横观各向同性油气藏水力压裂裂纹扩展规律研究

针对横观各向同性与各向同性油气藏水力压裂裂纹扩展的差异性,基于扩展有限元法建立水力压裂力学模型,通过ABAQUS子程序开发了各向同性和横观各向同性岩体的起裂判据。在各向同性岩体数值模拟结果与解析解以及现场压裂典型曲线对比吻合的基础上,得到了包含层理构造的横观各向同性岩体水力压裂过程中裂纹扩展规律。层理类岩体水力压裂的裂缝扩展方向由地应力状态、层理方向以及岩体与层理界面抗拉强度共同决定;水力压裂过程中,注水压力在裂纹尖端产生应力集中,层理面法向分量先达到界面抗拉强度时,裂纹沿层理方向开裂,反之裂纹沿垂直最小地应力的方向扩展;裂纹扩展速度随层理抗拉强度的增加而降低;由于地层的滤失,随压裂液的注入,裂纹长宽尺度增长速率降低。

Study on the law of transverse isotropic reservoir crack extension during hydraulic fracture

With the difference between transverse isotropic and isotropic reservoir during hydraulic fracture,mechanical model was established based on the extended finite element method.Crack criterions were developed by subroutine in ABAQUS.The law of transverse isotropic reservoir crack propagation was studied on the base of which numerical simulation results coincided well with analytical solution results and the field measured curve with isotropic material.It had drawn some conclusions.The crack propagation direction is codetermined by in-situ stress,bedding direction,rock and interface tensile strength.Injection pressure produces stress concentration at the crack tip during hydraulic fracturing.Crack extends along the bedding when the stress component normal to interface reaches the interface tensile strength first whereas crack extend vertical to minimum in-situ stress.Crack propagation rate decreased with increasing of bedding tensile strength.With fracturing fluid injection,the crack length and width growth decrease due to the leak-off of formation.