This method was employed to bore through the cap rock in well C-05, which has been drilled from the Gullfaks C platform.
The job was done by Halliburton, which has long experience internationally with underbalanced drilling, in cooperation with drilling contractor Prosafe.
"Oil worth about NOK 1.2 billion is unrecoverable from Gullfaks with conventional drilling methods," explains Johan Eck-Olsen, project manager for underbalanced operations in Statoil.
"Now that we've succeeded in applying this technology, the accessibility of these reserves could help to extend the producing life of the field."
A second underbalanced well is due to be drilled from Gullfaks C later this autumn.
The new technology is needed because the upper section of the cap rock in the Shetland structure has fractured as a result of water injection.
Since the difference between pore and fracture pressure is small, drilling this rock with conventional methods would be difficult.
During regular drilling, a well is filled with heavy mud which creates a downhole pressure higher than the formation pressure to prevent any inflow of water, oil and gas.
Underbalanced drilling uses a lighter mud, giving a downhole pressure below that of the formation being drilled. This allows formation fluids to flow into the well in a controlled manner.
The fluid can then be collected and conducted to the platform's process facilities during the drilling phase.
"In the longer term, adopting underbalanced technology could allow us to get more resources out of fields like Kvitebjørn in the North Sea," says Mr. Eck-Olsen.
"This development comes on stream during the autumn, and its reservoir pressure will have sunk so much after just one year that underbalanced drilling could be appropriate."
He adds that Statoil is looking at international opportunities for using the technique to optimize recovery and producing life on fields it operates in such countries as Algeria and Iran.
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