Hydraulic fracture (HF) propagation in naturally fractured formations is strongly influenced by local stress states near pre-existing natural fractures (NFs). The role of NF-induced shear deformation and stress redistribution in controlling HF trajectories remains poorly characterized. This study investigates how NF-induced stress redistribution governs HF-NF interactions through coupled laboratory experiments and poroelastic extended finite element simulations on intact and pre-fractured PMMA specimens under plane-strain conditions. Digital image correlation provides full-field measurements of displacement and strain evolution during mechanical loading and hydraulic fracturing. Under fixed-base, lateral confinement, and vertical compression boundary conditions, inclined NFs induce asymmetric stress redistribution and shear deformation, generating distinct local stress states prior to fluid injection. The results demonstrate that the HF trajectory is governed by the sign and spatial distribution of shear stress and shear strain components generated by NF orientation relative to the far-field maximum principal stress. Shear deformation that promotes compressive stress development adjacent to the NF causes the HF to deflect away, whereas shear deformation that reduces the effective normal stress along the NF promotes fracture attraction and linkage. Corresponding numerical reproduction of HF curvature in pre-fractured specimens requires mixed-mode (Mode I-II) fracture energy release criteria, while the intact specimen propagates in pure Mode I. Overall, the findings reveal a transition from tensile opening to shear-assisted mixed-mode propagation as local stress states evolve due to the presence of NFs, providing a mechanistic basis for predicting and controlling fracture trajectories in subsurface stimulation and storage applications.

翻译：水力裂缝在天然裂缝性地层中的扩展受到天然裂缝附近局部应力状态的强烈影响。天然裂缝引起的剪切变形和应力重分布在控制水力裂缝轨迹方面的作用仍不明确。本研究通过耦合实验室实验和多孔弹性扩展有限元模拟，在平面应变条件下对完整和预裂聚甲基丙烯酸甲酯试件进行了研究，探讨了天然裂缝引起的应力重分布如何控制水力裂缝与天然裂缝的相互作用。数字图像相关技术提供了力学加载和水力压裂过程中位移和应变演化的全场测量。在固定基底、横向约束和垂直压缩边界条件下，倾斜天然裂缝诱导出非对称应力重分布和剪切变形，在流体注入前产生了不同的局部应力状态。结果表明，水力裂缝轨迹由天然裂缝方向相对于远场最大主应力产生的剪应力和剪应变分量的符号及空间分布决定。促进天然裂缝附近压应力发展的剪切变形导致水力裂缝偏转远离，而降低沿天然裂缝有效正应力的剪切变形则促进裂缝吸引和连接。预裂试件中水力裂缝曲率的相应数值再现需要混合模态（I-II型）断裂能释放准则，而完整试件则以纯I型扩展。总体而言，研究揭示了随着天然裂缝的存在导致局部应力状态演化，裂缝从张性开启向剪切辅助的混合模态扩展转变，为地下刺激和储存应用中预测和控制裂缝轨迹提供了机理基础。