SHELL MARKS PROGRESS IN BRENT FIELD REDEVELOPMENT

Shell U.K. Exploration & Production expects to resume oil and gas production this month from Platform B in Brent field in the North Sea.

The platform has been shut in for I year so it could be upgraded as part of a massive field redevelopment pro-gram.

Three of four platforms in giant Brent field, which went on stream in 1976, are being refurbished under a 5 year, 1.3 billion ($2 billion) project that got under way in 1993 (OGJ, Apr. 12, 1993, P. 28).

Shell Expro, operating arm of the Shell U.K. Ltd.-Esso Exploration & Production U.K. Ltd. joint venture, will reengineer Brent platforms to allow a major change in managing the reservoir to increase recovery.

Walter Van Der Vijver, general manager of Brent field, estimated that 5 billion bbl of oil equivalent originally were in place in Brent reservoir, which lies about 3,000 m below the seabed.

Earlier this year, Shell-Esso had recovered about 40% of volumes in place, he said. Brent has I billion bbl of oil equivalent in place, he estimated, making Brent still the largest remaining asset base among U.K.'s fields.

"Brent's platforms were designed in the early 1970s, when companies were fairly naive about what they were trying to do in the North Sea," Van Der Vijver said.

"We did not even consider then what to do with Brent gas. This was an 'add on' consideration in the 1980s."

Brent currently produces 185,000 b/d of oil and as much as 500 MMcfd of gas from three platforms that serve 60 producing wells and 30 water injection wells.

Brent is under peak redevelopment this year, with 1,750 persons working offshore. Each of the field's four platforms have a dedicated flotel to house workers,

REDEVELOPMENT GOALS

Van Der Vijver listed Brent redevelopment objectives as:

Increasing recovery by 350 million bbl of oil equivalent.
Refurbishing to extend field life and bring platforms in line with U.K. safety legislation flowing from the Cullen Report of 1990.
Reducing long term operating costs.

"It would be impossible to do these three things separately," Van Der Vijver said. "So we opted to do them all in one go. However, Platforms B, C, and D can only be shut down one at a time because of gas contract commitments, so we had to opt for a 5 year project."

One of the keys to operating cost reduction is platform manning.

Brent C platform currently requires a crew of about 250, the same as Brent B before it was shut down. Brent B will require a crew of only 140 when it returns to operations this month:

Manning levels will be cut about in half for each of the platforms.

Shell-Esso reckons this will help halve operating costs, which have risen to about 3/bbl ($4.8/bbl) because of high maintenance requirements on the aging platforms.

Brent B, a three legged structure, had production facilities at the heart of its topsides before refurbishment.

"The old layout was not the optimum for safety," Van Der Vijver said. "There wasn't maximum distance between production and accommodation. After redevelopment, central process facilities have been decommissioned and a new reduction module has been installed as far from the new accommodation as possible."

RESERVOIR PROGRAM

Current production involves water-flooding of the reservoirs, with production plants on the platforms operating at 110-130 bar pressure.

To recover remaining liquids and transform Brent into a gas field, Shell-Esso plans to allow operating pressure in the reservoir to fall away, requiring production equipment working at only 36 bar pressure.

"There will be a period in which we will not have to inject water," Van Der Vijver, said. "Then we will go into a blowdown phase, when most of the remaining oil will be recovered through depressurization.

"We will pull as fast as we can on the wells and pump out aquifer. Water injectors will be used here as water producers. This will be a delicate, gradual process, which never has been tried on this scale."

Van Der Vijver reckons 40% of gas reserves would have been recovered without blowdown, and 80% with blowdown. Shell-Esso said Brent originally had 3.7 billion bbl of oil in place and 7.4 tcf of wet gas.

"Blowdown will liberate the gas cap and previously unrecoverable gas from residual oil," Van Der Vijver said. "The key is managing pressure downhole. Once we have depressurized we can then start up electric submersible pumps to continue production."

This process also is expected to increase recovery of oil reserves to 55% through depressurization from 40% of oil in place.

Oil production from Brent is expected to cease in 2005, while gas production is expected to continue until 2010. When B platform goes back on stream, C platform will be shut in for a similar program of work to take place mainly during next year's weather window. Then the process will be repeated a year later for D platform.

Brent A platform has been refurbished to meet new safety legislation but is not being redeveloped. Many of the staff who have gained experience on B platform will transfer to work on C, then to D.

Dick Parker, Shell Expro production director, said Brent redevelopment is expected to take 9 million offshore man-hr at a cost of 4275/hr ($120/hr).

This compares with the recent new development of the North Sea's Nelson field, where 2 million offshore man-hr were required at a cost of only (15/hr ($24/hr).

"These figures show the real impact of having to do much of the work offshore in brownfield engineering (reengineering)," Parker said. "Hence we need to minimize shutdowns and continue production during redevelopment wherever possible."

MOVEMENT OF EXPORTS

Brent's platforms are interdependent for exports. Oil moves through infield pipelines to C platform for export, while gas is routed in the field to A platform prior to export.

While B platform has been shut down, oil and gas were rerouted through crossover systems so the flow of production from other platforms could be maintained.

Shell-Esso had to shut down the entire field for only 17 days late in 1994 while crossover facilities were installed on B platform.

In the 56 weeks of B platform shutdown, Shell-Esso estimates it will have spent 2.8 million offshore man-hr and handled 15,000 metric tons of equipment involving a total 13,500 lifts, of which the heaviest was 3,080 metric tons.