页岩气藏水力压裂中应力-流压耦合效应及人工裂缝扩展规律

为探究页岩储层水力压裂作业过程中地层应力和流体压力之间的耦合作用及人工裂缝扩展规律,根据有效应力原理和格里菲斯-库伦破裂准则,建立颗粒流应力-流压耦合数值模型,开展基于离散元方法的孔隙超压和水力压裂数值模拟研究。结果表明:孔隙流体压力与水平应力场之间存在耦合作用,孔隙超压能够降低固相介质的有效水平差异应力,增加岩石的抗破坏能力,存在孔隙超压的岩石的刚度和剪切破坏峰值强度均有提高;水平应力场与水力压裂流体之间也存在耦合作用,初始水平主应力的大小及方向直接影响压裂流体的扩展方式,水力裂缝会以细小裂纹的形式向最大周向应力方向延伸,最小周向应力方向的裂缝延展被抑制;应力-流压耦合作用下,流体生成的裂缝能够改变岩石介质所受最小水平主应力的大小及分布,最小水平主应力在裂缝尖端处弱化,在与裂缝垂直的方向增强。
In order to study the coupling effect of rock formation stress-fluid pressure during a hydraulic fracturing process of shale gas reservoirs, according to principles of effective stress and Griffith-Coulomb fracture criterion, a numerical model of particle flow stress in coupling with fluid pressure was built for the numerical simulation of hydraulic fracturing via a discrete element method. The simulation results show that, due to the coupling between pore fluid pressure and the initial horizontal formation stress, the over pressure in the pores can reduce the effective differential stress and increase the peak shear strength of the rock formation. Due to the coupling effect between hydraulic fracturing pressure and the initial horizontal formation stress, the initial size and direction of horizontal principal stress can directly affect the extension of the fracturing fluid, and the hydraulic fracture can be extended as small cracks in the direction of the maximum circumferential stress, while in the minimum circumferential stress direction, the cracks extension can be inhibited. Under the effect of the rock stress and flow pressure coupling, the fractures created by the fluid pressure can change the minimum horizontal stress of the rocks. The minimum horizontal stress on the top of the fractures top will be reduced, but on the direction perpendicular to the fractures, it can be strengthened