Marginal fields don’t always turn out to be that marginal, especially when you have efficient topsides and an ability to find hidden pockets in your reservoir. Elaine Maslin reports.
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Hummingbird Spirit. Photos from Centrica. |
Centrica’s Chestnut field has been something of a fighter. Labeled stranded by Venture, the field, 200km northeast of Aberdeen, finally made it into production with the UK’s first cylindrical floating production, storage and offloading (FPSO) vessel in 2008.
Initially thought to have just three years and 7 MMbbl production life, the field has produced 20 MMbbl. Now, facing the possibility of being shut-in this year, the field is set to produce yet more. A new well, under a US$45 million (£35 million) investment project, is set to push field life out to 2020, increase production rates from 4000 b/d to 14,000 b/d, and quadruple Chestnut’s initial field life estimate.
It’s a cracker
“Venture branded [Chestnut] a stranded asset. It’s the smallest standalone development in the UK North Sea, and almost definitely in the (wider) North Sea,” says Nigel MacLean, Central North Sea manager, Centrica. “We never realized Chestnut would be as large as it is.”
“Chestnut is unlike any of our other fields. It’s an injectite reservoir, not primary deposition. It was found by accident, as they usually are, when a deeper Forties reservoir was being targeted,” says Gemma Campbell, Centrica’s development and production manager for the Central and northern North Sea. “Injectite sands come from a sand body deeper down in the sequence, for example a turbidite sandstone. The shale compresses, forcing the sand up through the over burden. Re-depositing the sand in a shallower part of the sequence,” Campbell explains.
It is difficult to assess the size of such reservoirs, due to their uncertain nature and that they’re hard to image with seismic. It’s also difficult to drill, in terms of finding reservoir, but not difficult to produce from, as “it’s pretty much the best reservoir quality you can get, with 34% porosity. It’s not really rock. It’s sand. If you take a core, it falls apart,” she says.
Discovery
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Hummingbird Spirit in harbor. |
Chestnut was discovered in 1986, and drilled in 1987-88, before being brought online with a pre-drilled water injector, then two production wells, via the Sevan-design Hummingbird Spirit FPSO in 2008. The field was developed on the assumption that it contained 7 MMbbl recoverable. There had been a concern about the connectivity of the reservoir, but this was proved unfounded, and with very good water sweep, via an injection well, maintaining initial pressure.
The Sevan floater fitted Chestnut because it was a new technology – a cylindrical FPSO vessel – which meant then-operator Venture was able to get preferential rates (which have since been updated to reflect market conditions), MacLean says. The idea for Hummingbird Spirit was for it to go around and work on small pools with this small production unit, much like its namesake bird.
MacLean was working for Bluewater at the time and says that many were bemused by the new cylindrical concept. MacLean, however, liked it and managed to get a job on Chestnut.
The 200,000 bbl capacity Sevan300 Hummingbird Spirit was Sevan’s second cylindrical unit and the first one in the North Sea. Sevan’s first cylindrical FPSO was the Sevan Piranema, now Piranema Spirit and operated by Teekay Petrojarl, launched in 2007, offshore Brazil, in 1000m water depth. Both were built at Yantai Raffles/Keppel Verlome.
Sevan’s third unit was the Sevan300 Sevan Voyageur, used by Premier Oil on the Shelley field, from 2009-2010, then on the Huntington field, since 2013, for EON, where it still resides. A Sevan1000 was then used to develop the Eni-operated Goliat field and a Sevan400, built at Cosco, will be used for the Dana-operated Western Isles development, due online this year. Sevan has also built cylindrical drilling units.
The Hummingbird Spirit was designed to last 20 years and built with aluminum instead of steel, glass-reinforced plastic (GRP), instead of steel, and high-quality cranes, and turbines, MacLean says. Even the topside surface was thicker, to reduce issues with wear.
At Chestnut, the Hummingbird Spirit is moored in 120m water depth, using nylon and chain, with suction piles, in three sets of four mooring lines. A handysize tanker is used for offloading.
The Hummingbird Spirit’s 90% uptime and continued reservoir performance has helped the unit outlast its planned stay, albeit to the detriment of another field. In 2012, Antrim Energy – which dissolved this year – signed a deal to use the unit on the Fyne field from 2016. But, contract renewals with vessel owner/operator Teekay in 2013 (when Chestnut produced 13 MMbbl, reserves were estimated at 18 MMbbl), and again in 2016, meant the unit stayed put. The Fyne license lapsed in 2016.
Renewal
In Q1 2017, with production sitting at 4000 b/d, Centrica had a decision to make: to cease production by the end of the year or reinvest. The firm reprocessed seismic data on the field, using production well data. The result was the discovery of an area the firm thinks has yet to be swept. MacLean says that a rather different question emerged: “Should we completely redevelop, have a new export route, or new wells?” Although with hindsight, Chestnut would have been a bigger development in the first place, given the economic backdrop, the decision was made to keep the FPSO and drill a new well.
The new development well will be drilled using Paragon Offshore’s MSS1 semisubmersible, built at the Barreras shipyard in Spain in 1979. It will be the rig’s fourth well on Chestnut, having drilled the second production well, the water injection well and a sidetrack water injector. The unit, which had been stacked since coming off contract with Nexen late-2016, will be drilling from early August to mid-October.
The new, long horizontal well, which will reach about 9200ft measured depth, or about 6850ft true vertical depth subsea, is in an area Centrica believes hasn’t been swept by the current well stock: comprising a vertical well and a long horizontal well. The new well will be drilled using Schlumberger’s Geosphere technology, which has been used on other injectite wells, but is relatively new.
The tool reads 60ft into the formation, which means as Centrica drills it can see the formation around it, with the data loaded into Petrel and able to give an almost real-time display on top of existing seismic data, to help target shallow sands.
“We are keeping it simple, the difficulty is finding sand,” Campbell says. “Injectites are difficult to predict. You can build a geological model, but every model has risk. Schlumberger’s tool will help mitigate that risk. If we do miss the sand, we can sidetrack. It’s quite exciting technology.”
For the new well, a subsea tree, which was already built (but, surplus to another operator) has been procured and its control system changed out for an Aker Solutions system, to match the existing control systems in the field. The existing wells are in a daisy chain, so the new well will integrate into existing infrastructure.
The one downside is that the flow rate will back out production from the other wells, MacLean says, But, 10,000 b/d of dry oil is preferable to 10,000 bbl of fluid where 80% is water, he adds.
With the additional production expected, Centrica extended its contract with Teekay for the Hummingbird Spirit out to 2020. It is planning some work on the FPSO, including the replacement of eight of the 12 mooring lines. Centrica plans to do another four later this year. To extend equipment use, the firm has been using Viper Subsea’s, an Oceaneering company, umbilical support technology. This is used to mitigate degradation in electric supply cores in the field’s umbilicals, i.e. keeping them open, instead of having to replace them.
There’s also the potential for more water injection capacity to be added, and a debottlenecking study will run from this year into next year. But, a decision will not be made until the third well has started producing, MacLean says. Centrica is also assessing the used of drag resistant agent (DRA) in its production pipelines to aid injectivity.
Life after Chestnut
With its learnings from Chestnut and the knowledge of the Hummingbird, Centrica is considering its next steps. “The key might be, in the future, to re-develop another location, maybe something near to Chestnut,” MacLean says. “We have organic opportunity for this type of solution and Hummingbird Spirit would be a great tool for us to use.” OE looks forward to telling you more.
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