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doi:10.2204/iodp.proc.308.212.2009

Description of breakouts

Borehole breakouts are compressional failures of the borehole wall produced in a vertical well by near-borehole amplification of in situ horizontal stresses. For a concise review of breakout formation and further references, see Goldberg and Janik (2006). Breakouts occur on opposite sides of the borehole along a direction parallel to the minimum far-field stress. In a vertical well, they generally occur at the azimuth of the minimum horizontal stress (S_Hmin) and perpendicular to the maximum horizontal stress (S_Hmax). These borehole failures are generated by compressive failure of the borehole wall and result in its enlargement in the S_Hmin direction. In resistivity images, the failed areas normally appear as low-resistivity zones because of enlargement and/or fracturing of the borehole. Salt water in the borehole is more conductive than sediment; where the borehole is enlarged, the resistivity imaging tool sees higher conductivity or lower resistivity (Fig. F2A). The resistivity imaging tool has a resolution of 2–3 cm horizontally and 3–5 cm vertically at the shallow level of investigation (2–3 cm) beyond the tool diameter (shipboard Schlumberger Engineers, pers. comm.).

The occurrence of breakouts at shallow depths in muddy sediments may seem surprising. Muds are typically considered highly anelastic (viscous) and thus would be expected to flow rather than fracture. The borehole failures we discuss must have formed quickly, within minutes of cutting of the hole. Penetration rates are typically ~30 m/h and the resistivity imaging tool is located ~3 m above the bit. Drilling of the borehole rapidly removes the lateral confining stress of the sediment, replacing it with a hydrostatic fluid pressure at the borehole wall. Thus, the borehole failures can be approximated by the low confining stress triaxial experiments carried out on the cores from the Ursa region (Urgeles et al., 2007). It is reasonable to interpret these features as borehole breakouts.

The breakouts shown in Figure F2A (100–110 m) are typical of those documented in resistivity images from other ODP and IODP holes. Other breakouts at IODP Site U1322, developed at ~190–200 m, are more complicated and characterized by a high-resistivity zone flanked by zones of low resistivity (Fig. F2B). These latter images may represent an incipient failure mode that leaves a ridge of high-resistivity sediment between adjacent highly fractured zones (the flanking low-resistivity areas). The zone of high-resistivity sediment may represent a block of more intact material caught between conjugate fractures and bulging into the borehole in the S_Hmin direction. The high-resistivity signal results from sediment extruded closer to the resistivity tool. These high-resistivity features are located on opposite sides of the borehole and show azimuths consistent with the more typical breakouts above. The high-resistivity zones are narrower and straighter than the typical breakouts. This geometry is consistent with an incipient stage of failure, which could evolve to the broader more irregular low-resistivity breakout with spallation of the central high-resistivity block (Moos et al., 2007).

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