The shale strata of Longmaxi Formation in the central part of Sichuan Basin are highly hard and strongly brittle. During the drilling process, the borehole wall collapses seriously and the pipe is stuck frequently. In order to solve the problem of borehole collapse during drilling, the anisotropy characteristics of deep shale, such as microstructure, physical and chemical properties of water, and mechanical properties, were tested, which was represented by Well Zi201 in Sichuan Basin. Theoretical model of horizontal well borehole stability in deep brittle shale was established considering the factors such as wellbore trajectory, bedding fracture occurrence, wellbore-stratum coupling seepage effect, and the mechanical weak plane effect of bedding fracture. The experimental results show that the content of brittle minerals in deep shale is as high as 70%, the hydration expansibility is extremely low, and the mechanical strength of shale matrix is high. While the strength among bedding fractures is low, so the shale is easy to slip and fall along the bedding fracture. The calculation results of the model show that the influence of mechanical weak plane effect of bedding fracture is obvious. When the angle between the wellbore trajectory and the normal direction of the bedding fracture surface meet a certain degree, collapse and shear slip of the wellbore will tend to occur. The wellbore-stratum effect cannot be ignored. The pressure penetration effect will reduce the effective radial support force of the drilling fluid, which may cause the initiation, extension and even collapse of bedding fracture. So reasonable drilling fluid density and effective plugging property can improve horizontal wellbore stability in deep shale. The research results reveal the collapse mechanism of deep brittle shale wellbore, which can provide a theoretical basis for the design of key engineering parameters of deep brittle shale horizontal wells.
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