页岩气储层中节理岩体水力压裂机理及与原生断层的地质力学响应研究

At present, the shale gas/oil is mainly obtained by the hydraulic fracturing (fracking) technology with horizontal wells. The fracking in the exploration of shale gas/oil will be taken as a research background. In this proposal, laboratory tests (rock and analog material specimens), three-dimensional physical model tests, theory innovations and advanced numerical methods would be used to investigate the cracking initiation conditions and the mechanisms of cracking propagation and coalescence. Firstly, the mechanical and deformation characteristics of original fractured shale is performed by three-dimensional compressive tests under different fissure water pressures. Meanwhile, the rock-like materials with high brittleness and transparency (special resin and mortar) are also studied, and the fissure bodies can be precast in the specimens. Then we will obtain the phenomena and principles of cracking initiation, propagation, coalescence and even failure on the specimens. In order to study the geomechanical response to the original discontinuities (mainly on faults) in the process of hydraulic fracturing, a three-dimensional physical model test will be conducted. Finally, a fluid-solid coupling constitutive model of fractured rock mass and the failure criteria of hydraulic fracturing will be established on a basis of the fracture and damage mechanics, fluid-solid coupling theories and so on, which can realize the numerical simulations on the cracking initiation, extension propagation, coalescence and failure of the specimens and its geomechanical response to original faults nearby under external complex stress state and inner fissure water pressures. Furthermore, the related research can establish good foundation to improve the shale permeability and exploiting rate of shale gas/oil and could also be expanded to study the safety analysis and grouting technology in underground projects.

目前页岩气获取手段为水平井结合水力压裂技术，本课题以国内复杂的页岩气储层中裂隙水压致裂提高透气性及开采率为研究背景，主要通过实验室页岩及相似材料试验、三维物理模型试验、理论创新及数值模拟手段来研究裂隙页岩在内水压力作用下的起裂条件、裂隙扩展及相邻裂隙之间的贯通机理。通过原样页岩在不同孔隙水压作用下的三轴压缩试验得到其力学及变形特性；同时研制脆性度和透明度较好的低温树脂材料或砂浆材料，在试件中预制空的裂隙体，获得在裂隙水压作用下裂纹起裂、扩展及岩桥贯通的现象及规律；然后开展三维物理模型试验来探索水压致裂裂纹扩展与天然不连续体(主要为断层)之间的地质力学响应特征；最后通过断裂、损伤及流固耦合理论来建立裂隙岩体的流固耦合本构模型及水压致裂破坏准则，实现其在应力及裂隙水耦合作用下的裂纹扩展、贯通及与原生断层之间地质力学响应的数值模拟，进而为页岩气开采及地下工程稳定提供前期研究基础。

本课题以大型地下岩体工程为背景，通过实验室试验、理论创新及先进的数值模拟手段来研究裂隙岩体在无水和含内水压力作用下的起裂条件、裂隙扩展及相邻裂隙之间的贯通机理。首先设计研发了一套水-岩耦合作用的模具，该模具能够将注入到试件内部的水流密封在试件内部不流失；制作了含不同裂隙的水泥砂浆类岩石试件，开展了裂隙试件的单轴压缩试验及在不同侧压下的双轴压缩试验，发现两种条件下试件内裂纹扩展规律类似，双轴受压时裂纹的扩展速率要晚于单轴受压，而且试件易发生劈裂破坏；开展了含裂隙的水泥砂浆和透明树脂试件在水-岩耦合作用下的破坏机理研究，得到了试件的次生裂纹形态和裂隙扩展过程，分析了其产生的条件与机制，对比了有水与无水条件下试件破裂规律；此外，借助AE技术分析了试件破裂过程中的声发射撞击率与主频变化及分形特征，以及低、高频信号占比与试件破裂状态的对应关系，进而得到了水力耦合条件下试件整体破裂的预警信息。使用基于张拉破坏-剪切残余准则改进的岩体弹-脆性本构模型，在FLAC3D中进行了二次开发，并模拟了含裂隙试件在不同侧压情况下裂纹起裂及扩展过程，计算得到的起裂压力和峰值强度与物理试验结果大致相同，而且数值模拟产生翼型裂纹扩展过程及最后的破坏模式与物理试验完全吻合；采用PFC2D分析了裂隙试件在不同内水压作用下的裂纹演化规律，采用改进的管域模型研究了关键域内的水压力变化规律，并采用应力场和位移场分析相结合的手段深入分析了裂纹在内水压作用下的演化机理；此外，在PFC3D的基础上，采用研发的三维水-力耦合模型对裂纹的三维扩展进行了模拟，分析不同内水压作用下的三维裂纹的空间状态；最后引进矩张量理论模拟了裂隙岩体在加载过程中微破裂的AE事件，在改进的管域模型基础上，分析了AE事件的震级在不同内水压作用下的时空演化规律，并与室内实验中的AE试验数据进行了对比分析。相关成果能广泛应用于研究大型岩体工程稳定、地下工程注浆以及水力压裂等领域。