Subsea challenges are in focus at this week’s Underwater Technology Conference in Bergen. While the conference examines future oilfield challenges, we take a look back at past innovations in the industry. In the final article in a three-part series, Elaine Maslin looks at how the Troll development concept has evolved over the years.
If there’s one thing that never changes in the oil industry it’s that the industry is constantly changing.
A development concept chosen for a field one year could change by the next year, driven by technical and engineering advances.
Troll, an oil and gas field, with the oil field sitting mostly in Block 31/2 in the Norwegian sector of the North Sea, is one such field, driven by advances in technology, which have been tracked through successive years’ presentations at Bergen’s Underwater Technology Conference (UTC) sessions, the archives of which have now been made available online.
Troll, 80km northwest of Bergen, was discovered in August 1979, and a statement of commerciality issued in 1983. Some 13 exploration and appraisal wells had been drilled on it by 1984, by which time it was calculated to span some 22km by 14km, with excellent porosity gas sands over laying a thin oil sand. Gas in place, in 1984, was put at 19.4 Trillion cubic feet, with 2.1 billion barrels of oil initially in place, according to a presentation by N. J. West, from Norske Shell, at the 1984 UTC.
The concept for Troll, then, comprised a single fixed, potentially Condeep-style or steel a jacket supported platform, in 334m water depth, to tap the field’s gas reserves via a single cluster of platform wells, and “up to 30 subsea producing wells,” clustered in four- and two-well templates, to draw out Troll’s thinly spread oil.
The challenges around producing Troll’s oil were multiple – soft seabed conditions, 80-100m diameter, 2-5m deep pock marks in the sea bed, the harsh North Sea environment, the shallow and thin oil accumulation – and conspired to make Troll “an immense technical challenge.”
Among the challenges to producing Troll’s oil were how to transport production fluids from the low pressure reservoir to production facilities along flowlines up to 12km-long. Well deviation at the time was limited to 45 degrees, allowing only four oil wells per template, says West, with the alternative – having multiple platforms – untenable in the 340m deep water, then considered to be deep by North Sea standards. Then there was the question of how to service this fleet of subsea wells. How these issues would impact Troll’s commerciality was key.
When the Plan for Development and Operation for Troll B, the field’s first oil producing facility, was agreed in 1991, it comprised 17 subsea wells from four subsea templates, tied back to the catenary anchored Troll B semisubmersible floating production facility.
By 1994, project execution was well underway, according to a presentation at that year’s UTC on contracting strategy lessons learned by A Liverud, VP Technology and Projects Division, Norske Hydro.
By the 1996 UTC, Anders Henriksson, Marine Operations Manager for Norske Hydro, the lessons learned focused on marine operations, then two seasons into a four season campaign. The project now comprised five clusters, which could accommodate up to 32 wells, Henriksson said. Out of those, 22 were planned and would will be horizontally completed wells, with one vertical gas injector well.
“The use of horizontal wells reduces the number of wells considerable and makes a field development such as Troll Olje economically possible,” Henriksson said at the time.
Finally, Troll oil production started in fall 1995. Troll gas started up in 1996 from the Troll A fixed platform.
There, the story, at least through the so far published archive papers of UTC ends. Troll’s story, however, continues. Technological advancements have meant the life-expectancy of Troll has been extended. Since 1996, a third platform, the Troll C floating production facility, was brought on stream, producing more Troll oil. The total number of well slots, continuously re-used for re-drilling, has risen to 125. The ongoing development of Troll has meant a number of new PDOs for the new subsea templates required.
Lovise Knutsen Hundsnes, who works on Troll today, says the new slots, increased well density, gas injection, branch control (multi-branched), and well completion technology along the branches, including inflow control devices, have all helped to increase recovery at Troll.
As a result, the field, which was originally expected to end production this year, is now expected to run beyond 2025.
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