水平井定向射孔裂缝起裂与穿层特征数值分析

Numerical analysis on hydraulic fracture initiation and penetration characteristics in directionally perforated horizontal wells

为了探索水力裂缝的起裂形式和受控因素影响以及水力裂缝扩展至产层隔层界面处的穿层特征,运用基于有限元的数值模拟方法建立数值模型对其进行分析和研究,重点探讨了压裂双裂缝形成的可能性和条件,模拟了裂缝延伸至产层与隔层界面处的穿层现象。结果表明,地应力差和射孔方位角对压裂裂缝的起裂形式影响显著,地应力差越大,则裂缝从射孔方向起裂的可能性越小,方位角越大,从射孔方向起裂的可能性也越小,此时若水泥环失效,裂缝可从井壁大主应力方向起裂;若满足岩层物理力学与压裂条件,裂缝同时从射孔方向和井壁大主应力方向起裂的双裂缝起裂形式是可能的;在裂缝延伸过程中,产层与隔层弱胶结界面可使裂缝钝化或滑移成"T"型裂缝,弹性模量较小的隔层有利于裂缝穿层并继续延伸。数值分析结果可对水平井射孔参数优化及水力压裂施工提供些许参考。

In order to reveal the initiation mode and controlling factors of hydraulic fracture as well as its propagation characteristics near the interface between production layer and barrier,a FEM-based code is employed and several numerical models are built to emphatically discuss the possibility and necessary conditions of the formation of double fractures.The penetration phenomenon of hydraulic fracture near the interface between production layer and barrier is also numerically simulated. Numerical results show that the differential stress and perforation azimuth play an important role in the initiation mode of hydraulic fractures. Either a higher differential stress or a higher azimuth can decrease the possibility of fracture initiation and propagation along the perforation direction. If the cement sheath is failed,a fracture near the well may initiate along the orientation of maximum principal stress. If both mechanical and hydraulic conditions are satisfied,a double fracturing mode,i. e. two fractures initiating synchronously along the orientation of perforation and maximum principal stress,could be obtained. During propagating,a weak interface between production layer and barrier may terminate the fracture or induce the formation of T-shape fracture. A barrier with lower elastic modulus may help a fracture penetrate through the interface and continue to propagate. The numerical results in this study are expected to provide some references for the perforating parameters optimization and fracturing design of horizontal wells.