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

Abstract

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 (20–80mm long, 10–50mm in diameter) with varying borehole diameters of 1–19mm 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 275MPa for samples with 1mm borehole diameter, to 89MPa for those containing a 19mm 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 0.15mm around the borehole wall in the fine-grained shale.