Statoil is planning a much more extensive operation on subsea wells on the Norwegian Continental Shelf (NCS) to optimize its subsea well operations work and ultimately to raise its overall recovery rate from around 43% today to an ambitious 55% in 2008.

One key to the whole process is a diverless installed subsea intervention lubricator stack (SIL) which allows intervention tools to be fed into a well on wireline while maintaining a pressure barrier. The lubricator stack is specifically designed for the 7-in. horizontal trees commonly used throughout Statoil's NCS subsea assets.

"The overall aim of wireline intervention instead of using a drilling rig is that these operations are done in a more efficient and faster way," said Oyvin Jensen, manager for drilling and subsea operations in Statoil's Tampen area fields in the North Sea, which covers the Statfjord, Gullfaks, Snorre, Tordis Vigdis and Visund areas.

Jensen also has a wider responsibility for maintenance and repair of subsea installations throughout the Tampen area involving around 150 subsea wells. Consequently anything he can do to cut intervention time and improve efficiency has a big impact on his balance sheet. By using wireline from a dynamically positioned intervention vessel instead of an anchored drilling rig, Jensen said immediate cost benefits become possible because of the reduced transit time between wells. Using wireline also eliminates the need to deploy a marine riser, reducing offshore operating time on each well still further.

"Not using anchors saves time in moving from well to well," he said, "We can go directly into a well after removing the tree cap without having to run a riser system, and after connecting the subsea lubricator stack."

The SIL was designed by Schlumberger in Aberdeen, Scotland, under a contract to CalDive's Well Ops division, and it provides the pressure barrier to allow well entry without a marine riser. Once a well tree cap is removed by remotely operated vehicle (ROV), the SIL can be installed using ROV assistance.
Statoil has quoted up to 50% cost savings on its intervention work compared with rig-based intervention operations. Currently the company has set a goal of US $161,748 target daily rate for light well intervention costs, Jensen said.

One of the highest costs of intervention, he said, is vessel mobilization costs, which are now reduced, and pre-mobilization costs can be spread across several wells. Well intervention has helped to identify new drainage points in wells, which, combined with through-tubing drilling techniques and slim hole drilling, can help towards attaining the higher recovery target, boosting the economics of the operations.
"We can isolate non-productive zones on wells with mechanical sliding sleeves if they have been installed, and we can set plugs to isolate part of a well if this is required."

Statoil carried out its first wireline light well intervention operations on Norwegian North Sea block 34/8. Subsequently operations followed on Statfjord wells and most recently on Åsgard, which began producing May 1999. "So we got experience in 2003," Jensen said.

Those first intervention operations involved using Prosafe's multi-service vessel Regalia, but when further intervention was required, Statoil put the work out for a competitive tender, and CalDive's Well Operations division won the deal for 2004.

Intervention work will be performed on seven wells during the 2004 campaign - ending January 2005 - with each well operation lasting between 8 and 14 days.

For 2005, Jensen said approximately 180 days light well intervention work is planned. But, he said, the operator is also considering a longer-term strategy for light well intervention going from 2006 onwards.
On Åsgard, which is a gas and condensate field, Statoil ran production logging tools (PLTs) to collect production performance data that could not be collected with any instruments on the Åsgard production vessel at surface.

"After that, on our second and third wells, which are high pressure gas injection wells - 5,880 psig (400 barg) - with those we also ran PLTs to check data and we are now re-perforating well number 4." Operations on a fourth well were hampered, said Jensen, with long periods spent waiting on weather. The operation is limited to significant wave heights up to 14.7 ft (4.5 m). But he went on, "We have proved we can do this kind of operation late in the fall."
Prior to the 2004 campaign, Statoil tested the SIL system offshore in sheltered water in Onarheims, Fjord near Bergen, for 14 days, operating in a water depth of 328 ft (100 m) on a test well to check that all systems worked correctly.

Working with the Seawell intervention ship, he said Statoil has achieved its health, safety and environmental objectives, too. "The health, safety and environment (HSE) record of the Seawell has been very good, and that is our main objective, to have all operations perform with a high degree of HSE focus."

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