超声波促进页岩储层裂缝发育机理

Development Mechanism of Ultrasound-Promoted Fractures in Shale Reservoirs

通过研究超声波对页岩裂缝扩展作用的影响,结合断裂力学理论,分析裂缝失稳、达到张开或伸展效果的临界状态。基于页岩裂缝两端所受力和超声波作用界面产生的能量之间的关系,得出若要使裂缝脱离力的束缚,达到促进裂缝发育的效果,只需超声波对裂缝所施加的最大作用力大于使页岩裂缝达到失稳状态的最小外加应力即可,进而计算出能促使裂缝失稳、断裂、发育的最小振幅。研究认为,超声波作用下,页岩内部裂纹尖端应力场应力强度因子增加,促进裂缝进一步发生扩展,并初步得到了页岩内部裂缝扩展的临界应力;超声波会在裂缝面发生反射,在裂缝面形成拉应力,拉应力越大,裂缝发育程度越大。在理想状态下,已知超声波频率和波速,当超声波振幅大于A1时就可以实现裂缝断裂,实现超声波促进裂缝发育的目的。

Combining with fracture mechanics theory,this paper analyzes the critical state of fracture unstability,opening or stretching bystudying the effect of ultrasound on crack extension in shale.Based on the relations between the forces on both ends of cracks in shale andthe energy of interface generated from ultrasonic action,it is considered that if the maximum force of ultrasound on cracks is greater thanthe minimum applied stress which could make cracks reach to an unstable state in shale,cracks will break away from the forces and crack-ing can be promoted as well,and the minimum amplitude to promote the unstability,cracking and development of fractures can be furthercalculated.The research shows that the increase of stress intensity factor in the stress field of crack tip could promote crack propagationand the preliminary critical stress is obtained.Ultrasound will reflect on the fracture surface and a tensile stress forms.The larger the ten-sile stress is,the better the fractures are developed.Under ideal conditions,with known ultrasound frequency and velocity,fractures willcrack when the amplitude is greater than A1.