基于细—宏观力学的页岩储层脆性破坏机理研究

The evaluation of shale brittleness is an important basis for the analysis of the mechanical properties of reservoir, perforating reconstruction selection and fracturing scale design. However, the existing brittle failure theory and evaluation method can not reveal the mechanism of shale in the development process of shale fracture network, also they can not reflect the essence of the brittle failure of shale. The study establishes the characterization methods of rock mechanic characteristics for shale in the meso and macro aspects, which is based on the theories and experiments of the previous evaluation of fracturing reconstruction, and also based on the relationship between mesoscopic mineral mechanics and macroscopic rock mechanics of shale. The elastic-brittle damage constitutive model is established with multi-scale homogenization method, which is based on the influence of mesoscopic damage element for macroscopic damage. Fracturing model for horizontal well of shale reservoir is established under the Multi-physics Fields with finite element method, which is based on fracturing model for horizontal well. Therefore, we can explore the brittle failure process in fracturing, reveal the mechanical mechanism of the controlled fracture morphology and the complex fracture network generated, so that we can get the brittle failure theories and evaluation methods based on the mesoscopic and macroscopic mechanics of the shale reservoir. The result can not only solve the selection of fractured interval and the uncertainty problems of evaluation on effectiveness after fracturing, but also enriches and perfects the nonlinear characteristics of Engineering Mechanics and key problems on theoretical prediction in the high-effective development of basic theory of shale reservoir, it will have important significance for scientific guidance in fracture reconstruction of shale reservoir.

页岩脆性评价是分析储层力学性质、遴选射孔改造层段和压裂规模设计的重要基础，但现有的脆性破坏理论和评价方法既不能揭示页岩在缝网发育过程中的作用机理，也无法反映页岩脆性破坏的实质。本研究将在以往压裂改造评价研究的理论与实验基础上，基于页岩细观矿物力学与宏观岩石力学关系，建立页岩细-宏观跨尺度岩石力学特征表征方法；基于细观单元破损对宏观破损的影响，运用多尺度均匀化方法构建页岩弹脆性损伤本构模型；基于水平井压裂模型，运用有限元方法建立多物理场条件下的页岩储层水平井压裂模型，探究压裂过程中页岩脆性破坏过程，揭示控制页岩储层压裂裂缝形态及形成复杂缝网的力学机制，形成基于细-宏观力学的页岩储层脆性破坏理论和评价方法。研究结果既可解决页岩储层压裂选层和压后效果评价的不确定性问题，又可完善并丰富页岩储层高效开发基础理论中页岩储层非线性工程力学特征与预测理论的关键问题，对科学指导页岩储层的压裂改造具有重要意义。

页岩脆性评价是分析储层力学性质、遴选射孔改造层段和压裂规模设计的基础，但现有的脆性破坏理论既不能揭示页岩在缝网发育过程中的作用机理，也无法反映页岩脆性破坏的实质。本项目采用实验测试、理论分析与模型构建相结合的方法，从含气性评价方法、细-宏观跨尺度脆性破坏机理、细观损伤本构模型和裂缝起裂与扩展规律4个方面进行了系统研究。结果表明：1.页岩“局部化破裂造缝”本质上是脆性矿物内部以及不同矿物间的弱接触面上高应力集中致使应变发生异常并超过极限值引起的；2.建立的考虑多因素耦合的页岩气-液-固三相界面吸附模型的计算误差为6.1%，与传统干燥吸附模型相比，计算结果更接近储层实际含气量；3.评价页岩脆性的力学参数按准确性依次为峰值应变、破裂时间、残余应变、峰值应力、差应力、残余应力、破裂面条数、围压、泊松比、弹性模量；4.岩石强度尺度效应受端部摩擦和非均质性共同制约。对于小尺度岩样，端部摩擦是决定强度尺度效应的主要因素；对于大尺度岩样，非均质性对强度尺度效应起决定性作用；5.细观损伤显著影响页岩的破坏模式，随着损伤数目从0增加到20，破裂压力从18.04MPa减小到15.36MPa，岩石更易发生脆性破坏；6.峰值压力与裂缝深度呈正相关，与裂缝条数呈负相关，随着裂缝角度增大呈现凹形变化的规律，45°裂缝的峰值应力最小；7.建立的考虑细—宏观缺陷的页岩损伤本构模型的应力-应变曲线与三轴压缩实验曲线具有很好的一致性，且裂缝深度、角度、条数对峰值应力的影响规律也与实验结果相同；8.单簇裂缝宽度和长度与射孔方位角和压裂液黏度分别呈正相关和负相关；水平应力差越大，越易形成“长、直、窄”状裂缝，反之则形成“短、宽”状裂缝，增大注入排量会形成“长、宽”状裂缝；9.两簇裂缝形态呈“哑铃”状；裂缝长度和转向位置与缝间距和水平应力差均呈正相关，且注入排量越大裂缝长度越长；三簇、四簇裂缝扩展时，诱导应力使得中缝长度受到抑制，而两侧裂缝相互排斥扩展延伸。