Drilling-Induced Tensile Fractures
Learning objectives
- Explain DITFs as tensile failure of the wall where the hoop stress goes negative
- Locate them at the maximum-horizontal-stress azimuth, perpendicular to breakouts
- Compute the mud weight at which the wall first goes tensile, and the breakdown pressure with tensile strength
- Use DITFs as a direct SHmax compass and a stress-magnitude constraint
The Opposite Failure
Breakouts form where the wall hoop stress is highest. Drilling-induced tensile fractures form where it is lowest, at the azimuth, and they form for the opposite reason: not too much compression but too little, tipping into tension. When the mud weight is heavy, the pressure inside the hole pushes the wall outward and reduces the compressive hoop stress at the azimuth; push it hard enough and that hoop stress goes negative, into tension, and when it reaches minus the rock's tensile strength the wall splits along a thin axial fracture. These DITFs run parallel to the borehole axis, appear as sharp lines on an image log at the azimuth, and are the perpendicular twin of breakouts: breakouts at , tensile fractures at , ninety degrees apart, both pointing at the same stress field from opposite sides.
Raise the mud weight in the figure and watch the axial cracks appear at the azimuth. The onset condition comes straight from the Kirsch minimum hoop stress: the wall goes tensile when falls to zero, giving an incipient-DITF mud weight of MPa on the canon well, with the tensile strength taken as zero for the first hairline crack. To open a fully developed fracture against a rock with real tensile strength , the pressure must reach the breakdown pressure , which for the canon MPa is 50.7 MPa. Between 40.7 and 50.7 the wall is cracking incipiently; above 50.7 it breaks down and takes a propagating fracture, which is exactly the hydraulic-fracturing threshold of Part 7.
A Cleaner Compass
DITFs are, in some ways, an even better stress indicator than breakouts, because they require no knowledge of rock strength to give the azimuth: wherever they appear, that is the direction, full stop. Their presence or absence also constrains the magnitudes, an over-heavy mud that produces DITFs where none should occur, or a mud that fails to produce them where the model predicts they should, both refine the stress estimate, which is the no-DITF bound the polygon used in Part 5.5. The elegant symmetry the course has been building is now complete: one set of equations, the Kirsch field, produces two perpendicular failures, breakouts and tensile fractures, that together read both the direction and the magnitude of the horizontal stresses off the wall of a single well. The next section assembles all of this into the diagram that decides whether a well survives: the mud-weight window.
References
- Aadnoy, B. S. (1990). Inversion technique to determine the in-situ stress field from fracturing data. Journal of Petroleum Science and Engineering, 4(2), 127-141.
- Brudy, M., & Zoback, M. D. (1999). Drilling-induced tensile wall-fractures. International Journal of Rock Mechanics and Mining Sciences, 36(2), 191-215.
- Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.