直接剪切下页岩的各向异性破坏特征和强度预测模型

Anisotropic Failure Characteristics and Strength Prediction Model of Shale Under Direct Shear

了解页岩剪切破坏特性对页岩气储层改造和井壁稳定性控制具有重要意义。然而,由于页岩本身的各向异性属性,页岩的剪切破坏行为复杂。本文对6种不同层理倾角(0°、30°、60°、90°、120°和150°)的立方体页岩进行了系统的直接剪切破坏测试,详细分析了在层理弱面影响下页岩的剪切应力–剪切位移、抗剪强度、剪切模量和破坏模式的各向异性特征。结果表明:页岩的剪切破坏表现为典型的非线性渐进破坏特征;页岩平均抗剪强度和剪切模量随层理倾角的增加均呈现“M”型变化趋势;在90°~180°的层理倾角区间,平均抗剪强度的极值差明显大于0°~90°区间,而剪切模量近似关于90°呈对称变化;当层理倾角从0°变化到180°时,页岩剪切各向异性行为呈现非对称演化规律。在层理倾角和直接剪切荷载作用下,页岩的破坏模式有3类:层理倾角为0°时,沿层理面剪切滑移破坏;层理倾角为30°、60°和90°时,表现为穿透层理和基质的剪切滑移破坏;其他倾角下呈现穿透层理和基质的剪切滑移与沿层理的拉伸劈裂相结合的复合破坏。基于试验结果和组构张量,构建了一个可考虑页岩本征各向异性和剪应力诱导各向异性影响的直接剪切破坏准则,其理论预测值与试验结果的平均偏差率低于3%,能够定量表征黏聚力和摩擦系数的各向异性程度,合理评估页岩直接剪切强度对外加法向应力的敏感性,可推广应用于理论表征所有层状岩土材料的直接剪切强度。

Understanding the shear failure characteristics of shale is of great significance to the drilling stability control of shale gas reservoirs and reservoir reconstruction designs.However,due to the anisotropic properties of shale itself,the shear failure behavior of shale is complex,and the isotropic shear strength model is no longer applicable.Therefore,a systematic direct shear failure test was carried out on six kinds of cubic shale with different bedding dip angles(0°,30°,60°,90°,120°and 150°).The anisotropic behavior of shear stress-shear displacement,shear strength,shear modulus,and failure mode of shale under the influence of a weak bedding plane was analyzed in detail.The results show that the shear fail-ure of shale is a typical nonlinear progressive failure.The average shear strength and shear modulus of shale show an‘M’trend with the increase of bedding dip angle.The extreme value difference of average shear strength in 90°~180°bedding angle range is significantly greater than that in 0°~90°,but the shear modulus changes approximately symmetrically.This indicates that when the bedding angle changes from 0°to 180°,the shear anisotropy behavior of shale shows asymmetric evolution.Under the action of bedding dip angle and direct shear load,the failure mode of shale is complex,and the shear failure along the bedding plane is 0°.When the bedding dip angle is 30°,60°and 90°,the shear slip failure through the bedding and the matrix;at other dip angles,the composite failure of penetrating bedding and matrix shear slip combined with tensile splitting along bedding is presented.Based on the experimental results and the fabric tensor,a new direct shear failure criterion considering the effects of intrinsic anisotropy and shear stress-induced anisotropy of shale is proposed.The average deviation between the theoretical prediction and the ex-perimental results is less than 3%.In addition,the new criterion can quantitatively characterize the degree of anisotropy of cohesion and friction coefficient,and reasonably evaluate the sensitivity of shale direct shear strength to applied normal stress,which can be applied to the theoretical characterization of direct shear strength of all layered geotechnical materials.