Breakouts

Part 6, Part 6: The Wellbore, Kirsch and the Window

Learning objectives

Explain breakouts as compressive failure of the wall where the hoop stress exceeds the rock strength
Locate breakouts at the minimum-horizontal-stress azimuth, and use them as an SHmax compass
Compute the breakout width from the hoop-stress and strength equations
Find the mud weight that just suppresses breakouts, the collapse-free pressure

The Wall Crushes Where the Hoop Is Highest

The Kirsch field of the last section makes the wall hoop stress highest at the minimum-stress azimuth. When that peak hoop stress exceeds the rock's compressive strength, the wall fails there, spalling out a pair of breakouts: enlargements of the hole, diametrically opposite, at the $S_{hmin}$ hmin azimuth. They are the single most common wellbore-failure feature and, because they always point along $S_{hmin}$ hmin, the most useful: a breakout is a compass needle for the stress field, reading the $S_{Hmax}$ Hmax direction as the perpendicular to itself. What a breakout is not is a runaway failure; it spalls back only until the enlarged, flatter wall carries the hoop stress below strength, then stabilizes. So the observable is the width of the breakout, the arc over which the wall failed, not a depth that keeps growing.

Lower the mud weight in the figure and watch the dog-ear arcs appear at the $S_{hmin}$ hmin azimuth and widen. The width follows directly from setting the wall hoop stress equal to the strength: the breakout spans the arc where $\sigma_{\theta\theta}\ge \mathrm{UCS}$ thetathetagemathrmUCS. For the canon well at mud $P_w = 36$ w=36 MPa, with $S_{Hmax} = 62$ Hmax=62, $S_{hmin} = 46$ hmin=46, $P_p = 35.3$ p=35.3, and UCS 65, the breakout is 27.8 degrees wide on each side. Raise the mud weight and the arcs shrink; at a high enough pressure they vanish entirely. That pressure, the breakout-free mud weight, is $P_w = 3S_{Hmax} - S_{hmin} - P_p - \mathrm{UCS} = 39.7$ Shmin−Pp−mathrmUCS=39.7 MPa: pump heavier than that and the wall no longer crushes.

Reading Breakouts Backward

Breakouts are prized because the relationship runs both ways. Forward, given the stresses and strength, you predict the breakout width, the drilling-stability calculation. Backward, given an observed breakout width and a known UCS, you constrain $S_{Hmax}$ Hmax, exactly the upper bound the polygon used in Part 5.5. And their orientation, picked from a caliper or image log, gives the $S_{Hmax}$ Hmax azimuth directly, the local refinement of the regional prior from the World Stress Map. A wider breakout at fixed mud and strength means a higher $S_{Hmax}$ Hmax; a rotated breakout means a rotated stress field, near a fault perhaps. This is why breakout analysis is the backbone of practical stress determination, and why the canon well's 27.8-degree breakout is one of the anchor numbers of the course. The next section turns to the opposite failure, the wall going into tension when the mud is too heavy.

References

Bell, J. S., & Gough, D. I. (1979). Northeast-southwest compressive stress in Alberta: Evidence from oil wells. Earth and Planetary Science Letters, 45(2), 475-482.
Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.
Barton, C. A., Zoback, M. D., & Burns, K. L. (1988). In-situ stress orientation and magnitude at the Fenton Hill geothermal site. Geophysical Research Letters, 15(5), 467-470.