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1 Department of Materials Science and Engineering and Geological Engineering Program, University of Wisconsin, Madison, WI 53706-1595, USA (e-mail: bhaimson{at}wisc.edu)
2 Present address: Mueser Rutledge Consulting Engineers, New York, NY 10122-0002, USA
Drilling of vertical wellbores in sandstone often results in stress-induced failed zones called breakouts. In the laboratory, miniature drilling under simulated far-field crustal stress conditions above a certain threshold produces breakouts that can be studied to enhance our understanding of deformation and failure mechanisms in rock. We report on drilling experiments in St. Peter sandstone, an Ordovician aeolian rock consisting of well-rounded bimodal quartz grains held together by sutured contacts. Breakouts here are slot-shaped and oriented at right angles to the far-field maximum stress direction (
H), resembling emptied compaction bands. We distinguish between two porosity ranges in this rock. In the high-porosity variety (16–22%), grains are bonded through narrow sutures. In the vicinity of the borehole at points aligned with the far-field least horizontal principal stress (h) direction, where the maximum compressive stress concentrates, grain sutures sever at relatively low stress levels, leading to intergranular cracking. Debonded intact grains repack and produce a reduced-porosity narrow compaction band. Loose grains in the band abutting the borehole wall are then flushed out, assisted by the circulating drilling fluid. This intensifies the stress concentration ahead of the breakout tip, advancing the compaction band and lengthening the slot-shaped breakout. The process continues sequentially. Grains in the lower-porosity sandstone (11–12%) are more substantially sutured over much larger surface areas. This makes the sutures nearly as strong as the grains themselves; hence, failure requires considerably higher stresses, sufficient to bring about intragranular cracking. Consequently, and in contrast to the high-porosity sandstone, breakouts here are preceded by compaction bands of shattered and crushed grains. Both types of compaction bands have been encountered in the field and in independent laboratory experiments.
