Influence of borehole diameter on the formation of borehole breakouts in black shale

We performed a series of borehole breakout tests on Posidonia shale to study the influence of borehole diameter on borehole stability in unconventional black shale. Thick-walled cylindrical samples (20–80 mm long, 10–50 mm in diameter) with varying borehole diameters of 1–19 mm drilled perpendicular to bedding were loaded under increasing hydrostatic pressures until formation of borehole breakouts. The critical hydrostatic pressure for breakout...