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Simulation of drilling-induced compaction bands using discrete element method
Rock failure is observed around boreholes often with certain types of failure zones formed which
are called breakouts. Drilling experiments in some high porosity quartz-rich sandstone have
shown that breakouts are formed by developing a narrow localized compacted zone 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. Therefore, investigation of the mechanism of
fracture-like breakout has received considerable attention in the past ten years in the oil industry
due to its potential impact on the performance of oil wells.
This paper presents the results of numerical simulations of borehole breakouts using three
dimensional discrete element method (DEM) to investigate the mechanism of the fracture-like
breakouts and to identify the role of far-field stresses on 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 to simulate the drilling experiments.
The results show that failure occurs in the zones of the highest concentration of tangential stress
around the borehole in the direction parallel to the minimum horizontal stress. It is concluded
that fracture-like breakout will develop as a result of non-dilatant micro-mechanism consisting of
localized grain debonding and grain crushing which leads 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 far-field stresses.
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- Article (Draft / Submitted)