DEM modeling of the propagation of stress-induced borehole breakout in shale sample

Shales make up more than 75% of drilled oil and gas formations worldwide, and cause most wellbore-instability problems during and after drilling. Understanding borehole failure mechanism is important in optimizing well production. In this study, the initiation and propagation of borehole breakout under monotonically increased far-field stress at reduced-scale are investigated using discrete element method (DEM). The micro-structure of Mancos shale is captured by explicitly representing the presence of weakness with a series of parallel smooth joint contacts. Square samples (50 mmx50 mm) with a 10 mm borehole in the center are loaded until the formation of breakouts. Effects of different factors on the borehole breakout process are evaluated systematically, including the borehole diameter, rock anisotropy and far-field stress conditions. Reduction of borehole diameter significantly increases the hydrostatic pressure. Rock anisotropy plays an important role in the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress. Under anisotropic far-field stresses, V-shapes borehole breakouts align with the direction of minimum principal stress in isotropic model and close to the minimum principal stress orientation in anisotropic model. Results from this study agree well with those obtained in laboratory on hollow cylinder tests and can provide in-depth understanding on the grain-scale mechanisms that control the previous observations in laboratory and field on the borehole instability in anisotropic rocks. Copyright 2016 ARMA, American Rock Mechanics Association.