Prediction Method of Shear Instability Region around the Borehole for Horizontal Wells in Bedding Shale

In order to understand the effects of strength anisotropy of bedding shale on shear instability region around the borehole of a horizontal well,the strength anisotropy characteristics for Longmaxi shale in the southern of Sichuan were researched based on direct shear tests,and the test result comparison was made with typical bedding shale data of some other countries,so as to establish the coordinate transformation model around the borehole in the bedding shale.In combination with stress solution around a borehole,Mohr-Coulomb weak plane strength theory,Mohr-Coulomb criterion,the prediction method of shear instability region around the borehole of a horizontal well in bedding shale was established,and the influence of borehole azimuth on shear instability region was also analyzed.It showed that strength parameters of weak plane and rock mass are basically in a linear relationship as the cohesion and internal friction angle of rock mass are about 2.10 and 1.20 times of those of weak planes,respectively.Strength anisotropy was more evident in Longmaxi shale.The shear instability region around the borehole was not in the commonly seen "dog ear" shape due to the weak planes.Instead,it had a square shape,which agreed with typical physical experimental results.If bedding planes in shale reservoir are significantly developed,the formation around borehole may experience shear sliding along the weak planes,which would increase the risk of instability in horizontal wells.The prediction method in this paper could be used to calculate the shear instability regions from weak planes shearing and rock mass shearing providing the basis in borehole stability analysis of bedding formation and the in-situ stress orientation inversion from imaging logging.