水力压裂裂缝三维扩展ABAQUS数值模拟研究

油井岩石的水压致裂过程是多孔介质下的流固耦合过程。建立了水力压裂流体渗流连续性方程与岩石变形应力平衡方程，引入了二次正应力裂纹起裂及临界能量释放率裂缝延伸准则，考虑流体在裂缝面横向、纵向流动，采用有限元计算软件ABAQUS中的Soil模块模拟岩石水力压裂的三维复合裂缝起裂与扩展。通过其黏结单元设定裂缝延伸方向，编写用户子程序并嵌入ABAQUS主程序中，以确定初始地应力场、渗流场、随深度变化的孔隙度及随时间变化的滤失系数。从数值模拟结果可以得到水力压裂泵注不同时刻裂缝几何形态、缝内压力分布、岩石变形及其应力分布、孔隙压力分布、压裂液滤失量以及压裂液流体特性、排量、上下隔层应力差、滤失系数等参数对裂缝几何尺寸的影响。

Research of ABAQUS numerical simulation of 3D fracture propagation in hydraulic fracturing process

The hydraulic fracturing treatment in oil well is a process of solid-fluid coupling in porous medium. In this paper, continuity equation of hydraulic fracturing fluid seepage and stress equilibrium equation of rock deformation are established, the criterion for fracture initiation of quadratic nominal stress and fracture propagation of critical energy release rate is introduced, horizontal flow and vertical flow of fluid along fracture is considered, and the Soil module in finite element tool ABAQUS is applied to stimulate 3D composite fracture initiation and propagation in hydraulic fracturing. The cohesive element is used to pre-define the fracture path, and the corresponding user subroutines are also developed and embedded into ABAQUS to determine the initial in-situ stress field ,seepage field, depth-dependent porosity and time-related loss coefficient. The result of numerical simulation can show fracture geometry, fluid pressure distribution along fracture, rock deformation and stress distribution, pore pressure distribution and fluid loss at different time points of pumping in hydraulic fracturing. Apart from that, the influence of fluid properties, volume, interlayer stress difference and loss coefficient on fracture geometry is analyzed.