Gulltopp is the world’s longest producing well drilled from an offshore platform.

StatoilHydro’s Gullfaks A platform lies offshore Bergen, Norway, in Northern North Sea Block 34/10 in 443 ft (135 m) of water. Originally discovered in 1979, Gullfaks and its satellite fields have been in production since the mid-1980s. The company’s most recent satellite discovery, Gulltopp (formerly known as Dolly), was identified in 2002 and has now been tied into the Gullfaks production facilities with the world’s longest producing well drilled from an offshore platform.

The Gulltopp discovery lies 32,810 ft (10,000 m) from Gullfaks A, which presented the operator with a major decision: should the prospect be drilled using a subsea template or directly from the platform? Faced with a challenge, an impressive potential cost savings, and a possibly faster return on investment, StatoilHydro opted for the latter.

Planning and analysis

The development of Gulltopp was approached with two basic scenarios: placing a template on the seafloor or drilling the formation directly from the platform — a challenging feat for the shallow reservoir.

Based on economics, drilling the well from the platform showed an estimated cost savings of around 70% provided the well was successfully completed. Estimated cost for a subsea well was US $393.9 million. Although a subsea well had a higher chance of success, drilling from the platform would cost considerably less — $98.5 million.

In addition to cost, time was also a factor. According to an SPE paper presented in Bergen in 2004 (Hallstein M. Anes, Lasse Stoltenberg, and Per Lars Sande, Statoil), the company estimated that production would start about five and a half months earlier if drilled from the platform. With a lower initial investment and the benefit of earlier production, the higher risk of drilling from the platform was considered the best option for Gulltopp.Financial risk aside, part of the reason for choosing to drill from the platform was the opportunity it presented to learn about this drilling method.

“If we were able to drill Gulltopp, we could then learn how to reach other prospects in the area. We wanted to prove it was possible,” said Reidar Helland, Subsurface Manager for Gullfaks field. With a “learning experience” in mind, the operator set out to plan what would become the most difficult well in its history.

Putting it on paper

According to Ina Kristine Garvik, drilling superintendent for the Gullfaks A platform, the company began its planning for Gulltopp by compiling an extensive “criticality analysis” that outlined every facet of the operations involved.

The detailed analysis included all equipment and related systems and was documented in a matrix that took a full year to compile. The matrix provided improved maintenance routines, closer attention to stock level on spare parts, and identified necessary recourses required to solve problems upfront and during the drilling operation.

The extended planning phase gave the operator time to perform a thorough training program for extended reach drilling operations, which encompassed key personnel and crew members. “StatoilHydro demanded that only the best people be used on this project,” Helland said. Throughout the project, the matrix analysis was constantly monitored and improved.

According to Garvik, “This was a new process for StatoilHydro, learned through contacts with other operators who had completed extensive extended reach drilling projects.” Both Garvik and Helland reported that the lengthened planning process added value to the overall project. “Gullfaks has some of the best drilling rigs in Statoil’s portfolio regarding uptime on rig equipment. The company is now looking at ways to further improve operations,” said Garvik.

Estimating the challenges

StatoilHydro’s project team estimated the Gulltopp well would be approximately 34,500 ft (10,500 m) from the Gullfaks A platform to the reservoir. Reaching this distance was compounded by a shallow target at <9,843 ft (<3,000 m) sub-sea.

“Changing the main break on the drawworks was necessary to control the increased loads of the well,” Garvik said. The Gulltopp well also required the use of oil base mud in all sections below a 20-in. shoe. This increased the need for improved HSE requirements and logistics. A mud cooler was installed at the surface to lower its temperature by about 50°F (10°C) to keep the temperature below 122°F (50°C). The cooler reduced associated vapors and decreased the amount of required personal protective equipment for the drilling crew.

The Gulltopp well presented risks, many of which were identified and remedied in the operator’s planning stage. With its “criticality analysis” in place, StatoilHydro informed its partners of the risk associated with drilling Gulltopp from the platform.

Gulltopp contains an estimated 28 million bbl of recoverable oil. StatoilHydro estimated the overall success rate for drilling the well from the platform at 85%. “Due to the high upside of succeeding with drilling from the platform, management decided to approve the project despite the risk of failure,” Garvik said. In April 2005, three years after the initial discovery, drilling began from the Gullfaks A platform.

The outcome

The 6.2-mile (10-km) drill pipe proved difficult to control from the rig at the sea surface, Garvik said. The distance involved required greater attention to the process.
Gulltopp is a relatively shallow oil deposit. For each 3.3 ft (1 m) that was drilled vertically, it was necessary to drill 13 ft (4 m) horizontally. When the well was drilled far enough into the seabed to be slanted, it had to be drilled almost horizontally to hit the target. Most of the well has a downward inclination of just 7°.

Thin drilling fluid was used to avoid high equivalent circulating density. Increased evaporation and cuttings accumulation was experienced using the thin mud. Another challenge was barite sagging. Maintaining a constant mud weight was a persistent challenge during the operation.

Casing the well was also extremely difficult. To reduce friction for Gulltopp, the 103?4-in. casing was filled with air. This made the casing lighter, allowing it to be lifted from the borehole floor. The operator also had to use a new deep barrier element once the casing was in place.

With no documented experience to set and retrieve mechanical plugs using wireline or coiled tubing beyond 22,967 ft (7,000 m), the operator chose a specially designed glass plug that could be removed with pressure. “It was difficult to get downhole, and there was not as much control,” Garvik said. “The glass plug made it possible for us to open the well with pressure rather than wireline or coiled tubing. We had used glass plugs before, a few times, but we had to make a redesign, and this was the first time we used the redesigned plug.”

Gas lift required three levels of sub plate mounted valves in the completion string. Two of the valves were set in the horizontal section at 10,827 ft (3,300 m) and 23,295 ft (7,100 m). The 7-in. monobore tubing required upgraded rubber centralizers to reach the required depth at 22,951 ft (7,300 m).

“We almost lost the well when we couldn’t manage to release the running tool for the screen. It took ten days of work to successfully release. There wasn’t any known equipment in the world that could have helped us,” Garvik said.

The well was finally brought onstream in April, 2008. In spite of extensive planning, Gulltopp proved to be an extremely difficult well. Originally StatoilHydro estimated drilling would take 164 days and completion would take 33 days (approximately 8 months total). The operation would ultimately take 540 days (spread out over three years).

According to Helland, “The original plan estimated a major time savings, but the operation was set back due to several accidents.” Gulltopp’s original cost was estimated at $98.5 million. The actual cost of the well was $177.3 million.

Looking ahead

Cost overruns not withstanding, the story does not end badly. Today, Gulltopp is producing 22,000 b/d of oil. “The well was paid back after two and a half months of production,” Helland said.

Garvik added, “The y axis on the production chart for the Gullfaks A platform has now been changed from 0 to 6,290 b/d to 25,159 b/d. This is a very impressive development.”

Now onstream, Gulltopp is the world’s longest producing well drilled from a platform with a total measured depth of 32,515 ft (9,910 m) and a total vertical depth of 7,973 ft (2,430 m). Even though the project cost more than anticipated, StatoilHydro sees the Gulltopp experience as valuable.

As Helland said, “We have deeper prospects, which make it easier for extended reach drilling. We have a lot of self confidence now. We are going to expand.”