页岩裂缝非平面扩展行为的控制机制研究

Commercial development of shale gas is an effective way to relieve the increasing shortage of natural gas in China. The formation of non-planar and asymmetrical fracture networks, which can increase the volume of fractured reservoir, is a key technology to achieve high efficient development of shale gas. The key scientific issue of this project is the complex mechanical behaviors and control mechanisms of fractures with different modes in non-planar propagation. The main research contents to be carried out are as follows: Firstly, the inclined angles of fracture are determined when pure tensile, pure shear and tensile-shear failure occur for Notched Semi-Circular Bend (NSCB) specimens, according to the mode-I and mode-II dimensionless stress intensity factors of fractures for different inclined angles. Then the anisotropic fracture toughness of bedding planes and shale matrix are measured for three different modes, namely, mode-I, mode-II and mixed mode (I–II), based on three-point bending method of NSCB specimens with different inclined angles of fracture. The bedding directivity effect of the non-planar propagation behaviors of fractures with different failure mechanisms is also revealed as per the difference of fracture propagation trajectories. Finally, the relationship between the micro fracture mechanism and the macro rupture is established, according to the surface morphology scanned by scanning electron microscope (SEM). The complex mechanical behaviors and the bedding directivity effect in the non-planar extension fractures with different modes are analyzed using comparison method. Furthermore, the control mechanisms of bedding and stress state on the non-planar propagation of fractures are revealed. The results can provide a theoretical support for the formation mechanism and control methods of complex fracture networks in shale fracturing.

页岩气的商业化开发是缓解我国天然气日益短缺的有效途径，形成能增大储层改造体积的非平面、非对称裂缝网络是实现页岩气高效开采的关键。本项目针对页岩不同模式裂缝非平面扩展中的复杂力学行为及其控制机制等关键问题，拟开展以下工作：通过半圆盘三点弯曲时I型及II型无量纲应力强度因子随裂缝倾角的变化规律，指出纯拉伸、纯剪切及张-剪复合破裂时的裂缝倾角范围；进而通过不同裂缝倾角及裂缝与层理成不同方位的半圆盘三点弯曲试验，探明页岩层理及基质体I型、II型及I-II复合型断裂韧性的各向异性及不同破裂机制裂缝非平面扩展行为的层理方向效应；最后根据局部断口微观形貌的SEM扫描电镜观察，建立微观破裂机制与宏观断裂间的关系，并对比分析不同模式裂缝非平面扩展中的复杂力学行为及其层理方向效应，进而揭示层理和应力状态在裂缝非平面扩展中的控制机制。研究成果可为页岩压裂复杂缝网的形成机制及调控方法等提供理论支撑。

本项目以页岩气储层压裂改造中如何形成复杂裂缝网络通道的关键难题为研究背景，以四川盆地页岩气示范区块的龙马溪页岩为研究对象，在断裂参数有限元分析的指导下，通过不同倾角裂缝半圆盘三点弯曲加载、高速摄像机观测、DIC数字图像相关和SEM扫描电镜等试验，指出了不同倾角裂缝发生纯拉伸、纯剪切及张-剪复合破裂的裂缝倾角范围，分析了不同模式裂缝的扩展演化行为及其层理方向效应，识别了不同模式裂缝的宏观扩展形态及其层理方向效应，获得了断裂韧性和裂缝扩展行为的各向异性特征；基于断口形貌的微观观察、断裂面的宏观形态、声发射事件的时空演化特征和DIC数字图像观测到的试样表面位移场特征，从微观和宏观角度探讨了不同模式裂缝在层理处穿过、转向等非平面扩展行为的层理方向效应，指出了层理和应力间的竞争扩展行为；基于主应力突变、Cook–Gordon效应及泊松效应和力学性质的不匹配性揭示了层理和受力条件对裂缝非平面扩展行为的控制机制，厘清了层理和受力条件在裂缝非平面扩展中的主控作用。研究结果进一步丰富和完善了页岩基础力学参数和断裂理论，对页岩气储层井壁稳定性控制、水力压裂设计及压裂改造中复杂网状裂缝的调控方法等具有一定的指导价值。