页岩储层裂缝形态特征及缝网压裂的损伤力学机制研究

The shale reservoir has obvious heterogeneity due to the developed natural fractures, which have important effects on reservoir stimulation volume during hydraulic fracturing. Consequently, it is very important to study the development and spatial distribution characteristics of natural fractures, and then investigate the evolution mechanism of fracturing network. The existing literature shows that, few reports have been published comprehensively involving the macroscopic and microscopic heterogeneity of shale reservoir, and by virtue of the damage mechanics to investigate the evolution machismos of fracturing network. In this project, a combination of core measurement, outcrop description, paleomagnetic test and subcritical crack growth method will be conducted to study the natural fractures in the marine and continental sedimentary shale reservoir, China; then the discrete fracture network model is developed to represent the natural fractures. Considering the microscopic characteristics of the gas shale, eventually, the numerical model is developed, which involves the macroscopic and microscopic heterogeneity of shale reservoir. Based on the above obtained results, a serious of studies are conducted using the numerical simulation approach based on finite element theory and statistical damage mechanics. The interaction between natural fractures and hydraulic fractures is analyzed, influence of the treatment type on hydraulic fracturing effectiveness is numerical investigated; sensitivity analysis for the factors affecting the fracturing network is done; and then optimize the hydraulic fracturing design for naturally fractured shale formations. The above results can provide theoretical basis and technical support for the exploration and development of shale gas.

页岩储层具有明显的裂缝非均质性，天然裂缝的存在对储层体积压裂改造具有重要的影响，所以探明页岩储层中天然裂缝的形态特征，并基于此研究其对压裂缝网激活演化的机制具有重要意义。查阅现有的文献表明，目前尚未见到同时对储层的宏微观非均质性进行研究，并采用损伤力学方法进行压裂缝网形成演化机制研究方面的报道。为此，本项目拟通过储层岩芯量测、野外露头描述、古地磁测试、页岩亚临界裂纹扩展机制相结合的方法，查明我国典型海相和陆相沉积页岩储层中天然裂缝发育特征及空间展布形态，开发表征裂缝宏观特性的离散裂隙网络模型；并考虑到储层岩石的微观特性，最终建立能够充分体现储层宏微观非均质特性的数值模型。在此基础上采用基于有限元理论和统计损伤力学的数值计算方法，分析水力裂缝沟通天然裂缝活化的力学机制，评价压裂方式对储层改造效果的影响，并对复杂缝网形成的多因素进行评价和优化，从而为页岩气勘探开发提供理论依据和技术支持。

项目聚焦页岩中的天然裂缝对储层压裂改造的影响这一关键科学问题，系统地研究了页岩储层中天然裂隙的发育特征，并对储层压裂缝网形成演化多尺度机制进行了研究。重要结果体现在以下几方面：.（1）针对典型的海相页岩，进行了不同层理面倾角的力学性质测试，包括单轴压缩变形试验、三轴压缩变形试验、巴西劈裂抗拉强度试验和声发射监测试验。抗拉强度测试考虑了加载速率对试样宏观抗拉强度及破坏形态的影响，较系统研究了层理面倾角对应力应变曲线的影响。同时，进行了实时微米CT扫描单轴压缩试验，样品考虑不同层理面的情况，深入分析了页岩破裂过程中微细观结构变化，对裂纹空间构型，矿物成分进行了提取。根据扫描电镜结果，对样品中矿物进行了微分区与识别，结合CT图像构建了数字岩心并进行了不同加载路径下的细观数值计算。.（2）针对储层非构造型裂隙发育这一地质特征，进行了非均质材料水力压裂的模型试验。压裂模型采用一定水泥和砂浆配比，通过高温压热、空气冷却预置随机裂缝来模拟储层中的非构造性裂缝，重点分析了水力裂纹扩展、破裂应力和裂缝沟通面积。提出采用重建主破裂面的三维形态，计算分形维数进行压裂效果评价的方法。同时，针对我国典型陆相页岩的纹层发育特征，制备了横观各向同性物理模型试样，进行了大尺度真三轴物理模型试验。.（3）探讨了水力裂缝沟通天然裂缝活化延伸形成缝网的机制，计算模型包括单一天然裂隙和多条天然裂隙情况，计算变量考虑天然裂缝与水力裂缝的逼近角、主应力差和地层的弹性参数；同时，编写了储层离散裂隙网络模型，考虑了海相页岩储层应力差、裂缝倾角、注入速率、裂缝内摩擦角等因素，采用数值试验研究了缝网形成演化机制。.（4）对比分析了海相页岩储层和陆相页岩储层中天然裂隙发育形态及岩石力学性质的差异，并对两种页岩储层的可压裂性进行了深入的对比分析，提出了用于表征可压裂性的物理力学指标，通过数值试验和大型物理模型试验验证了指标的可行性。