Laboratory experiments were conducted to study the micromechanics leading to fracture-like breakouts in vertical holes drilled into Mansfield sandstone and the relationship between their geometric characteristics and the applied far-field state of stress (sh, sH, sv). Fracture-like breakouts are tabular, long, and very narrow, originating and extending along the sh springline, where the maximum compressive stress concentration occurs. Several series of tests revealed a clear correlation between the applied stress regime and breakout length, indicating a potential use of these breakouts as stress magnitude indicators. The average breakout width in all the tests is constant regardless of the far-field stress conditions, suggesting that this is a material property related to the lower porosity narrow band developed ahead of the breakout tip. The latter is viewed as a zone of localized compaction formed by the debonding of weakly-sutured grains and their repacking as a result of the high stress concentration along sh springline. Fracture-like breakouts are produced by the removal, with the help of the circulating drilling fluid, of loosened intact and cracked grains from within this compaction band.
1. INTRODUCTION
Cross sections of deep vertical holes drilled into the Earth's crust are often found to have elongated as a result of the formation of two diametrically opposed compressive failure zones. These zones are commonly termed 'stress-induced breakouts'. The occurrence of such breakouts was observed in the field [1,2,3], and their occurrence verified in laboratory experiments conducted on crystalline and carbonate rocks [4,5,6]. Breakout orientation in vertical-boreholes is typically along the diameter aligned with the direction of the minimum horizontal in situ stress, called the sh springline, where the highest stress concentration occurs. Hence, stress-induced breakouts have been utilized as a reliable indicator of the in situ stress direction.
Breakouts in crystalline and carbonate rocks were found to be typically broad at the borehole wall and short in depth, resembling dog-ears. Micromechanical studies revealed that dog-eared breakouts are formed by episodic spalling of thin rock flakes bounded by tightly spaced dilatant extensile microcracks that are simultaneously subparallel to the borehole wall and to the maximum far-field horizontal stress, sH. Laboratory studies in granites and limestones have also revealed a direct correlation between breakout dimensions (breakout length, and angular span at the borehole wall) and the far-field principal stress magnitudes [5,6]. An important practical application of this finding was the field use of logged angular spans of breakouts at the borehole wall together with the true-triaxial rock strength
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Figure 1. Micrographs of Mansfield sandstone texture. (a) Back scattered electron (BSE) image, showing subrounded quartz grains (Q) as the dominant constituent mineral, and some clay (altered mica) (C). Grains are bonded together primarily by suturing (indicated by arrows) over narrow contact areas. (b) Same field of view as (a) but with Cathodo-luminescence. Note the quartz overgrowth (QO), which is not recognizable in (a). (c) Thin-section image showing sutured grain contacts (indicated by arrows).
criterion to constrain the maximum horizontal in situ stress, sH, magnitude at Cajun Pass and KTB scientific deep wellbores [7,8].
