基于零厚度黏聚力单元的水力压裂裂隙空间分布影响分析

作为页岩气开采关键技术，水力压裂在目标岩层产生大量裂隙，构成页岩气有效运输通道。因此，研究裂隙的空间分布和影响因素具有重要意义。采用连续-非连续方法，通过ins-coh程序在ABAQUS有限元模型全域内批量嵌入零厚度黏聚力单元，得到计算模型。考虑到水力压裂中的流固耦合，编写用户子程序确定孔隙压力初始场分布，并给出压裂阶段压裂液的流动准则和流固耦合方程。随着压裂液的注入，黏聚力单元发生破坏，连贯的张开区域形成页岩气运输通道。在验证该方法可行性基础上，结合西南某地区页岩气储层典型地质条件，进行三维水力压裂数值模拟。模拟结果显示:该方法能够合理模拟高水压下压裂液在岩体中流动，岩体裂隙面萌生、扩展、贯通的完整破坏过程。最后，通过设计多组模型进行对比分析，探究注液速度、初始裂隙间距和长度对裂隙空间分布的影响，为实际工程的方案设计提供指导意见。

Analysis of spatial distribution of cracks caused by hydraulic fracturing based on zero thickness cohesive elements

As the main mining method of shale gas extraction,hydraulic fracturing produces a large number cracks in the target rock,forming effective transportation channels for shale gas.Therefore,it is of importance to study the spatial distribution and factors of cracks.In this paper,zero thickness cohesive elements are inserted between adjacent solid elements by ins coh program based on continuous discontinuous method,obtaining computational model.Considering the fluid solid coupling in hydraulic fracturing,user subroutines are developed to define the initial field distribution of pore pressure.The flow criterion of fluid and fluid solid coupling equation are given in the fracturing stage.With the injection of fluid,cohesive elements break down,resulting in coherent space which constitutes transportation channels for shale gas.After verifying the correctness of the method above,a 3D numerical model of hydraulic fracturing is established,combining the geology of shale gas reservoir in some region of southwest China.The results show that this method can simulate the flow of fluid in rock mass under high pore pressure,and the complete failure process of rock mass properly.Finally,through the analysis and comparison of multi group models studying the effect of injection rate,initial spacing and length of cracks on the final distribution,some guidances on the design of projects are given.