一种用于水力裂缝扩展数值计算的材料断裂能反演方法与应用

A Method and Application of Material Fracture Energy Inversion for Numerical Calculation of Hydraulic Fracture Propagation

为研究预置裂纹对水力裂缝扩展行为的影响,利用基于扩展有限元法的虚拟裂纹闭合技术计算能量释放率,借鉴PFC参数标定的思想,提出一种依据水压与裂尖能量释放率的关系反演材料断裂能的方法.将此方法计算出的断裂能作为材料参数,得到的起裂水压模拟值,与相关文献及水力压裂试验值对比,误差均不超过7%,证明了方法的可靠性.在此基础上研究水压注入预置裂纹时水力裂缝的力学行为,采用最大能量释放率准则,分析了围压应力和预置裂纹初始倾角对水力裂纹起裂与扩展的影响.结果表明,对于类岩石内斜裂纹,在水压作用下发生拉剪破坏,水力裂纹朝向裂纹前端拉应力等值线距离裂纹尖端最近的方向进行扩展,该方向径向切应力为0;相比于地应力,预置裂纹倾角对水力裂纹起裂水压力的影响较大,因此应主要通过改变注射角或者选择45°~60°的天然弱面进行压裂,从而降低工程成本.

To study the effect of the initial crack on hydraulic fracture propagation behavior,the energy release rate is calculated using the virtual crack closure technique based on the extended finite element method.Based on the idea of PFC parameter calibration,a method for inverting the fracture energy of materials based on the relationship between water pressure and energy release rate of the crack tip is proposed.The fracture energy calculated by this method is taken as the material parameter.The obtained hydraulic pressure simulation value is compared with the experimental data and related literature,errors not exceeding 7%,so the reliability of the method is proved in this way.Based on this simulation,the mechanical behavior of hydraulic fracture is studied when water pressure is injected into the pre-existing crack.Using the maximum energy release rate criterion,the effects of in situ stress and the pre-existing crack on hydraulic fracturing propagation behavior were analyzed.The results show that for rock-like internal oblique cracks,tensile-shear failure occurs under the action of water pressure.The hydraulic fracture propagates in one direction in which the tensile stress contour in front of the crack is closest to the crack tip,and the radial shear stress in this direction is 0.Compared with the in situ stress,the pre-existing crack inclination has a greater influence on the crack initiation water pressure.Therefore,fracturing should be performed mainly by changing the injection angle or selecting the natural weak surface of 45°~60°to reduce the engineering cost.