不稳定流体注入时裂缝内压力和尖端应力分析

Analysis of Pressure and Tip Stress in Hydraulic Fracture under Unstable Fluid Injection Condition

为了解不稳定流体注入时水力裂缝内压力和尖端应力变化规律,采用流体力学理论建立了不稳定流体注入条件下井筒内波动压力的计算模型,根据波动力学理论建立了水力压力振动波在裂缝内的传播方程,运用断裂力学理论分析了水力压力振动波在裂缝内传播时裂缝尖端的应力变化。分析结果表明:不稳定流体注入条件下,裂缝内的波动压力随时间呈明显的衰减现象;相同裂缝长度下,随波长的增加,裂缝内的波动压力衰减幅度加剧,压力能量损失加快;随着裂缝长度的增加,裂缝尖端的应力强度因子、峰值应力和总势能均相应地增加;采用不稳定流体的注入方式可以提高水力压裂的改造效果;水力压力振动波沿缝长方向传播时呈现出压力振幅衰减的规律;考虑水力压力振动波效应,裂缝一旦开启和扩展后,裂缝只需在较小的净压力作用下延伸更长的距离。研究结果可为变排量压裂工艺的提出和科学设计提供理论依据。

To understand the pressure and tip stress change in hydraulic fracture during unstable fluid injection,the calculation model of fluctuating pressure in wellbore under unstable fluid injection condition was established using fluid mechanics theory.The hydraulic pressure vibration wave propagation equation in the fracture was established according to wave dynamics theory.The theory of fracture mechanics was used to analyze the stress change at the fracture tip under the hydraulic pressure vibration wave propagation in the fracture.The analyses results showed that,under the unstable fluid injection,the fluctuating pressure in the fracture shows significant attenuation with time.Given the same fracture length,as the wavelength increases,the attenuation of the fluctuating pressure and the pressure energy loss both increase.The stress intensity factor,peak stress and total potential energy at the fracture tip increase with the fracture length.The unstable fluid injection method can improve the effect of hydraulic fracturing.The pressure amplitude attenuates as hydraulic pressure vibration wave propagates along the longitudinal direction of the fracture.Considering the hydraulic pressure vibration wave effect,once the fracture opens and extends,the fracture could propagate under a small net pressure.The study results could provide theoretical bases for the propose and scientific design of the variable displacement fracturing process.