附加应力场和滤失作用下页岩水力压裂开裂机理

Shale Reservoir Fracture Mechanism Under Impacts of Additional Stress Field and Leak-off

为了研究含有天然微裂隙和层理的页岩储层在压裂液产生的滤失和附加应力场共同作用下的开裂机理,采用复变函数保角变换方法修正了传统附加应力场解析解,基于最大拉应力强度理论,通过解析分析比较储层基质、天然裂隙和层理的复杂应力状态及相应的强度,得出水力压裂主裂纹遇天然裂隙或层理后的开裂机理和扩展方向.基于莫尔强度理论分析讨论被裂纹穿过后的储层次裂纹形成机理.结果表明:水力压裂主裂纹尖端拉应力集中,发生张性破坏;被主裂纹穿过的天然裂隙或层理在附加应力场和压裂液产生的滤失作用下发生剪切破坏,形成滑动次裂纹;附加应力场增大裂纹附近的最小地应力、降低裂纹附近的最大地应力,不利于储层次裂纹的形成;降低排量、增加压裂作用时间,提高裂纹附近储层孔隙压力,降低有效应力,可促进储层次裂纹的形成.

In order to study the fracturing mechanism of shale reservoir with many natural cracks and beddings under the combined impacts of leak-off and additional stress field,additional stress field analytical model was adjusted by complex function conformal transformation.Based on the maximum tensile stress strength theory,the major crack fracturing mechanism and propagation direction when hydraulic fracturing meeting natural cracks and beddings was obtained by a comparison of complex stress state in reservoir matrix,natural crack and beddings with corresponding strength.The formation mechanism of secondary cracks in reservoir with penetrating cracks is discussed based on the Mohr strength theory.The results show as follows.The stress concentration appeared at major crack tip of hydraulic fracture,which makes major crack propagate due to tensile failure.The secondary cracks emerged because of natural cracks and beddings sheared sliding failure under the combined impacts of leak-off and additional stress field.The additional stress field made the in-situ minimum stress increase and the maximum decrease,which restrained reservoir shear failure.Reducing injection velocity and increasing the fracturing time can increase pore pressure around the major crack and effective stress,which promoted reservoir secondary cracks formation.