Glossary

Bit size

The diameter of a drill bit, usually given in inches.

Variations in hole size around (above or below) this value are important for a number of reasons. Under gauge well sections (i.e. hole widths less than the bit size) may indicate mudcake build up, sloughing shales, squeeze and hole collapse. Holes larger than the nominal bit size may mark ovalisation, borehole breakout, washout and generally rugose sections.

Proposed bit size through a section is an important consideration in the predicted utility of acoustic borehole imaging tools, as large boreholes results in rapid attenuation of the acoustic signal often with little data of geological merit imaged.

In dipmeter and image processing if a caliper is not available to define hole shape then bit size can be used as a proxy for the hole diameter.

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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.