Boreholes rarely drill as perfect cylinders, but can become ovalised (or elongated) in cross-section during or after drilling. Inclined holes frequently are elongated in the direction of drilling as an effect of the tool string seeking the bottom of the hole (key seating). The effect of in-situ stress can also change hole shape through borehole breakout with the development of hole ovalisation in the direction of minimum horizontal stress (Shmin) and the orientation of borehole elongation can been interpreted as reflecting the orientation of Shmin In the absence of borehole breakout and tension fractures, hole elongation may be used as a low confidence indicator of minimum horizontal stress (Shmin) direction. The elongation direction can be derived from oriented four and six arm calliper data and borehole imaging tools. Care must be taken, however, to distinguish hole ovalisation due to in-situ stress and that arising from key-seating.
See also key seating, borehole breakout, in-situ stress and drilling induced tension fractures.