页岩力学特性的层理效应及脆性预测

Influences of bedding planes on mechanical properties and prediction method of brittleness index in shale

为系统研究层理面对页岩力学性质的影响机理,以四川盆地焦石坝地区龙马溪组页岩为例,首先使用偏光显微镜及扫描电镜等对页岩岩样进行微观分析,可见页岩的层理、层间微裂缝及微孔隙等较发育,黏土矿物呈定向排列,层间胶结作用较弱;再通过岩石单轴压缩物理实验和RFPA(真实破裂过程分析)数值模拟,系统研究不同的层理角度、层理密度及层理面力学性能等对页岩单轴抗压强度、弹性模量、泊松比及脆性等参数的影响;最后,采用统计学方法提出一种基于层理密度、层理角度、单轴抗压强度及层理面力学性能等参数的页岩脆性预测新方法。结果表明:页岩的弹性模量随着层理角度的增加呈现先减小再增加之后再次减小再次增加的趋势。页岩的单轴抗压强度和脆性指数的特征曲线均近似为U型,随着层理角度的增加而先减小后增大,且在层理角度为45°左右时,两者均达到最小值,在层理角度为0°和90°附近时,两者的值均相对较大。随着层理角度的增加,页岩的泊松比整体上呈现先降低再增加而后再降低的趋势,并在30°左右出现最小值,在70°左右出现极大值。页岩的弹性模量、单轴抗压强度和脆性指数等参数总体随层理密度的增大而降低;泊松比随着层理密度的增大在不同的层理角度表现出不同的状态。不同层理角度岩石的破裂类型不同。强度参数、弹性参数、脆性及破裂模式等均表现出很强的各向异性特征。整体而言,针对页岩力学特性的层理效应研究及脆性预测,可为页岩气的合理开发及页岩气井的井壁稳定提供一定的理论参考。

In order to study the mechanism of the effects of bedding planes on the mechanical properties of shale,we carried out the following related experimental researches. After observing the shale samples taken from Long-maxi Formation in Jiaoshiba area in Sichuan Basin by polarizing microscope and scanning electron microscopy,we got that the shales are rich in bedding planes,micro-fractures and micro-pores. The clay minerals are arranged in a directional arrangement,and the cementations of the layers are weak. RFPA numerical simulation basic parameters were acquired by the results of uniaxial physical compression experiments at different bedding plane angles. These are the basis on which the influence of bedding plane angles,the densities of bedding panes and the mechanical properties of bedding planes on uniaxial compressive strength,elastic modulus,Poisson’s ratio and brittleness can continue to be studied by using digital simulation method. A new prediction method for shale brittleness index was presented by statistical methods when we only know the densities of bedding planes,the angles of bedding planes,uniaxial compressive strength and the mechanical properties of bedding planes. Numerical simulation experiments results indicated that the characteristics of uniaxial compressive strength and brittleness index at different angles of bedding plane roughly showed the U-shaped changing trend. The maximum value occurred at the bedding plane angles of 0° and90°,whereas the minimum value occurred at the bedding plane angle of 45°. The elastic modulus of shale decreased with the increase of bedding angles,then increased and then decreased,finally increased. With the increase of bedding plane angles,the value of Poisson’s ratio firstly decreased,then increased and finally decreased. And Poisson’s ratio got the minimum value at bedding plane angle of 30°,whereas the maximum value at bedding plane angle of 70°.Elastic modulus,uniaxial compressive strength,and brittleness index overall decreased with the increase of the density of bedding planes. Poisson’s ratio showed different states at the different bedding plane angles with the increase of the density of bedding planes. The types of rock fracture of uniaxial physical compression experiments are different at different bedding plane angles. The strength parameters,elastic parameters,brittleness and fracture modes all showed very strong anisotropy. The study about the influence of bedding planes on the mechanical properties and the prediction of brittleness in shale can provide a necessary technical foundation for the stability of shale gas wells and rational development of shale gas.