层理性泥页岩井壁失稳机制的FLAC3D 数值分析

 在层理性泥页岩钻井过程中经常出现井壁失稳，影响钻井的顺利进行。考虑层理性泥页岩的岩块和层理特性，采用层理岩石单元和层理面单元的组合模型描述层理性地层的破坏模式。利用有限差分数值模拟软件FLAC3D建立层理性泥页岩井眼的有限差分计算模型，模拟了实际钻井过程。和均质地层对比，分析了层理性泥页岩井眼的塑性区分布、井眼位移和次生应力分布特点。数值模拟再现了层理面的变形破坏现象，描述了层理面的滑移特征。研究结果表明，数值模拟结果和实际钻井现象相吻合，层理面上下两侧位移差最大为0.8mm，层理面滑移位移是错动位移的8.13倍，证明了层理面的滑移是层理性泥页岩井壁失稳的根本原因。分别模拟了地应力、层理面空间分布、井眼轨迹参数和井眼尺寸对倾斜层理直井和水平层理斜井的变形破坏规律。

Numerical Analysis of Borehole Instability Mechanism in Layered Shale Formation Based on FLAC3D

The layered shale formation is an extremely troublesome formation of borehole instability, with a significant influence on drilling. This paper studies the effect of joint weak planes in the rock mass and proposes a combined model, which combines both the bedding plane model and the intact rock model to describe the characteristics of the layered formation, especially the characteristics of the bedding plane. The finite difference software FLAC3D is used to establish the numerical model of the layered shale formation and to simulate the drilling process. To compare with the isotropic shale formation, the distributions of the failed area, the borehole displacement and the secondary stress in the layered shale formation are obtained. The failure process of the bedding plane and characteristics of the deformation are well simulated. The difference of the displacements on the two sides of the bedding plane is up to 0.8 mm and the slip displacement is 8.13 times the shift displacement. The results agree with the drilling practice. The slip behavior of the bedding plane is shown to be the central cause of the borehole instability in the layered shale formation. The effect of the geo-stress, the occurrence of the bedding plane, the parameters of the well track and the borehole size on the well deformation in inclined and horizontal layered formations are also simulated.