深部页岩拉伸与剪切各向异性破断机制及理论模型研究

Deep reservoir complex environment including high temperature and high pressure results in the non-linear mechanical properties and fracturing response of deep shale which is different from shallow rock. Since currently the mechanical inverstigation of deep shale in China lags badly behind the engineering practice, it is urgent to study the failure mechanism and theoretical model of deep shale under tensile and shear conditions considering thermal-mechanical coupling effect. To this end, by investigate the "double factors" effect of deep in-situ environment and layer orientation, this project adopts new specimen configurations to carry out direct tensile and direct shear tests of shale, and mainly explores the coupling effect mechanism of temperature, stress and bedding on the tensile and shear failure of shale. Furthermore, the time-space evolution path of anisotropic damage and fracture during the whole failure process of shale is tracked by DIC and AE monitoring, and the complex morphology of shale fracture is analyzed based on laser scanning and SEM observation, thus the physical mechanism of anisotropic failure of deep shale under thermal-mechanical coupling condition can be revealed in essence. Theoretically, an anisotropic strength model of deep shale will be proposed by combining stress tensor, fabric tensor and temperature parameter. At the same time, the real damage evolution process inside shale samples can be revealled by AE parameter analysis, so as to construct an anisotropic damage evolution model of shale which is available in deep thermal-mechanical coupling environment. The proposed new model is expected to provide guidance for fracture pressure prediction, fracturability evaluation and hydraulic fracturing design of deep reservoirs.

深部储层高温高压环境致使深部页岩呈现出不同于浅部岩石的非线性力学特性与压裂响应，当前我国深层页岩力学研究严重滞后于工程实践，迫切需要对温度-应力耦合下深部页岩拉伸与剪切破断机制及其理论模型展开重点研究。为此，本项目考虑深部原位环境和层理结构“双重因素”的影响，采用新的试件构型开展页岩直接拉伸和直接剪切试验，重点探索温度、应力和层理对页岩拉伸与剪切破断的耦合影响机制。利用DIC和AE监测实时追踪页岩破断全过程中各向异性损伤破裂的时空演化路径，结合激光和SEM扫描剖析页岩的复杂破断形貌，从本质上揭示热力耦合下深部页岩各向异性破断的物理机制。理论上联合应力张量、组构张量与温度参量提出深部页岩的各向异性强度模型，同时利用AE参数刻画页岩内部真实的损伤演化过程，以此构建适用于深部热力耦合环境的页岩各向异性损伤演化模型。新模型有望为深部储层破裂压力预测、可压性评价、水力压裂设计等提供指导。

深部页岩油气开采是我国能源转型发展的重大举措和必然趋势。然而，深部高温高应力复杂环境使得层状页岩呈现出非常规力学特性与压裂响应，当前对于温度-层理耦合作用下页岩的基本破断行为机制研究甚少。为此，本项目以丁山区块龙马溪页岩为研究对象，综合考虑高温环境和层理结构“双重效应”的影响，系统开展了不同层理倾角下页岩直接拉伸、圆环劈裂和直接剪切各向异性力学破坏实验，全面分析了页岩拉伸和剪切过程中受力变形特性、峰值强度、破坏模式的各向异性变化规律及其温度效应；结合声发射监测、DIC数字图像、断面激光扫描、XRD分析、波速测试等手段，揭示了温度效应下层状页岩渐进损伤破坏演化过程及其各向异性力学破断机制，探明了温度对页岩矿物组分、细观结构等物理性质的影响，解释了高温热处理引起页岩力学强度变化的内在机理；通过分析页岩宏观破坏模式，考虑基质和层理的联合贡献，构建了层状页岩各向异性破坏强度准则，并对比验证了其适用性。研究结果表明，高温环境和层理弱面对页岩拉伸与剪切破坏行为（破坏强度、各向异性度、破坏模式等）均具有重要影响。就层理效应而言，页岩圆环劈裂和直接剪切峰值荷载所呈现的层理效应较显著，破坏荷载均随层理倾角增大而呈非线性减小趋势，但高温下页岩破坏荷载差异性较小；就温度效应而言，600℃范围内同一层理页岩破坏强度随温度升高而增大，但其强度各向异性程度呈整体下降趋势，表明温度有助于减小层理效应引起的强度各向异性；高温环境将使页岩矿物组分、细观组构等发生变化，这些是导致页岩物理力学特性发生变化的本质原因。新建的数学连续性强度准则，参数简单且具有显式表达式，能够很好地预测高温页岩各向异性破坏强度变化趋势。研究成果进一步丰富和完善了我国海相页岩的基础力学参数和强度理论模型，对深部页岩油气开采、油页岩高温热激增透、储层高温注采及核废料处置等深地工程提供重要数据支撑与理论指导。