低渗透性砂砾岩水力压裂裂纹三维扩展行为的研究

Hydraulic fracturing is one of the core technologies for coal gas extraction from low-permeability coal seams and oil/gas recovery from the low- permeability oil fields. The formation mechanism, distribution modes and influencing factors of hydraulic fractures or cracks in low-permeability rocks under geological stresses and hydraulic pressures are of critical significance to the design of hydraulic fracturing, reservoir stimulation, and enhancing gas and/or oil resource recovery in low-permeability reservoirs. This study focuses attentions on the challenge that it is intractable to accurately determine and forecast the hydraulic fracturing effects of low-permeability conglomerate rock reservoirs being deeply buried underground, heterogeneous, in a complex stress state, and difficult to detect. A few promising measures including fractal reconstruction, numerical simulation, true triaxial fracturing tests and on-site computer tomography are employed to investigate the fracture initiation, propagation and convergence in heterogeneous low-permeability conglomerate rock. The formation mechanism, propagation modes, distribution characteristics, and influencing factors of hydraulic fractures are discussed as well. A novel fractal description and the corresponding parameters of the three-dimensional crack propagation and distribution are proposed. A theoretical model relating the fractal descriptors to the rock properties, heterogeneous gravel structures and hydraulic pressures is established in this study. This study could provide a promising way to better understand and to quantify the formation mechanism and propagation modes of hydraulic fractures of low-permeability conglomerate rock reservoirs for accurately evaluating the hydraulic fracturing effects and efficiently implementing hydraulic fracturing in practice.

水力压裂是低渗透煤层瓦斯增透抽采与低渗透油气田开采的核心技术之一，地应力与水压力共同作用下岩层压裂裂纹的形成机理、展布模式和影响因素是低渗透储层水力压裂开采设计、储层改造和提高抽采率的基础与核心科学问题。本项目针对深埋低渗透砂砾岩储层埋深大、非均质、地应力复杂、探测难度高、难以准确地评判岩层水力压裂效果等难题，运用分形重构、数值模拟、真三轴水力压裂与实时CT扫描实验等方法，研究地应力和水压力作用下非均质砂砾岩压裂裂纹在三维空间中的起裂、扩展和汇聚，揭示压裂裂纹的形成机理、扩展模式、展布特征和影响因素；提出三维裂纹扩展与展布规律的分形表征方法与描述参数，建立压裂裂纹定量表征与岩石物性、非均质结构和水压力之间关系的数学力学模型；为认识和表征深埋低渗透非均质砂砾岩储层水力压裂裂纹的形成机理与扩展模式，准确评判深埋砂砾岩储层的水力压裂效果、有效地实施水力压裂技术提供新理论和有效方法。

针对低渗透油气储层改造中对压裂裂缝的起裂与扩展机制以及地质环境影响认识不足、缺乏定量分析与评价压裂效果方法等问题，开展了低渗透砂砾岩水力压裂裂纹三维扩展行为的研究，提出了低渗透储层岩石复杂结构的分形重构方法与分形重构模型；利用自主设计研发的真三轴水力压裂装置、CT成像系统以及改进的数值模拟方法，分析揭示了低渗透岩石压裂裂缝的形成与扩展机制及主控因素；提出了低渗透岩石可压裂性的数学模型及定量评价方法，开发了具有自主知识产权的储层岩石分形重构与可压裂性分析的计算软件。研究成果为认知和定量表征地应力环境与压裂改造方式对低渗透储层压裂缝网的形成与三维扩展机制及其主控因素、改进低渗透储层压裂的数值模拟方法、科学地实施低渗透储层压裂改造以及提高资源采收率提供了新的理论方法与有效途径，上述成果在工程应用中取得了良好的经济与社会效益。项目执行期间，出版专著1部，发表SCI论文21篇（其中Q1区11篇、Q2区7篇）、EI论文6篇；授权美国发明专利7项、国内发明专利2项、软件著作权1项；获教育部自然科学一等奖、中国煤炭工业协会科技进步二等奖各1项。项目负责人入选国家“万人计划”科技领军人才、国家百千万人才计划和有突出贡献中青年专家。项目骨干1人入选教育部新世纪优秀人才计划，1人获全国煤炭青年科技奖。研究团队入选中组部、科技部“万人计划”重点领域创新团队。培养出站博士后1名、毕业博士生7人、硕士生14人，选派了8名博士生赴美国和澳大利亚等国际著名大学联合培养。项目负责人及骨干成员担任国内外学术会议大会或分会主席2次，做大会特邀报告11次、专题报告5次。出国（境）开展学术交流5人次，接待外国专家来访7人次，组织申报获批“111引智计划”项目1项。本项目全面完成了预期目标。