Master workovers, coax out more from infrastructure, look with fresh eyes at seismic studies, think big, and then think again. All of these capabilities help operators to gain a competitive advantage when working with mature assets.

Maersk Oil, a medium-sized oil company that has exploration operations from Norway to Brazil, is an old hand in the art of teasing out oil from aging fields in the North Sea. Two examples illustrate the application of technical know-how, innovative thinking, and the ability to seize opportunities to make good on its promise of long-term commitment to the Danish North Sea and the UK North Sea.

In the case of the Gorm field in the Danish North Sea, an irreparable well led to an operation that doubled production. At the Gryphon field in the UK North Sea, seismic data led to the decision to extend operations and put off abandonment.

To cement or drill?
The Gorm field – named after a 10th century Danish king – was the second Danish field to come onstream in 1981.

Nearly three decades later, the field has 40 producing wells. During a routine check, the company discovered that the N-40 well had leaks that could compromise safety. The decision was made to shut in the well immediately.

The Gryphon FPSO is the longest-serving permanently moored FPSO in Europe. (Images courtesy of Maersk Oil)

Faced with a loss of 2,000 b/d from this single well, engineers tried to salvage the tubing and repair the casing, but it was clear that this would not be possible because the casing had collapsed.

"It was then we had to make a decision,” Per Bak, section head in petroleum engineering, said. “Either we fill the well with cement, or we redrill it. It was either abandonment or spending money.”

After investigating further and reviewing well log data, the team came up with a hypothesis – that the well never had produced from a reservoir section and that redrilling a new well in parallel to the original one actually could access the reservoir.

Drilling began ahead of schedule in the summer of 2009, thanks to the arrival of Maersk Drilling’s new jackup Maersk Resolve.

"In a relatively short planning time, we took a new rig straight from the shipyard with new rig crews and new technology and executed a workover and redrill that contained significant uncertainty,” Bak said.

This was possible through thorough risk assessments, partner reviews, and smooth decision-making among a diverse group of employees from the company’s petroleum engineering, drilling, and well services departments.

The result was somewhat surprising. Although the team suspected parts of the reservoir had remained undrained, when the new well was drilled, production doubled to 4,000 b/d.

Wells drilled into injection wings have increased production.

“What I learned from this experience is – know your wells. Do your reservoir surveillance, well tests, and reservoir modeling,” Bak said. “It sounds pedantic. People say, ‘You don’t make oil from data.’ Well, this shows you can.”

Lessons learned
Three significant learnings resulted from this project. The first dealt with the difficulty of redrilling a well through sand fractures stemming from the original well. “Deep underground, it is difficult to tell where sand fractures end 100 m (330 ft) or more away from their points of origin. Understanding and recognizing where sand fracs are has become a high priority because this is a huge challenge in our maturing fields,” Bak said. “We have incorporated a number of logging tools measuring electrical resistance and atom density so that we are not just playing a guessing game.”

A second lesson was that when dealing with aging fields, the decades of knowledge, history, and experience with the specific geology, infrastructure, and equipment were valuable. To ensure this type of information is not lost for subsequent projects, Maersk Oil is completing a data management project that has employed a Google-like web crawler to trawl through decades of data and collate them into an easily researchable archive.

Another result of the Gorm workover was the establishment of a multidisciplinary workover team that looks at each well in need of maintenance to see how additional benefits could be gained.

“You need to stand on a good foundation of reservoir knowledge and make the most out of these challenges by turning them into opportunities,” Bak said. “What I have learned is the importance of being ready – when the next well goes, I will be there for it.”

Extending life
Scheduled for abandonment in 2012, the UK Gryphon field, with its adjacent floating production, storage, and offloading (FPSO) vessel, has been producing since 1993 and has overshot original estimates of ultimate oil recovery. Recent increases in identified recoverable reserves have changed expectations for the field.

“Various factors led to the identification of more oil,” Martin Rune Pedersen, managing director, UK, said. “These were a better understanding and technical knowledge of the field by geologists, geophysicists, and engineers working on Gryphon and the use of advancements in seismic imaging combined with four-dimensional time-lapse evaluation of the reservoir.”

Maersk Oil, which has gained a reputation for its pinpoint horizontal wells, improved its drilling techniques, enabling it to access so-called “injection wings” that are characteristic to Gryphon.

Injection wings are sand structures thrust up through layers of younger geological formations. The sand is more porous and permeable than the formation surrounding the wings, trapping oil. The difficulty is drilling precisely enough to hit the slender wings.

Last year, three new wells were drilled and brought onstream, accelerating development of the northern field extension. Drilling continues this year.

“This year, we have seen production back up to the levels of 10 years ago of 35,000 b/d. It is extremely rare to see stable production levels in these kinds of mature assets,” Pedersen said.

But neither the new seismic data nor precision drilling would have been worth the effort if Maersk Oil had not taken on the challenge of upgrading the FPSO.

The Gryphon FPSO is the longestserving permanently-moored FPSO in Europe. This vessel was designed to operate for about 20 years. So the question was, “Could it be safely used beyond that lifespan?”

Maersk Oil decided to carry out a major upgrade to ensure the FPSO would continue to operate safely.

Traditionally, such an upgrade entails halting production for months as the vessel is moved to a dockside location. The only option to taking the FPSO out of service was to contract a new FPSO at a price tag of US $500 million to $700 million.

Again, Maersk Oil opted for an unconventional solution. “We decided we could drive a car while having it fixed at the same time,” Pedersen said. “We effectively dry-docked the FPSO on station at Gryphon and maintained it while it was producing oil.”

Upgrading in this manner is painstaking work that has to be done carefully, but successful completion of the work has resulted in Maersk Oil’s expectation that Gryphon will produce until 2018.

“Compared to a new field development plan, where there is a lot of exposure to financial risk, we have a good feeling of what we are operating. We don’t need to disconnect the FPSO, so we can keep the revenue stream flowing. Strategically, this is a fairly low-risk strategy,” Pedersen said.