Reading the Well: Images and Calipers
Learning objectives
- Read breakouts and tensile fractures on an unwrapped borehole image log
- Pick the breakout azimuth from a four- or six-arm caliper as the enlargement direction
- Confirm the internal consistency: breakouts and tensile fractures sit ninety degrees apart
- Turn a well's failure log into an SHmax orientation with a rose diagram
The Wall Records Its Own Failure
Everything this part predicted, breakouts at , tensile fractures at , is written on the borehole wall, and two tools read it. A borehole image log wraps a resistivity or acoustic picture of the entire wall and unwraps it into a flat strip, azimuth across, depth down. On that strip, breakouts appear as broad dark bands at two azimuths 180 degrees apart, the two dog-ears of the enlarged hole, and drilling-induced tensile fractures appear as thin sharp lines, also paired 180 degrees apart but offset 90 degrees from the breakouts. A caliper, with four or six arms measuring the hole diameter in several directions, reads the same breakouts more crudely: the arms that fall into the enlargement record the long axis, giving the azimuth directly. The mechanics of these image tools belong to the Petrophysics course, which covers image logs in its Part 14; here we use them only to harvest the stress information.
Pick the features on the unwrapped image in the figure. Set your azimuth picks on the breakout bands and the tensile-fracture stripes, and the tool checks the physics for you: breakouts must sit 90 degrees from tensile fractures, because one marks and the other , which are perpendicular by definition. A consistent well shows exactly that orthogonality, and the picks combine into a rose diagram of the azimuth, tight where the stress field is coherent, scattered where it is disturbed. This is the local measurement that refines the regional prior from the World Stress Map: read enough depths and the well tells you its own direction to within a few degrees.
From Failure Log to Stress Model
This section closes the loop the whole part opened. The Kirsch equations turned the stress field into predicted wall failures; the image and caliper logs read those failures back; and the inversion, breakout width plus rock strength for the magnitude, breakout and fracture azimuth for the direction, recovers the stress state from a well that has already been drilled. It is the same constraint-intersection method the polygon used in Part 5.5, now with the observations in hand rather than assumed. And it is exactly the workflow Part 8 will run on the Ogbon-1 well: read its breakouts and fractures, invert them through Kirsch against the measured , and assemble the full mechanical earth model. Part 6 has built the wellbore's stress machinery end to end, from the Kirsch field to the failures it predicts to the logs that read them. Part 7 turns from the failures we try to avoid to the one we cause on purpose: hydraulic fracturing.
References
- Zoback, M. D., et al. (2003). Determination of stress orientation and magnitude in deep wells. International Journal of Rock Mechanics and Mining Sciences, 40(7-8), 1049-1076.
- Plumb, R. A., & Hickman, S. H. (1985). Stress-induced borehole elongation: A comparison between the four-arm dipmeter and the borehole televiewer. Journal of Geophysical Research, 90(B7), 5513-5521.
- Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.