Long live chalk!

Research into the chalk reservoirs in the North Sea promises to extend the producing life not only of older Norwegian fields but also of discoveries in other parts of the world.

Chalk research on the Norwegian continental shelf dates back to the early 1970s, and marks the beginning of improved oil recovery (IOR) efforts in these waters.

After development of Ekofisk started in 1971, it quickly became clear that the huge chalk reservoirs in this part of the North Sea had a very low recovery factor. Calculations suggested that up to 83 per cent of the oil in place would remain underground ? something regarded as far from satisfactory.

When the NPD was established the following year, the question of whether recovery from Norwegian fields could be improved quickly came up. The initial development plan envisaged that all recoverable reserves in the Ekofisk area would be produced by 1996. Instead, these fields could outlast all the others on the NCS.

"Only a small part of the oil in place in a reservoir can normally be recovered," the NPD's annual report for 1977 noted. "A lot will depend on the way production is planned, the pace of recovery selected and so forth. Various methods, including gas or water injection, can increase the recovered amount in many cases.

"Difficult economic judgements often have to be made here, where the interests of licensees and the authorities will not always coincide. As the guardian of the government's interests, the NPD has an important and complicated job to do in this area."

"The Norwegian authorities saw at an early stage that something had to be done on Ekofisk," recalls Sigurd Heiberg, who then headed the agency's reservoir department.

"When the field development was planned, some NPD specialists believed it would be possible to achieve a recovery factor of 40-60 per cent. The idea was to study the whole chalk basin at the southern end of the NCS in order to acquire as much information as possible for application to the individual fields."

New provisions on acceptable utilisation of Norway's petroleum resources were adopted in 1978. Mr Heiberg says it was the fourth Norwegian offshore licensing round the following year which really highlighted the need for IOR from chalk reservoirs.

"The Ekofisk licensees were asked what they were doing to increase the recovery factor, and appreciated that boosting the resource base was an important part of Norwegian oil policy. To achieve that, research into IOR had to be pushed ahead."

A joint chalk research (JCR) programme was established in 1980 by the NPD, the Danish Energy Agency and the licensees in the North Sea's chalk fields. Focused on issues and challenges posed by such reservoirs, this joint government-industry collaboration was special even in an international context.

"The JCR was launched after the properties of North Sea chalk fields proved to differ very considerably from the industry's initial assumptions," says Tom Andersen. His duties with the NPD include supervising fields in the southernmost part of the NCS.

"Productive areas of such structures are soft and highly porous, and well stability and productivity present constant challenges."

The Ekofisk licensees resolved in 1983 to start a waterflood programme to press more oil out of the field. Three years later, crude output had doubled and estimated recoverable reserves were substantially increased.

"This underlines the importance of a constant government focus on IOR and research," says Mr Andersen. "We can benefit today from the NPD's early work on chalk, but continued attention is needed. A great deal of crude remains in place. The oil companies have reported more than 600 million barrels in possible additional volumes in chalk reservoirs.

"These fields; Ekofisk, Valhall, Eldfisk, Tor and Hod collectively represent over 30 per cent of the IOR potential on the NCS. Output from Edda, Albuskjell and West Ekofisk has ceased, but increased understanding of their reservoirs through chalk research could provide new opportunities for them."

The NPD takes the view that production from some of these fields could be resumed, he explains.

"In the early years of the JCR, its prime movers were the NPD and the Energy Agency. But the initiative has shifted over time to the oil companies. The programme is now in its sixth phase, although it has been hard at times to get new phases started because of the resource position for the companies.

"As long as we see the work being done, there's no problem with the authorities staying on the sidelines. However, it's important that both the companies and independent institutions are able to work on both short- and long-term projects."

The chalk fields in the North Sea still have a long producing life and opportunities for increasing their recovery factors are substantial, he says.

(This article was first published in the NPD journal Norwegian Continental Shelf)