Borehole breakout evolution through acoustic emission location analysis

A series of thick-wall cylinder tests were performed on Bentheim sandstone to investigate the nucleation and propagation of borehole breakouts. Isotropic compression tests showed a significant decrease of the critical pressure required to nucleate breakouts with increasing borehole diameter. Advanced analysis of acoustic emission radiation and optical microstructures indicate a three stage process of breakout nucleation and growth. Relatively few acoustic emissions are observed during the elastic loading stage of the specimens. Once breakout nucleation has occurred at the borehole wall a drastic increase of acoustic emission activity is observed. A close spatial correspondence between located acoustic events and the breakout indicates formation of two symmetric cusp-shaped breakouts on opposite sides of the borehole. With increasing isotropic pressure the breakouts grow forming parallel-sided slots that are surrounded by a process zone revealing grain crushing and pore collapse as typically observed in experimentally produced compaction bands. We apply a simple fracture mechanics approach to predict the observed size effect of the critical pressure required to initiate breakouts.