A program to model the initial shape and extent of borehole breakout

Abstract

A FORTRAN program has been developed to model the initial shape and extent of borehole breakout, based on linear elasticity and various rock-failure criteria. From the examples presented for the study of the potential breakout orientation, it is shown that the previously held perception that borehole breakout orientation is aligned always with the azimuth of δh (the least horizontal principal stress) may not be true, because borehole breakout orientations have been shown to depend on the tectonic stress regime or the relative magnitudes of the in-situ far-field horizontal principal stresses with respect to vertical stresses. In addition, it is demonstrated that compressive failure criteria for rocks influence both the breakout azimuth and the extent of the initial failure zone, and, in general, the Mohr-Coulomb criterion tends to predict larger breakouts than are predicted by other criteria taking into account the effect of the intermediate principal stress. This program can be applied to study the relationship between breakout azimuth, breakout width, the in-situ stress field, and wellbore stability in relation to rock strength and the effects of mud weight and pore pressure in the rock formation.