Hole that is drilled ahead of the main wellbore and which is of a smaller diameter than the bit used in the main borehole (refer to "Pilot Bit"). (Bul D20)
The Mudblogger (Oilfield, Energy and Related Photographs), Oil Field Glossary, TX, USA 2014. http://mudblogger.com/oilfield-glossary/, (Oil - Amer) {777}