裂缝充填物对脉动水力压裂应力扰动效果的数值模拟

利用线性滑移模型、有限差分及交错网格技术研究脉动水力压裂过程中应力在含裂缝地层中的传播与分布规律,分析不同缝间距和震源频率下裂缝充填物对脉动水力压裂应力扰动效果的影响机制。结果表明:含矿物质裂缝对脉动水力压裂应力扰动影响微弱,可近似看作均质区域;含液体裂缝可通过反射作用引起强透射区与弱透射区介质速度的差异,引发剪应力形成拉伸破坏,但当距离震源较近时,反射波易抵消压力波,削弱压裂效果;含气体裂缝类似含液体裂缝,但反射作用更强烈,距离震源更远才能提升压裂效果;脉动水力压裂可利用裂缝的非均质性促进压裂效果,缝间距为初始人造裂缝长度的2~3倍时可获得较好的压裂效果。

Numerical simulation of influence of crack fillers on stress disturbance effect during pulsating hydro-fracturing

A linear slip deformation model, a finite-difference method and staggered-grid were used to investigate the stress propagation and distribution in fractured formation during the pulsating hydro-fracturing (PHF) process, and the influence of crack fillers was analyzed at different crack intervals and source frequencies. The numerical simulation results show that the influence of mineral-filled cracks on PHF stress disturbance is weak, so that its fractured zone can be considered as homogeneous. When the crack is filled with fluid, it can cause the reflection of pressure waves, which will form a great velocity contrast between strong and weak reflection zones to generate shear stress, then causing tensile damages. However, if the crack is too close to the pulsating source, the reflected and propagated pressure waves will disturb each other and reduce the fracturing effect. The influence of gas-filled cracks is similar to that of the fluid-filled, but its reflection of pressure waves is much stronger, which means a longer crack interval can be applied to promote the stress disturbance effect. The PHF technique can be used in coal seams with fracture anisotropy to increase their permeability. When the interval between initial artificial fractures and natural fractures is 2-3 times larger than the length of initial artificial fractures, a better PHF stress disturbance effect can be acquired.