低渗透煤岩水力压裂复杂缝网的形成机理及可压裂性评价方法

Aim at the issue of shale fracability evaluation on unconventional oil and gas resources exploitation in China, the project attempts to research the growth law of hydraulic fracturing cracks and fracability evaluation method on shale using mathematics modeling, experiment and Hydraulic fracturing is one of the core technologies for coal gas extraction from low-permeability coal seams. The formation mechanisms of the complex fracture network induced by hydraulic fracturing and the fracability of coal are of critical significance to the design of hydraulic fracturing, reservoir stimulation, and enhancing gas resource recovery in coal gas reservoirs. This study focuses attention on the challenge that it is intractable to accurately determine the formation mechanism of fracture network and establish the effective evaluation method of fracability in coal gas reservoir. Based on self-developed true triaxial hydraulic fracturing equipment and CT scanning technique, the hydraulic fracturing tests and CT scanning tests of coal are carried out to reveal the effect of geological condition, coal properties and fracturing technology on spatial geometry form of fracture network on coal. The formation conditions and main influence factors of complex fracture network induced by hydraulic fracturing is exposed. The statistics principle and fractal theory is employed to analyze the distribution characteristics of spatial form of fracture network. The mathematic description method and characterizing parameters of fracturing network morphology on coal are established. Based on the forming mechanisms of fracture network of coal, an innovative fractal evaluation method and corresponding mathematic model on fracability are proposed and established by use of fuzzy mathematics and fractal theory. The research can provide theoretical foundation and guidance for optimizing hydrofracture design proposal and effectively implement hydraulic fracturing technology in coal gas reservoir.

水力压裂是低渗透煤层瓦斯增透抽采的核心技术之一，煤岩水力压裂复杂缝网的形成机理及可压裂性评价方法是煤层气开采设计、储层改造和提高采收率的基础和核心科学问题。本项目针对煤层气储层水力压裂复杂缝网形成机理认识不清、缺乏有效的煤岩可压裂性评价方法等问题，利用真三轴水力压裂实验装置和CT扫描设备开展煤岩水力压裂裂缝扩展实验和CT扫描实验，分析地质条件、煤岩物性和压裂工艺等因素对煤岩水力压裂裂缝扩展模型、展布特征的影响规律，探明煤岩水力压裂复杂缝网的形成条件和主要影响因素，揭示煤岩水力压裂复杂缝网的形成机理；利用统计学原理和分形理论等方法研究压裂缝网空间形态分布特征，建立煤岩压裂裂缝网络空间形态的数学描述方法和表征参数；结合水力压裂复杂缝网的形成机理，利用模糊数学和分形等理论提出煤岩储层可压裂性的分形评价方法和数学模型。项目研究为优化煤层气水力压裂设计方案和有效实施水力压裂技术提供理论依据和指导。

针对储层岩石水力压裂缝网形成机理及可压裂性评价方法的难题。本项目提出了一种塑性岩石人工物理岩心的制备方法，研制的人工物理岩心和天然岩石相比，在全岩矿物成份和脆性、塑性等力学性质方面具有很好的一致性；开展了塑性储层岩石水力压裂裂缝扩展实验，构建了水力压裂裂缝空间展布形态的三维可视化模型，揭示了岩石塑性和地应力差对储层岩石水力压裂裂缝扩展的影响规律。建立了基于三维扩展有限元法(XFEM)的水力裂缝扩展模型，提出了裂缝扩展角、扩展速率、分形维数、张开度和曲率来表征裂缝空间形态，分析了塑性和原位水平差应力(HDS)对裂缝扩展行为和表征参数的影响；建立了不同储层/夹厚度岩石水力压裂有限元模型，阐述了不同储层厚度岩石裂缝贯穿储层/夹层界面的扩展规律；重构了非均质和均质岩石的三维数值模型，分析了不同射孔条件下裂缝起裂时井筒周边的应力分布特征和裂缝起裂位置，提出了利用长度、宽度和分形维数来表征裂缝特征，研究了射孔参数、地应力差等因素对非均质和均质岩石水力压裂裂缝的起裂压力、延展形态以及裂缝几何参数特征的影响；构建了多井筒压裂现场尺度的地质模型，研究了布井密度、布井方式、地应力比、地层物理力学性质对现场水力压裂裂缝的起裂压力、延展压力及裂纹展布规律的影响，提出了利用长度、宽度来表征裂缝特征，分析了不同条件下裂缝起裂压力、井筒周边的裂缝分布特征、裂缝最终扩展形态；提出了一种基于组合赋权法的可压裂性多因素耦合综合评价方法，可压裂性多因素耦合综合评价模型对工程现场具有更科学的指导意义，为压裂设计和预测提供了新的参考。. 项目执行期间，发表研究论文10篇，其中：SCI收录7篇，EI收录2篇；授权发明专利5件（其中：美国专利1件、中国发明专利4件）；获软件著作权1项；培养了毕业硕士生7人，在读博士生2人；项目负责人及骨干成员受邀参加学术会议并做分会场专题报告1次，出国（境）开展学术交流1人次。