Almost ten kilometres long and almost completely horizontal. Never before has StatoilHydro drilled a well as complicated as the Gulltopp well on the Gullfaks field. The well provides the company with valuable knowhow and great revenues.
Tuesday 8 April was a red-letter day on the Gullfaks A platform in the North Sea. The most complicated well in StatoilHydro's history was successfully completed and hydrocarbons were flowing up through the well at 9910 metres. This is thus the longest producing well in the world drilled from an offshore platform.
"This is a day of rejoicing both for Gullfaks and StatoilHydro. We were aware of the risk that Gulltopp drilling from the platform might fail. This makes it extra great that we today have successfully completed the company's most demanding drilling operation," says Arne Sigve Nylund, the head of Operations West in StatoilHydro.
The experience gained by StatoilHydro is very valuable to the further development of both remote prospects at Gullfaks and on other fields in the company's portfolio.
"The increased range that we now envisage for platform drilling opens up new perspectives for effective exploitation of existing infrastructure, and thus increased producing life," Nylund says.
Gulltopp will, together with other prospects in the area, secure continued Gullfaks operations towards 2030.
An extensive plan for how to extend the life of the field, which came on stream back in 1986, has been developed.
The drilling of Gulltopp has been a very demanding job that requires a go-ahead spirit, as well as innovation and perseverance.
The 10-kilometre drill pipe was controlled from the drilling rig at the sea surface. It was run 150 metres down to the seabed, and then kilometre after kilometre through various types of rock strata.
The longer the drill pipe is, the more difficult it is to control the forces that are transferred to the drill bit down in the deep, thousands of metres away. This requires great attention and skills by personnel in charge of drilling.
"The Gulltopp well has been a great technological challenge, and was possible thanks to high professional skills among our own drilling and well personnel, in addition to crucial contribution by the involved suppliers," says Geir Slora, head of drilling and wells in StatoilHydro.
"There have been project delays due to subsurface conditions and demanding technical and operational challenges related to the well, brake system in the drilling rig and platform's power supply. It has therefore been necessary to upgrade the brake system and power supply," Slora says.
Successful filling of air into the casing
The working conditions below the seabed are always challenging. The conditions are unstable, with varying pressure and mixture of rocks.
For Gulltopp the conditions were even worse. This is an exceptionally shallow oil deposit. For each metre to be drilled down, it was necessary to drill four metres horizontally. Most height meters must be used at the top of the well.
When the well was drilled far enough into the seabed to be slanted, it must be drilled almost horizontally in order to hit the target.
Most of the well has a downward inclination of just seven degrees.
This poses great challenges with regard to the increasing friction between the drill strings and rocks underway when the well is extended.
The platform equipment has been reinforced several times during this process. But in order to drive the drill forward in the slack incline the friction in the well must be reduced.
Pulling an eight-kilometre casing that rubs against the bottom of the slack well was not possible. The drilling management decided to fill the casing with air. The casing would then hover in the hole rather than lying along the seabed. This was the key to success.
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