层状页岩水力压裂裂缝与岩体弱面的干扰机理研究

Mechanism of Interaction between Hydraulic Fractures and Weak Plane in Layered Shale

为了进一步认识水力压裂裂缝在层状页岩中的扩展行为,以四川盆地龙马溪组页岩为研究对象,根据Mohr-Coulomb准则,研究了水力压裂过程中层状页岩压裂裂缝与弱面的干扰机理,以及基质破坏、弱面剪切破坏、弱面张开破坏所对应的临界液压;根据断裂力学,研究了弱面在张开和剪切破坏下的剪切滑移量,建立了水力压裂裂缝与岩体弱面干扰理论模型,并对其进行了应用分析。实例计算分析表明:0°<α<20°时(α为弱面法向矢量与最大水平主应力方向的夹角),水力压裂裂缝中的液压容易促使岩石基质发生破坏;25°<α<75°时,水力压裂裂缝中的液压容易促使页岩沿弱面剪切破坏;80°<α<90°时,水力压裂裂缝的液压容易促使页岩沿弱面张开;剪切滑移量受弱面上摩擦系数、剪应力、正应力的共同影响,其中剪应力是剪切滑移的驱动因素。研究认为,当弱面发生张开或剪切破坏后,弱面滑移量与裂缝长度成正比,与弹性模量成反比;建立的理论模型与现场压裂微地震监测结果基本吻合,可用来预测水力裂缝与弱面的干扰行为。

Taking Longmaxi Formation of the Sichuan Basin as an example,the mechanism of interaction between hydraulic fractures and weak plane was investigated to further understand the hydraulic fracture propagation in layered shale.Such mechanism in fracturing was analyzed based on Mohr-Coulomb Criteria,and the critical pressure for rock matrix failure,weak plane shear and opening failure were also studied.According to fracture mechanics,the shear displacement under opening and shear failure of a weak plane were considered,and then an interaction theoretical model was developed and applied in shale reservoir.The calculation example showed that,the pressure in hydraulic fracture could stimulate the rock matrix-failure when 0°〈α〈20°,the weak plane shear-failure when 25°〈α〈75°,and the weak plane openingfailure when 80°〈α〈90°;the shear displacement was jointly affected by frictional coefficient,shear stress（as the main factor）and normal stress.After the shear or opening failure of weak plane occurs,its displacement is proportional to fracture length and inversely proportional to elastic modulus.It was suggested that the theoretical model could be used in predicting the interaction between hydraulic fracture and weak plane,which was very consistent with the micro-seismic crack monitoring.