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 formation decreased significantly from about 275 MPa for samples with 1 mm borehole diameter, to 89 MPa for those containing a 19 mm borehole, approaching the uniaxial compressive strength of the material. The observed scale effect was fitted to various failure criteria predicting fracture initiation at a depth of about 0.15 mm around the borehole wall in the fine-grained shale. (C) 2013 Elsevier Ltd. All rights reserved.