水力压裂裂缝尖端应力强度因子研究
Research on Stress Intensity Factor at the Tip of Hydraulic Fracture

基于断裂力学理论，通过分析裂缝尖端应力场的裂缝扩展临界条件来确定裂缝可能发生破坏的方向和储层所能承受的极限荷载条件，从而针对不同工况提出促进裂缝扩展的设计方法。当裂缝尖端的应力强度因子大于材料的断裂韧性时，储层中的裂缝被认为是可以起裂并且延伸的。对于单裂缝来说，随着初始裂缝角度不断增大，裂缝尖端的K值不断减小，当水力裂缝呈水平状态时，裂尖位置的应力强度因子要大于倾斜状态下裂缝尖端的应力强度因子。当裂缝尖端在应力强度因子的作用下，会造成裂缝改变其原有扩展方向。随着初始裂缝角度的不断增大，裂缝偏转角度会不断增大。当应力强度因子较大时，裂缝偏转角的角度变化较大，当应力强度因子较小时，裂缝偏转角的角度变化较小。对于垂直双裂缝来说，当两条裂缝处于同轴状态下时，裂缝两个尖端受裂缝相互作用的影响较小。此时，当应力强度因子呈指数级上升时，会造成裂缝尖端发生转向。当两条裂缝处于异轴状态下时，crack1受裂缝相互作用的影响小于crack2。相近的裂缝尖端因缝尖相互作用，导致其应力强度因子的变化幅度大于较远的裂缝尖端。但此时，crack1的偏转角度变化大于crack2。
Based on the theory of fracture mechanics, the direction in which fractures may fail and the ultimate load conditions that the reservoir can withstand are determined by analyzing the critical conditions for crack propagation at the crack tip stress field, thereby proposing design methods to prevent or promote crack propagation for different working conditions. When the stress intensity factor at the crack tip is greater than the fracture toughness of the material, the fractures in the reservoir are considered to be capable of initiating and extending. For a single fracture, as the initial fracture angle continuously increases, the K value at the crack tip continuously decreases. When the hydraulic fracture is in a horizontal state, the stress intensity factor at the crack tip is greater than that in the inclined state. When the crack tip is subjected to the stress intensity factor, it will cause the crack to change its original propagation direction. As the initial fracture angle continuously increases, the crack deflection angle will also continuously increase. When the stress intensity factor is large, the angle change of the crack deflection angle is large, and when the stress intensity factor is small, the angle change of the crack deflection angle is small. For vertical double fractures, when the two fractures are in the coaxial state, the influence of the interaction between the fractures on the two crack tips is small. At this time, when the stress intensity factor increases exponentially, it will cause the crack tip to turn. When the two fractures are in the eccentric state, the influence of the interaction between the fractures on crack1 is less than that on crack2. The stress intensity factor of the nearby crack tips changes more significantly due to the interaction between the crack tips than that of the distant crack tips. However, at this time, the angle change of the deflection of crack1 is greater than that of crack2.