界面非对称剪切滑移诱导缝网裂缝扩展机理研究

The key problems restricting the exploitation of unconventional shale is lack of clear cognition on network fracture propagation mechanism and can not scientifically design the fracture technology. Through the complete of national natural science funds"The extension features research of full three dimension hydraulic fracture in sand/shale interface", we find weak interface non-symmetric shear slip is an important factor affecting the fracture propagation, but the conventional mathematical model of hydraulic fracture is established based on mode I fracture, which can not be used to study of network fracture of unconventional shale. Through collect unconventional shale cores and make experimental specimens, This project study the abnormal extension mechanism of the mode I fracture when the fracture tip decouple or blunt under the action of weak interfacial shear slip, using digital speckle correlation technology. The mode I fracture abnormal extension mathematical model is established and calculating method is formed by using the method of combining the damage mechanics and fracture mechanics. The weak interface or natural fracture is represented by the second order damage tensor form,so a network fracture model for shale seepage - damage multi-physics coupling which under the continuous - discontinuous framework is established. From the new viewpoint of interface layer asymmetric shear slip to reveal the formation mechanism of network fracture，form the network fracture numerical simulation method of unconventional shale, discuss the mechanism of mode I fracture abnormal extension on formation of network fracture, put forward the conditions for the formation of network fracture. Its will lay the foundation for the effective exploitation of unconventional shale.

制约非常规页岩开发的关键问题是对形成的水力缝网裂缝扩展机理认识不清，不能科学的设计压裂工艺。通过完成的国家基金"全三维水力裂缝在砂/泥岩界面扩展特性研究"发现弱界面层非对称剪切滑移是影响裂缝扩展转向的重要因素，而常规水力裂缝数学模型是基于Ⅰ型裂缝建立的，无法用于非常规页岩缝网裂缝的研究中。本项目通过采集非常规页岩岩芯并制备实验试件，采用数字散斑相关技术，研究弱界面非对称剪切滑移作用下Ⅰ型裂缝尖端解耦钝化的变态扩展机制，建立Ⅰ型裂缝变态扩展数学模型及解算方法，采用损伤力学与断裂力学相结合的方法，应用二阶损伤张量形式表征弱界面层或天然裂缝，建立连续-非连续框架下页岩基体的渗流-损伤多物理场耦合缝网裂缝模型。从界面层非对称剪切滑移这一新视点揭示缝网裂缝形成机制，形成非常规页岩缝网裂缝的模拟方法，探讨Ⅰ型裂缝变态扩展对缝网裂缝形成的机制，提出缝网裂缝的形成条件，为非常规页岩的有效开发奠定基础。

该课题针对非常规页岩储层开发过程中遇到的缝网扩展机理认识不清这一问题展开的研究，基于页岩储层层理界面发育的特点，从界面非对称剪切滑移对水力裂缝的作用机理，揭示缝网裂缝的扩展规律。本课题采用实验研究与理论分析相结合的方法，采用数字散斑相关方法，获取裂缝扩展过程中试件表面位移场和应变场的变化，发现I型裂缝扩展至界面后，I型裂缝尖端钝化，裂缝由I型裂缝转变为I-II型复合裂缝，导致界面剪切滑移诱导裂缝拐折扩展，因此界面剪切滑移是引起裂缝拐折扩展的主控因素，侧向载荷越大，界面角度越大，界面粘结强度越弱，界面剪切应变就越大，裂缝在界面处拐折角度越大，裂缝越容易沟通界面扩展；在以上基础上建立了裂缝的起裂准则及诱导界面剪切破坏的判断准则，基于储层流固耦合数学模型，形成缝网裂缝起裂与扩展机理的数学模型；借助数值模拟分析技术，采用二阶损伤张量描述地层中不规则分布的界面层，建立基岩—水力裂缝—界面耦合的二维缝网裂缝模型，形成页岩储层缝网裂缝数值模拟方法，并通过现场井验证数值模拟方法的合理性；数值模拟结果得到：水力裂缝与弱层理交汇后，在水力裂缝尖端钝化与远场地应力的共同作用下，界面发生剪切破坏，形成分支裂缝。综合各因素对剪切破坏的贡献及对裂缝扩展速度的影响，其水平主应力差越小、注入流量越大、储层岩石弹性模量越大及合适的压裂液粘度下，可有效激活弱层理形成缝网裂缝，获得更大的压裂改造面积，该项目的可为非常规储层压裂施工设计提供技术支撑，以提高压裂改造效果。