基于直剪试验的页岩强度各向异性研究

 层理面的存在是页岩地层力学性质、强度特征和破裂模式表现出明显各向异性特征的根本原因，也是引起水平井井壁易失稳的重要原因之一。为分析层理面的力学性质及其影响下页岩的抗剪强度各向异性特征，开展不同角度页岩的直接剪切试验，并根据剪切破坏机制的各向异性和剪应力集中系数，从不同角度分析抗剪强度各向异性的原因，试验和理论分析结果表明：(1) 层理面是页岩地层的薄弱面，其黏聚力和内摩擦角明显小于页岩基质体，抗剪强度也最低，其剪应力–剪切位移曲线并没有表现出岩石剪切强度随滑动而弱化的特点，而是其残余摩擦力甚至还略大于抗剪强度。(2) 0°，30°，60°和90°四个方向中，页岩抗剪强度的最大值在60°时取得，且0°，30°和60°试样的剪应力–剪切位移曲线均表现出剪切强度随滑动而弱化的现象。(3) 页岩剪切破坏机制可分为沿页岩本体的剪切破坏、沿层理面张拉和本体剪切的复合破坏、以及沿层理面的剪切滑移3种模式；页岩抗剪强度的各向异性是由其剪切破坏机制的各向异性控制的。(4) 剪应力集中系数在一定程度上反映了岩石直接剪切时剪切承载力的强弱，可用来分析页岩抗剪强度的各向异性特征；不同方向页岩直接剪切时，剪应力集中系数仅与沿剪切方向的弹性模量和剪切层的厚度有关；相同法向应力下，90°试样的剪应力集中系数最大，抗剪强度最小，而60°试样的剪应力集中系数最小，抗剪强度最大。该试验和理论分析结果可为深入分析岩质边坡中滑动面的运动特征和页岩气水平井井壁稳定性等提供一定参考。

ANISOTROPY OF SHEAR STRENGTH OF SHALE BASED ON DIRECT SHEAR TEST

The existence of bedding planes is the fundamental cause of the anisotropy of the mechanical behaviors，the strength and the failure modes of shale formations and is also one of the main reasons for borehole instability of horizontal wells. In order to analyze the mechanical properties of bedding planes and the anisotropy of shear strength，direct shear tests on shale sample with different bedding angles were carried out. The causes for the anisotropy of shear strength were analyzed based on the anisotropy of the mechanisms of shear failure and the concentration factor of shear stresses. (1) The cohesion and the internal friction angle of bedding planes were found to be significantly smaller than those of the matrix of shale；the bedding planes are weak ones in shale formations and have the smallest shear strength. The shear stress-shear displacement curves do not exhibit the characteristic of slip-weakening when shear failure occurs along bedding planes and the residual friction is even slightly higher than the shear strength of the bedding planes. (2) The largest shear strength occurred if the angle ? between the bedding planes and the normal stress is 60°among the four angles of ? = 0°，30°，60°and 90°. The shear stress-shear displacement curves exhibit the characteristic of slip-weakening when the shear failure occurs for the shale samples with ? = 0°，30°and 60°. (3) The failure mechanism of shale is divided into three types: the shear failure across matrix，the sliding across matrix together with the tensile splitting of the weak planes, and the sliding along the bedding planes. The anisotropy of shear strength of shale is governed by the anisotropy of shear failure mechanism. (4) To some extent，the shear stress concentration factor can reflect the shear strength of rock, and it can be used to analyze the anisotropy of shear strength of shale. In the direct shear tests of shale，the shear stress concentration factor is related to the elastic modulus in shear direction and the thickness of shear layers. Under the same normal stress，the shear stress concentration factor is the largest with ? = 90° and the smallest with ? = 60°. But the shear strength of shale is the smallest with ? = 90° and the largest with ? = 60°.