Glossary
Borehole breakout
The drilling of a borehole leads to a change in the local in-situ rock stress state in the vicinity of the well bore. For a vertical hole drilled parallel with a principal stress axis a stress concentration occurs in the area of the borehole in the direction of minimum horizontal stress (Shmin). Shear failure can occur where tangential stresses are concentrated, in the direction of Shmin. Localised stress concentrations results in a failure triangle in cross section that is enclosed by conjugate shear planes. This effect is confined to the immediate vicinity of the borehole wall. They are recognized on images as twinned, vertical, irregular and relatively wide and deep patches approximately 180º apart. These features are also commonly identifiable with caliper measurements alone. The use of borehole breakouts is a recognised method for determining in-situ stress direction within boreholes.
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About the glossary
This listing is not exhaustive or definitive, do not expect Oxford English Dictionary standards; it is aimed at those who need to understand basic principles and those that read reports on dipmeters and borehole images requiring some technical help to get the most from such documents.
We have attempted to make the descriptions and explanations generic, giving the glossary a wide application and appeal, with minimal, if any, commercial bias. If you spot any mistakes, omissions or any problems with the explanations, please do not hesitate to contact us, and we will try to accommodate your comments.
Acknowledgements
This listing has been derived by Task Geoscientists. All external sources are fully acknowledged.