页岩直剪破断的各向异性特征及破坏模型研究

Anisotropic shear fracture is an important failure pattern in both areas of hydraulic fracking and wellbore stability in shale reservoirs. However, the physical failure mechanism of such a significant fracture mode of shale remains unclear and, a suitable theoretical model, which is able to effectively describe the anisotropic shear fracture, is missing. Therefore, more efforts should be devoted to addressing the related issues. Aiming at this, direct shear tests, accompanying the real-time micro-fracture monitor via acoustic emission and in situ X-ray microtomography techniques, will be performed to approach the anisotropic shear failure behaviors of shale. According to the experimental research, the understanding of the anisotropic micro-crack initiation and propagation mechanism (sliding along the bedding plane, failure of matrix or both of them) and anisotropic damage evolution behavior under direct shear loadings will be significantly improved. Concerning the anisotropic shear failure model of shale, it will be established by considering the whole loading process including the strain softening stage. Firstly, an idempotent and continuous tensor will be constructed through a detail research on the micro-structures of shale. Combining the damage mechanics and nonlinear fracture mechanics, the directional damage gradient function will be then investigated. Finally, a shear damage anisotropic evolution function will be proposed to ultimately determine the theoretical model which can properly describing the anisotropic shear fracture of shale. With the efforts of this project, the anisotropic shear fracture evolution behavior of shale in direct shear tests will be figured out, and moreover, the anisotropic shear failure model will be presented. It is believed that the modeling of hydraulic fracking, fracturing design optimization and disaster prevention in drilling can significantly benefit from this research result.

页岩各向异性剪切破坏是储层裂缝扩展及井壁失稳破坏的重要模式。然而，对页岩剪切破断机理的认知和理论模型的建立都严重落后于工程实践，迫切需要对此展开科研攻关。为此，本项目利用AE和加载式CT扫描两种先进的实时探测技术，以页岩剪切裂缝各向异性起裂和扩展机制研究为基础，重点探索页岩剪切破断全过程中内部不连续位移场及各向异性损伤时序演化行为，从本质上揭示页岩层理与基质耦合变形破坏的物理机制。页岩各向异性剪切破坏模型方面，本项目突破以各向异性强度模型研究为主的思路，转变为以页岩层理构造特征的微结构张量为切入点，并联合损伤力学和非线性断裂力学等学科理论，重点探索页岩各向异性剪切破坏全过程的势函数，以此构建符合页岩结构特征的各向异性剪切损伤软化方程，进而获得符合页岩结构特征的各向异性剪切破坏模型。新模型有望用于页岩储层裂缝扩展建模和压裂优化、钻井工程灾害防治、动力失稳控制对策研究。

页岩各向异性剪切破坏是储层压裂改造和井壁失稳破坏的重要模式。然而，对页岩剪切破断机理的认知和理论模型的建立都严重落后于工程实践，迫切需要对此展开科研攻关。为此，项目针对“页岩各向异性剪切破断机理不清楚”和“页岩各向异性剪切破断模型不清楚”的现状，协同采用AE和DIC两种先进的实时探测技术以及CT扫描技术，研究了页岩各向异性剪切裂缝扩展行为，明确了页岩中裂缝穿透层理扩展和转向层理面扩展的竞争机制；以页岩剪切裂缝各向异性起裂和扩展机制研究为基础，重点探索了页岩剪切破断全过程中内部不连续破裂及各向异性损伤时序演化行为，从本质上揭示页岩层理与基质耦合变形破坏的物理机制。页岩各向异性剪切破坏模型方面，本项目为以页岩层理构造特征的组构张量为切入点，定义页岩各向异性状态参数，并联合损伤力学和非线性断裂力学等学科理论，分析了页岩裂缝尖端的应力场，建立了页岩裂缝扩展沿层理偏转的竞争性扩展准则，提出了符合页岩层理构造特征的各向异性剪切破坏强度准则。项目研究成果应用于丁山地区页岩储层压裂优化，并有望延伸用于其他区块页岩储层裂缝扩展建模和压裂优化、钻井工程灾害防治、动力失稳控制对策研究。