Well bore breakouts and in situ stress

Abstract

The detailed cross-sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data. We show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations. Both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression. This work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ.

Implementation of a Flaw Model to the Fracturing Around a Vertical Shaft

“…For a linear elastic material and for an isotropic stress situation, Fjaer et al (1992) show that the growth of the shear fractures can result in pairs of shear surfaces intersecting parallel to the shaft axis. Zobak et al (1985) considered an anisotropic horizontal stress field and they came to similar conclusions. They also suggest that the dogearing is a cyclic mechanism in which new fracture surfaces develop outside the existing ones.…”

Hydraulic fracturing stress measurements at Yucca Mountain, Nevada, and relationship to the regional stress field

“…There are two types of black bands: very regular, continuous black bands with fuzzy, gradational edges and more irregular bands, often discontinuous, with sharp, jagged edges. These were examined by reprocessing the recorded signal in travel time mode to produce a horizontal cross section of the hole [see Zoback et al, 1985]. The travel time cross sections of these bands demonstrate that the regular, continuous bands are due to the tool being off center in the hole.…”

Determination of minimum and maximum stress profiles using wellbore failure evidences: a case study—a deep oil well in the southwest of Iran

“…However, at the mid points of the breakout and the induced fractures, the circumferential effective stress is exceeded the formation strength. Through some laboratory experiments, Zoback et al (1985) observed that after initial formation of a breakout, the stress concentration around the wellbore is such that the breakout tends to deepen while the breakout width remains constant. The fallowing equation was developed by Barton et al (1988) to estimate in situ horizontal stresses using breakout width (W bo ):…”

“…Therefore, breakouts can be detected using the (four-arm) caliper log tool that provides two diameters of the borehole cross section (Reinecker et al 2003). The Formation mechanism of the breakout was first discussed by Gough and Bell (1981) and later expanded by Zoback et al (1985). Breakouts are formed in symmetric zones around the borehole at the azimuth of least horizontal principal stress (h = 90°, 270°) where the circumferential stress (r hh ) is the largest and exceeds the formation compressive strength.…”