Case study Gulltopp

Traditionally, during casing installation, the casing is filled with drilling mud and rubs against the well floor

In order to reduce the friction for Gulltopp the casing was filled with air. This makes the casing lighter, allowing it to be lifted from the bore hole floor. (Drawings: Jan Ulriksen)

Platform Drilling Services

Region: North Sea, Norway
Customer: Statoil
Field: Gullfaks Alpha Platform
Well type Exploration: Development well named Gulltopp

Challenge
To drill a 10,000 meters long, extended well to a remote part of the reservoir previously felt to be beyond the reach of the platform.

The challenges faced during this task made Gulltopp considerably more expensive than initially assumed. But the experience gained from the project is invaluable - and will be used to develop similar oil reservoirs at a lower cost.  Statoil may thus save NOK billions by using this solution.

Why drill such a long well? The record-length of the Gulltopp well is due to the distance from the nearest platform: it's10 kilometres from Gullfaks to the oil discovery. Drilling the well was very demanding; needing innovation, perseverance and a never-give-up spirit.

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 concentration and skill in personnel in charge of drilling.

Geir Slora, head of drilling and wells in StatoilHydro said: "The Gulltopp well has been a great technological challenge and was only possible thanks to high professional skills among our own drilling and well personnel, in addition to the crucial contribution of the suppliers involved. There have also been project delays due to subsurface conditions and demanding technical and operational challenges related to the well, the brake system in the drilling rig and the platform's power supply. As a result, it was necessary to upgrade both the brake system and the power supply."

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 and for each metre to be drilled down, it was necessary to drill four metres horizontally. When the well was drilled far enough into the seabed to be slanted, it had to be drilled almost horizontally in order to hit the target.

Most of the well has a downward inclination of just seven degrees. This posed great challenges with regard to the increasing friction between the drill strings and rocks as the well was extended. The platform equipment was reinforced several times during this process. But, in order to drive the drill forward in the slack incline, the friction in the well had to reduced.

Pulling an eight-kilometre casing that rubbed against the bottom of the slack well was not possible. So 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, ultimately, was the key to success.

What we did
Before commencing drilling, a detailed plan for critical equipment was made  - based on the equipment's history and downtime data. Extended maintenance work was also carried out, in addition to allowing time  between each drilled section. The upgrade of critical drilling equipment to facilitate the requirement for higher power and pressure included using the latest technology in well fluids.

Results
Tuesday 8th April 2010 was red-letter day on the Gullfaks A platform. The longest, most complicated well in Statoil's history was successfully completed and hydrocarbons were flowing up through the well at 9910 metres. This is how the longest producing well in the world was drilled successfully from an offshore platform.

"This is a day of rejoicing both for Gullfaks and Statoil. We were aware of the risk that Gulltopp drilling from the platform might fail. This makes it extra great that, today, we have successfully completed the company's most demanding drilling operation," says Arne Sigve Nylund, the head of Operations West in Statoil. "The experience gained by Statoil is very valuable to the further development of both remote prospects at Gullfaks and other fields in the company's portfolio."

Benefits
The alternative to drilling this distance would be to locate a subsea template on the seabed. This, however, would be far more expensive than drilling from an existing platform. "The increased range that we now envisage for platform drilling opens new perspectives for effective exploitations of existing infrastructure and thus increasing producing life,"  Nylund says

Gulltopp key facts

• This is StatoilHydro's longest well on stream on the Norwegian continental shelf  
•  9, 910 metres long down to 2,430 metres below the sea surface 
• Gulltopp was drilled from the Gullfaks A platform in the North Sea outside Florø 
• The oil and gas discovery was proven in 2002, and drilling started in April 2005 
• The recoverable resources are 4 million standard cubic metres of oil and 500 million standard cubic metres of gas
• The expected production plateau is16 000 barrels of oil per day 
• Licensees are Statoil 70 percent; Petoro 30 percent