable for the HF to completely open the intersecting NFs; instead, it typically opens only a segment or one of them. Consequently, it may be challenging to establish a fracture network with the simultaneous development of primary and branching fractures. Additionally, some water leakage into the inactive NFs gives rise to a decline in both the fracture length and fracture area. The findings can offer theoretical insights for refining the fracturing design in shale reservoirs with NFs. [ABSTRACT FROM AUTHOR]