Amoco Norway boosting reserves recovery at Valhall

Amoco Norway Oil Co. is using improved recovery programs to boost total oil reserves in Valhall field in the Norwegian North Sea to a targeted 1 billion bbl. When Amoco brought Valhall on stream in 1981, total reserves were estimated at 250 million bbl of oil. Now the estimate is a total of 700 million bbl of oil, including 50 million bbl in Hod satellite, which came on stream in 1990. Hugo Halvorsen, Valhall resource manager at Amoco, said the Valhall reservoir is high-porosity chalk, and

March 30, 1998

4 min read

Amoco Norway Oil Co. is using improved recovery programs to boost total oil reserves in Valhall field in the Norwegian North Sea to a targeted 1 billion bbl.

When Amoco brought Valhall on stream in 1981, total reserves were estimated at 250 million bbl of oil. Now the estimate is a total of 700 million bbl of oil, including 50 million bbl in Hod satellite, which came on stream in 1990.

Hugo Halvorsen, Valhall resource manager at Amoco, said the Valhall reservoir is high-porosity chalk, and initial expectations were that only 10-20% of oil in place would be recovered.

Now Amoco plans to squeeze the most out of this asset, through increasing reserves, reducing operating costs, and persuading third parties to use spare capacity on Valhall platform for processing of oil and gas.

Halvorsen said Valhall produces 100,000 b/d and Hod 10,000 b/d of oil: "In the last 2 years, we have been adding production as rapidly as we were in the previous 10 years. This is particularly through use of multiple-fractured horizontal wells."

Meanwhile, the company is working to reduce operating costs to $2.64/bbl this year from $3.35/bbl in 1997. One way has been an alliance with BP Norge AS to share helicopters and boats serving their nearby fields.

Improving recovery

Halvorsen said that, to reach the 1 billion bbl reserves target, Amoco will need to employ secondary recovery techniques, but he added that the key to enhanced recovery is improved reservoir characterization.

In 1996, Amoco formed an alliance with Baker Hughes Inteq, Stavanger, to bolster production in all its North Sea fields. Among techniques employed as a result of this alliance in Valhall are horizontal drilling, coiled tubing, multiple fractures, and reaming while drilling.

Janne Saurdal Kvernstrøm, Valhall drilling engineer, said Amoco has been performing simultaneous drilling operations in Valhall, enabling drilling, completion, and stimulating of wells to take place at the same time.

Cost savings

The Maersk Guardian jack up is stationed alongside a wellhead platform installed in Valhall in 1996. The rig carries out drilling work while a coiled tubing package on the platform is used for well service work.

"This was the first simultaneous operation of its kind," said Kvernstrøm,"and the 23/8-in. diameter, 6,000-m coiled tubing is the longest in the world and has been in use since 1996.

"The benefit of simultaneous operations is the accelerated rig schedule, because we don't have to stop drilling to wait for wells to be stimulated. Since 1996, we have saved 220 rig-days this way."

Kvernstrøm said a further benefit is that 2 million bbl of oil production has been brought forward to date, while coiled tubing is also faster and cheaper than snubbing wells using the rig.

Amoco reckons to have saved $18 million in Valhall through simultaneous drilling operations. A second coiled tubing unit is ready to be delivered to Valhall's main platform when a current well is completed.

The second coiled tubing unit will be used to redrill existing wells as multilaterals. The main platform's 30 well slots are taken up by existing producers. Nine wells have been drilled through the wellhead platform, which has 19 slots.

Frac solution

Lilly Skartvert, multiple-fracturing specialist in Valhall, said Amoco began drilling horizontal wells in Valhall in 1991 but experienced severe stability problems as boreholes in chalk collapsed.

The company began studying the feasibility of deploying multiple proppant fracture techniques in Valhall in 1995 and subsequently boosted production significantly.

"Since 1995," said Skartvert, "we have increased the lengths of horizontal sections in Valhall wells. We have successfully performed up to eight fracs/well, and the cycle time for fracs is down from 10 to 3 days.

"Production rates in Valhall's multiple-frac wells are up to three times as high as our earlier horizontal wells; they are achieving typically 7,000 b/d production compared with 3,000 b/d for non-prop frac wells."