Subsea programs slotted for maturing UK prospects
Amoco Norway announced its first major find as an operator for over two decades through exploration well 6507/5-1 in the northern Haltenbanken. This followed sizeable gas discoveries last year by BP and Norsk Hydro in the Voering Basin just to the north.
Block 6507 is part of a 15th Round licence awarded to Amoco in February 1996. The discovery, within the Donatello structure, is sandwiched between the Heidrun and Norne oil fields in relatively undrilled acreage. Partner Enterprise Oil credits its geoscientists with having identified Donatello as a new play.
The semisubmersible drilling unit Maersk Jutlander spudded the well in the block's north-east corner in 327 meters water depth, reaching a true vertical depth (TVD) of 4,200 meters within the lower Jurassic. The drilling contractor conducted three productive drill stem tests in different formations. The lower Jurassic and Cretaceous tests yielded similar flow rates (5,425-5,800 b/d of oil; 5.3-7.8 MMcfd of gas), both through 38 mm chokes, while the middle Jurassic (Garn and Ile) formations produced over 26 MMcf/d and 1,120 b/d of condensate.
Downdip unclearThe composite picture suggests an oil-bearing Cretaceous reservoir overlying a thick Jurassic oil leg with a large gas cap. But reserves are hard to quantify at this stage, with only one well having been drilled. The middle Jurassic sequence gas/oil contact was not identified, and the down-dip size of the lower Jurassic oil-bearing reservoir remains unclear. Two other prospective segments within the structure, B and C, were not investigated.
These unknown factors account for the vague reserves estimate of 200-650 million BOE. Further analysis of the Cretaceous reservoir could determine the final count. Although thin, at around 8 meters thick, this turned out to be highly productive, and may extend over a wide area.
Analysts Wood Mackenzie point out that the structure lies on a north-east to south-west trend between Norne and Heidrun within a liquids-rich belt. Other minor discoveries to the west suggest a gas-prone province. Enterprise also revealed that further prospects had been identified in adjoining block 6507/3, named Clara, Sara and Tara.
Following appraisal of the Donatello well data, Amoco and partners plan a second well on their licence acreage early in 1999. An encouraging point for the partners is the current drive to commercialize associated gas from Heidrun and Norne. Wood Mackenzie views a floater as the likely development outcome.
Voering plateauThere was mixed news from the other Norwegian sector stalwarts. Statoil announced plans to drill a second exploration well in the 1,200-meter-plus water depths of the Voering plateau in 1999, despite a dry kick-off well on the Vema Dome early this year. Following BP's gas strike on nearby Nyk High, Statoil is curious to establish whether oil has been generated in source rocks, and if so, where it migrated to. However, Statoil is drawing back from plans to access thin oil layers from its Midgard gas field in the Asgard complex. If the oil is not produced early, it may become inaccessible as the reservoir pressure drops.
Saga plans a year-long production test on its H Central accumulation west of the Tordis Field in the North following completion of two appraisal wells. One (26A) encountered oil at two levels in good quality late Jurassic sandstone, testing 5,000 b/d through a 14.3 mm choke and 8,500 b/d through a 17.5 mm choke.
The semisubmersible Scarabeo 5 drilled the wells through the Tordis East subsea template - oil from H Central is being piped to Gullfaks C for processing, via the Tordis subsea manifold. In the same block, Saga also made a small oil discovery west of the Vigdis Field in 250 meters of water.
Well numbers downDespite the fertility, Norwegian exploration and appraisal well numbers were down 50% in the first quarter of 1998, compared with the same period in 1997, according to Arthur Andersen Petroleum Services. On the UKCS, levels are down 40% and 42% respectively.
Exploration successes have been moderate of late in the UK, and mainly close to existing fields. But that seems to suit the development-minded operators, who are keen to fill spare well slots on their production installations.
Three suitable accumulations were recently approved for fast-track development by the UK government. These comprise:
- Phillips' Delilah, discovered by jackup Maersk Enhancer in the southern gas basin in November 1997. The well tested 25 MMcf/d from Rotliegendes sands. Its surface location was 200 yards from Phillips' Della subsea manifold, and production will be routed via the Della facilities to the Hewett A platform. Peak output of 15,000 MMcf/d is expected shortly after the impending startup.
- Flora was discovered in the central North Sea last July by semisubmersible Kan Tan IV. The well tested 34!API oil at 6,500 b/d in the main Jurassic horizon. An overlying chalk horizon was also tested following use of acid treatment to stimulate flow. Operator Amerada Hess is undertaking tiebacks of a single production well and one water injector to its Uisge Gorm FPSO 10 km to the south via a subsea manifold and a new pipeline. ABB Vetco Gray subsea trees and wellheads are being deployed under a frame agreement. Production starts late this year, keeping the floater operating at its full processing capacity of 50,000 b/d.
- In the northern North Sea, Total's Grant gas/condensate discovery finally gets an outlet, 21 years after its discovery. It will be tied back to the Dunbar platform through a single well connecting with a new subsea manifold, which will also accommodate further development wells from the Ellon Field. The 10,000-psi FMC subsea tree was installed by the semisubmersible unit Sovereign Explorer. Grant is the costliest of the three projects, with development drilling alone put at #22 million.
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