The alternative would be to pull out the existing tubing and re-enter the well to drill back into the reservoir, explains staff engineer Stein Kristian Andersen in Technology.
"That represents a much more expensive option, both because the tubing has to be replaced and because a longer well must be drilled," he says.
Already boosting recovery on such fields as Gullfaks, this method was adopted by Statoil in 2000. The group has so far drilled 12 through-tubing wells.
The technique has been deployed from platforms on the Gullfaks, Statfjord and Veslefrikk fields, with savings of NOK 10-20 million per operation.
Drilling through the production tubing means that the well bore will be smaller – typically about 5.5 inches as against a normal 8.5 inches.
An existing well which has ceased to produce is often used, with further drilling to reach another part of the reservoir where hydrocarbons can still be found.
"The cost savings involved make it economic for us to investigate a larger number of potential hydrocarbon pockets in each formation," Mr. Andersen says.
That yields extra revenue from oil and gas which, in many cases, would not have been recovered. The technique also helps to extend the producing life of offshore installations.
Statoil is now exploring opportunities for through-tubing drilling from a mobile rig in a subsea well on its Norne field in the Norwegian Sea. Savings could reach NOK 50 million per well.
But this requires drilling through a riser, where the constant motion of the rig could pose a problem.
Another challenge relates to the behaviour of the small-bore drill string in the large riser and other seabed equipment used for conventional drilling.
The aim is to start drilling through the production tubing in subsea wells next September.
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