A laminated and fissured formation around a horizontal borehole is modeled as a combination of intact rock blocks and permeable fractures with a DEM code. The model is used for understanding the mechanism of rock failure problems observed in a shale formation of an offshore field. Different mud weights and stress conditions are applied to the model to simulate the rock failure around deviated holes in laminated and fissured shale.

The numerical simulation shows that shear failure along the bedding planes is the predominant mode of collapse of the hole when mud under pressures penetrate into bedding planes on which shear stress is applied. The severity of wellbore instability varies with stress states and wall conditions. The effect of a plugging agent in mud is verified by comparison between permeable and impermeable wellbore models.

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