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Borehole Breakouts

Experimental Studies of Anisotropy on Borehole Breakouts in Mancos Shale

Fracture-like Borehole Breakouts in Highly Porous Sandstone:Are they Caused by Compaction Bands?

Simulation of Borehole Failure Phenomena using Discrete Element Modeling

Fracture Permeability and In Situ Stress in the Dixie Valley, Nevada, Geothermal Reservoir

Fracture-like Borehole Breakouts in Highly Porous Sandstone:Are they Caused by Compaction Bands?

Extensive field evidence and laboratory experiments suggest that borehole breakouts, defined as borehole cross-section elongations resulting from preferential rock failure, is a direct consequence of the in situ stress in the rock. One of the early observations of breakouts was in the quartzite and conglomerates of the Witwatersrand gold mine in South Africa (Leeman, 1964). The spalling was observed to occur at diametrically opposed points on the borehole wall perpendicular to the direction of the maximum principal stress.

The most publicized observation of breakouts was in the 3 m diameter drift at 420 m level in the Underground Research Laboratory (URL), Canada. Two diametrically opposed breakouts were approximately aligned with the vertical stress, which is the overall least principal stress at URL.

The breakout phenomena was also observed in the non-welded Paint Brush tuff along the spring line of the Exploratory Studies Facility approximately 250 m below the Yucca Crest, Nevada. The measured stress regime reveals that both horizontal stresses are smaller than the vertical stress, suggesting the overall maximum principal stress is the vertical stress.

Three different modes of failure have been suggested as the mechanism leading to the breakout.

Extensile failure model
(V. Maury, 1987; Haimson and Herrick, 1986; Lee and Haimson, 1993; Song and Haimson, 1997)

A family of subparallel microcracks was induced without any obvious shear displacement behind the borehole wall in the two zones aligned with the minimum horizontal stress direction. The cracks are densely spaced and subparallel both to the borehole wall and the maximum horizontal stress direction. Progressive spalling of detached flakes bounded by these extensile cracks leads to deep and pointed breakouts.

Shear failure model
(Zoback et. al, 1985)

Cracks initiates at the borehole wall, were intergranular and propagated along a pth of high shear stress. Moreover breakouts were formed by the intersection of two distinct conjugate fractures, rather than by clusters of flakes between micorcracks.

Compaction band model
(B.C. Haimson, in Phys. Chem. Earth(A), Vol. 26, no.1-2, pp. 15-20, 2001)

In high porosity granular rock, breakouts initiate at the borehole wall in the two zones aligned with the minimum horizontal stress direction. In previous models, propagation of breakouts was limited by the diminishing length of the flakes (extensile failure model) or by the domain defined by crossing conjugate fractures. However, in highly porous (approximately 20 to 25% or more) rock, initiation of breakouts provides seed for the propagation of anti-mode I fracture in the form of compaction band. The tip of the breakout or anti-mode I fracture advances orthogonally to the maximum horizontal stress. The typical shape of the breakouts created under this condition is in the form of long fracture with compaction band ahead of the fracture tip.