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Underbalance Drilling using Snubbing unit


This gas producer well in Poland was drilled overbalance in 2011 to a depth 1528 mAH. To prevent reservoir damage caused by overbalanced drilling fluids the company decided to drill the well further in underbalance mode to a depth of 1858 mAH. Snubbing was identified as the most safe and efficient method to drill underbalance into this fragile gas bearing formation.


The objective was to mobilize the Snubbing unit to Poland and to drill the well deeper to a target depth of 1858 mAH in underbalance mode.


  • Pre-job engineering was done to optimize operational efficiency.
  • 600k Snubbing unit was rigged-up on top of the drill floor of the conventional rig. The mast of the rig was tilted horizontal.
  • A 13m high 7-1/16” 10M Snubbing BOP stack was installed directly on top of the wellhead to strip in/out drilling BHA’s under pressure.
  • The Snubbing unit was equipped with a rotary table in order to enable rotation of the pipe.
  • The drilling BHA consisted out of; 5-7/8” tricone bit, 4-3/4” Mudmotor, floats, stabilizer, MWD system, 2x jar, HWDP, Drill collars and 3-1/2” drillpipe.
  • Drilling fluid consisted out of a mixture of fresh water, nitrogen and chemicals. De-foaming agents were used to control foam in the return line.


  • Good planning and pre-job engineering contributed to an efficient operation.
  • The well was drilled slightly underbalance with a SIDP of 130 PSI.
  • The hole was drilled from 1528 to 1858 mAH in a single trip without problems.
  • No downtime.
  • No incident and accidents.