页岩各向异性特征的试验研究

Experimental research on anisotropic properties of shale

为研究彭水页岩气区块储层的各向异性特征,开展了石柱县龙马溪组页岩的单轴和三轴压缩试验,分析了其力学特性、强度特征和破裂模式的各向异性,并揭示了其破坏机制的各向异性。结果表明:(1)龙马溪组页岩具有明显的各向异性特征,弹性模量在平行层理方向最大,垂直层理方向最小,且围压的增加使其增加速率不断减小;0°、30°和60°、90°页岩的泊松比随围压的增加呈现出了相反的变化规律,这可能与页岩层理间孔隙和微裂缝的良好发育有关。(2)相同围压下,0°试样强度最高,90°次之,30°最低,总体上呈现出两边高、中间低的U型变化规律,而不同角度的Hoek-Brown强度准则能较好地反映其强度的各向异性特征。(3)页岩破裂模式的各向异性是由破坏机制的各向异性引起的,而强度的各向异性是由破坏机制的各向异性控制的。单轴压缩时,0°页岩为沿层理的张拉劈裂破坏,30°为沿层理的剪切滑移破坏,60°为贯穿层理和沿层理的复合剪切破坏,90°为贯穿层理的张拉破坏。三轴压缩时,0°为贯穿层理的共轭剪切破坏,30°为沿层理的剪切滑移破坏,60°和90°为贯穿层理的剪切破坏;页岩地层的层状沉积结构和层理间的弱胶结作用是破坏机制各向异性的根源。研究结果为水平井井壁的稳定性分析和水力压裂施工设计等提供了技术参考。

This paper studies the anisotropic properties of shale formations in the shale gas blocks of Pengshui in Chongqing city.Uniaxial and triaxial compression tests on specimens of Longmaxi formation in Shizhu county were carried out. The anisotropy of mechanical properties, strength and failure modes were analyzed. The anisotropic failure mechanisms were revealed. The results show that：（1） Significant anisotropy is observed on the specimens. The elastic modulus parallel to bedding planes is the maximum,and the modulus perpendicular to bedding planes is the minimum. The increasing rate of elastic modulus decreases gradually as the confining pressure increases, while the variation trend of Poisson＇s ratio with bedding orientations of β=0°, 30°, 60° and 90° is opposite. The well-developed pore and microcrack in the bedding planes maybe contribute to these results.（2） The compressive strength of the specimen for β=0° is the maximum, the value for β=90° is a little less, and the minimum is in the orientation of β=30°at the same confining pressure. As the bedding orientation changes, the compressive strength curves present U-shape. The anisotropic compressive strength variations can be better described with the Hoek-Brown failure criterion of different orientations.（3） The anisotropic failure modes are mainly resulted from the anisotropy of failure mechanisms. The strength anisotropy is governed by the anisotropy of failure mechanisms. Under the condition of uniaxial compression, tensile splitting along bedding planes occurs for β=0°,sliding failure along bedding planes occurs for β=30°, composite shear failure across and along bedding planes occurs for β=60°,tensile splitting across bedding planes occurs for β=90°. But under the condition of triaxial compression, conjugate shear failure across bedding planes occurs for β=0°, sliding failure along bedding planes occurs for β=30°, shear failure across bedding planes occurs for β=60° and 90°. The layered sedimentary structure and weak cementing strength of bedding planes are the causes of the anisotropy of failure mechanisms. The results can provide references for the wellbore stability analysis of horizontal wells and hydraulic fracturing design in the shale gas exploitation.