Rock failure is observed around boreholes often with certain types of failure zones, which are called breakouts. Laboratory-scale drilling tests in some high-porosity quartz-rich sandstone have shown breakouts in the form of narrow localized compacted zones in the minimum horizontal stress direction. They are called fracture-like breakouts. Such compaction bands may affect hydrocarbon extraction by forming barriers that inhibit fluid flow and may also be a source of sand production.
This paper presents the results of numerical simulations of borehole breakouts using 3D discrete element method to investigate the mechanism of the fracture-like breakouts and to identify the role of far-field stresses on the breakout dimensions. The numerical tool was first verified against analytical solutions. It was then utilized to investigate the failure mechanism and breakout geometry for drilled cubic rock samples of Castlegate sandstone subjected to different pre-existing far-field stresses.
Results show that failure occurs in the zones of the highest concentration of tangential stress around the borehole. It is concluded that fracture-like breakout develops as a result of a nondilatant failure mechanism consisting of localized grain debonding and repacking and grain crushing that lead to the formation of a compaction band in the minimum horizontal stress direction. In addition, it is found that the length of fracture-like breakouts depends on both the mean stress and stress anisotropy. However, the width of the breakout is not significantly changed by the far-field stresses. Copyright © 2013 John Wiley & Sons, Ltd.