During an exploration drilling in Sweden rotary air drilling was used to penetrate the very hard and abrasive gneiss basement right outside the city of Lund. Lund is situated in the southernmost province of Sweden, called Scania. Total drilled depth was 3701.8 m. The project, conducted during 2002 and 2003, was a part of an exploration effort of the Tornquist Zone. TZ is a major basement deformation structure traversing large parts of northern Europe. The aim was to investigate the water production properties of the faulted and fractured deep-seated rock sequences. The reason for using rotary air drilling was mainly to achieve reasonable rate of penetration trough the very resistant basement rock.
The observations made during drilling and testing provide for a framework of discussions if and when underbalanced drilling is applicable in low temperature and low-pressure conditions.

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