Jeremy Beckman , London
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Troll facing pressure dilemma
De-pressurization at Troll will impact other nearby fields in the Norwegian North Sea, according to a study by Hydro and Statoil. The Norwegian Petroleum Directorate (NPD) requested a review, due to concerns over the companies’ plans to step up Troll’s gas output.
The study, based on a regional reservoir solution model, confirms that production from Troll also affects pressure development in an area extending from the Gjoa field to the northeast, to Brage to the southwest. Current pressure maintenance techniques in place on Troll such as water and/or gas injection also benefit these neighboring fields.
NPD considers Troll one of Norway’s prime long-term energy resources, housing 10% of the country’s oil and gas. From now on, however, the reservoir pressure decline will gather pace, so developing Troll’s remaining resources is time-critical. However, if gas extraction surges too quickly in relation to the field’s oil output, the resultant pressure drop could also have repercussions elsewhere.
Another Hydro stronghold in this region, Oseberg, has also experienced a pressure drop after 18 years producing oil and six years of gas - down from 170 bar in 1988 to 100 bar at present. To eke out a further 20-25 MMbbl, Hydro plans to modify the processing equipment on the Oseberg B platform to handle lower pressure production.
Hitherto, oil and gas has entered this installation’s first-phase separators at 67 bar, but following modifications, to be performed by Grenland Group, the oil will be directed straight to the second-phase separators, where pressure is only 20 bar.
The program includes adapting up to 10 wells into second-phase separators, with an extensive network of low pressure manifolds and pipework on Oseberg B, some connected to separators on Oseberg A via the platforms’ bridge link. Additionally, Grenland will upgrade gas compressors and production trains to handle lower pressure output.
Brent’s long-term future unclear
Shell has taken delivery of a new 2,000-metric ton (2,204-ton) compression module from Heerema Hartlepool for its Sean PD platform in the southern gas basin. The new equipment should help Shell deliver supplies to the UK mainland during peak demand periods in winter.
During a briefing at Offshore Europe, Executive Director of Exploration and Production Malcolm Brinded said the southern North Sea was still “quite exciting” for the company, with more pockets of gas to develop. However, the future of Shell’s Brent platform complex in the northern North Sea looked less clear.
Sean compression module.
The $2 billion Brent de-pressurization project of the 1990s extended the field’s life by over a decade, he pointed out, increasing gas recovery by 1.5 tcf. Now the company must plan ahead for the next 15-20 years on Brent, but that planning must also take into account decommissioning.
While Shell has been actively marketing other tail-end UK assets to new operators, a full or partial sale of Brent looks unlikely, he explained. “From our understanding of the subsurface, we think that these are assets we ourselves can add most value to.”
Chevron offloads heavy oil
Hydro has also been active in the UK northern North Sea, amassing stakes in various stranded heavy oil fields in Quadrant 9. The main transaction (pending UK government approval) brought Hydro Chevron’s operating interests in the Bressay and Mariner/Mariner East fields. Chevron has toiled in vain to find commercial options for these large but geologically complex structures. Quantifying the recoverable reserves has been one of the main problems, despite extensive appraisal drilling.
Hydro, however, has experience developing large heavy oilfields such as Grane, not far away in Norwegian waters, and more recently Peregrino offshore Brazil. Like Chevron, it will look to establish a production hub in Quadrant 9, also tying in current and future finds from three other licenses currently held jointly by Silverstone Energy and Wilderness Energy. Hydro has agreed to fund further exploration on these licenses in exchange for up to 50% of the equity, with a further option to take on operatorship.
An established UK North Sea operator, Venture Production, has just completed two appraisal wells on Pilot and Narwhal - two more heavy oil discoveries in blocks 21/27a and 28/2a. Oil samples recovered recently from Pilot were heavier and more viscous than those from wells drilled previously on the field. Venture had earmarked a Sevan floating production platform for Pilot, but will now take its time to re-assess the field’s recoverable reserves. To the south of Pilot, another well adjacent to Narwhal was a dry hole.
Tormore extends gas base
Total has discovered gas and condensate in a new location on the Atlantic Margin, 100-km (62 mi) northwest of the Shetland Islands. The Tormore well was drilled in 610 m (186 ft) of water in block 205/5a, testing 32 MMcf/d of gas, with a condensate-gas ratio of 75 bbl/MMcf.
Tormore is 15 km (9 mi) southwest of Laggan, another stranded gasfield which Total successfully appraised in 2004. Laggan is one of numerous accumulations in this area being linked to a prospective gas production hub. But progress on the project has slowed, due in part to weakening UK gas prices. The various consortia are also thought to be petitioning for tax breaks to offset the high costs of building gas export infrastructure in this region from scratch.
No such concerns for ConocoPhillips, which has another discovery in the southern North Sea to plug spare capacity in its Caister Murdoch System. The Harrison well in block 44/9b encountered a 50-ft (15.2-m) gas column in the targeted Carboniferous Lower Ketch formation.
In the same area, Venture brought on stream the first well in its platform-based Chiswick development, which is tied back to the Markham J6A platform in the Dutch sector. Also connected to Markham is Grove, which started up in April, but operator Newfield has decided to sell its stake in this and other UK gas interests to Centrica for $486 million.
Firmly within the Dutch North Sea, Wintershall aims to start acquiring 3D seismic shortly ahead of planned developments of the E18A and P9-A fields. The company has also been compiling new survey data across acreage bordering the Dutch, Danish, and German sectors. This summer, the German government awarded Wintershall exploration licenses for three blocks and two part-blocks 80-km (50-mi) northwest of the East Friesian island of Borkum.
