Theoretical development on morphology of wellbore toroidal breakout
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Copyright 2016 ARMA, American Rock Mechanics Association. Few researchers have conducted experimental investigations on the occurrence of wellbore toroidal breakout; also this breakout mode has been observed in the field. Unfortunately, there is no published theoretical study that adequately describes the occurrence of this unique feature. Thus, we have developed a theory that answers some of the questions related to the occurrence of wellbore toroidal breakout: (a), is the failure mode toroidal? (b). is the deepest depth for a homogeneous and isotropic formation at the center? (c). location of the deepest depth for a heterogeneous formation? To consider the impacts of support pressure on the breakout pattern, the two failure modes were considered, namely: shear fracture and extensile-splitting induced breakouts. Minimum strain energy criterion was used in determining the direction of the propagating extensile cracks; and the trajectories of the cracks were tracked with Fourier series of piece-wise linear functions. In the numerical experiments conducted, it was observed that the extensile cracks propagated in the r-z plane through mode II (shear) predominantly, and by mode I in the r - plane. By rotating the cracks profile in the r-z plane through the vertical axis by the breakout width, the breakout volume can be approximately determined; the volume generated by this rotation is usually greater than the actual volume.
