Aker BP has reported that its four-well development of the Symra Field on the Utsira High region of the North Sea has presented one of its toughest drilling challenges offshore Norway in recent years. 

Symra, a 7-km subsea tieback to the Ivar Aasen platform, came onstream in April. Production is part-processed at Ivar Aasen before being exported to the Edvard Grieg platform for final processing.

The drilling campaign was critical to bringing the Symra subsea development online. The field ties back to existing North Sea infrastructure, allowing Aker BP to commercialize reserves through the Ivar Aasen and Edvard Grieg facilities while maximizing use of established regional processing capacity.

Why Zechstein carbonates are difficult to drill

Jan Roger Berg, Aker BP's vice president of Drilling & Wells, Eiga and Ula, explained that the field’s Zechstein carbonate reservoir “remains one of the most challenging formations" the company drills in. "Large cavities and many faults mean we experience significant losses when we encounter them," he added.

Carbonate geologist Niels Rameil added that carbonate reservoirs are far less predictable than the typical sandstone reservoirs the company is used to working with. “Oil is found here in the rock, fractures, cavities and dissolved channels,” he said.

But the very high porosity and open cavities also provide good production properties for oil, according to Magnus Holm-Gjerde, vice president of Project Execution Eiga asset.

For the development, TechnipFMC supplied the subsea systems, Moreld Apply performed modifications on the Edvard Grieg platform, and Aibel implemented upgrades on the Ivar Aasen installation.

Managing fluid losses and maintaining well control

Odfjell Drilling and Halliburton co-managed drilling operations as members of Aker BP’s drilling and wells alliance.

While drilling through open fractures and channels, large volumes of drilling fluid disappeared straight into the formation.

“We experienced periods with significant fluid losses,” Berg explained. “In such situations, we have to stop work, pull back to the last casing shoe, and pump in SentinelCem, an acid-soluble cement used in wells to seal leaks. The cement finds its way to the leakage points and sets quickly, so that losses are reduced. Before we can continue drilling, we have to wait until the cement has set, and this must be repeated for every new loss zone we encounter.

“As the number of loss zones increases and we pass them, it becomes increasingly challenging to maintain well control.”

The situation demanded regular assessments throughout the process, with close collaboration across drilling disciplines.

Technology toolkit helps complete Symra wells

The team used a Controlled Mud Level (CML) system to reduce hydrostatic pressure against the loss zone, along with wired drillpipe with built-in cables for real-time data transfer from well to surface.

Based on this experience, Aker BP has identified the following measures as key to a successful Zechstein carbonate drilling campaign:

• Having strong plans in place, with decision trees for loss types and sizes, and understanding of which solution to use in each individual case;
• Use of a CML system for better pressure control and reduced overbalance in the hole, minimizing loss and loss rate to the reservoir;
• Drilling with treated seawater in certain parts of the reservoir;
• SentinelCem cement system effectively stops losses and can also be dissolved later for production; and
• Adapting completion solutions to facilitate safe completion with losses.

“The toolbox we have developed for Symra has worked very well," concluded Holm-Gjerde. "Together with a good understanding of which operational limitations we need to stay within, we have to a large extent cracked the carbonate code. Here, it is just as important to know when to stop as when to push further.”