Drilling into shallow gas pockets is one of the most dangerous situations that can be encountered.
In a shallow well, gas can travel to the surface very rapidly, giving little warning. While drilling shallow hole, the short surface casing string is set in a relatively weak formation. It is normally necessary to divert the flow rather than shut the well in, risking fracture at the casing shoe and the possibility of gas coming up around the outside of the well.
As the 'bottom-hole' times involved are short, the drill crew should be alert for signs of a kick.
The flow sensor may be the only item of equipment able to give an early enough warning of a shallow gas kick in progress which allows the diverter to be put into use. This sensor should be kept working whenever possible. If in doubt shut off the pumps and carry out a flow check.
Pit level gains, although a valuable indication, are generally noticed too late.
Most shallow gas pockets are found in exploration wildcat wells, through shallow gas-charged sands may be found in field development wells.
In latter case the shallow formations have been charged with high pressure gas from deeper zones in nearby wells, which has migrated due to a failure on the previous well. Poor cement jobs, casing failures, inadequate abandonment procedures, downhole blowouts and injection well operations are all possible causes.
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