页岩力学参数测试及脆性各向异性研究

天然页岩层理结构发育,不同方向力学特征差异显著,并且不同方向断裂前出现的弹性变形及破坏特征也有明显不同,使得页岩的脆性表现出各向异性特征。文中通过声波各向异性试验标定了天然岩心的水平主应力方向,开展不同取样方向页岩岩样的力学参数测试,对比不同层理角度以及不同主应力方向页岩力学参数的差异,分析了不同方向页岩的脆性破坏特征;推导了各向异性脆性指数B_(1θ)的理论公式,通过分析对比理论计算的B_(1θ)与试验测试的脆性指数B_1,证实天然页岩脆性特征具有显著各向异性;以强度参数计算了不同方向脆性指数B_2,与岩样破坏特征进行对比分析,表明以往研究应用拉压比直接表征页岩脆性具有一定的局限性。文中计算页岩不同方向脆性指数的方法,为改进现有页岩脆性评价方法提供了新的思路。

Measurements of mechanical parameters and brittleness anisotropy of shale

The bedding structure of natural shale is developed,and its mechanics characteristics in different directions are different significantly.The elastic deformation and failure characteristic in different directions before fracture are also obviously different,which make the shale brittleness show anisotropic property.In this paper,the horizontal principal stress directions of natural cores were identified by means of acoustic anisotropy test and then mechanical parameter tests of different sampling directions of shale rock were carried out.By comparing the differences of the mechanics characteristics of different bedding angle and different principal stress directions,the brittle failure characteristics of shale samples of different directions were analyzed.The theoretical formula of anisotropic brittleness index B 1θ was derived,and the brittleness of natural shale with significant anisotropy was confirmed by the comparison between brittleness indexes B 1θ and B1.The brittleness index B2 of different directions was calculated by strength parameters.It suggests that the shale brittleness represented by tension-compression ratio directly in previous studies has some limitations.In this paper,a method for calculating the shale brittleness index in different directions was obtained,which will provide new ideas for the improvement of the existing shale brittleness evaluation methods.