Prudhoe’s oil and gas future

July 27, 2009
Forty-one years after discovery on Alaska's North Slope, Prudhoe Bay field remains the largest oil-producing field in the US.

Forty-one years after discovery on Alaska’s North Slope, Prudhoe Bay field remains the largest oil-producing field in the US.

It is approaching time for the field, producing 400,000 b/d, to begin a transition to gas sales, said Scott Digert, waterflood/full field resource manager for BP Exploration (Alaska) Inc. BP’s Thunder Horse field in the Gulf of Mexico is a close second.

Prudhoe has produced 12.5 billion stb of oil and appears headed towards 14 billion bbl of ultimate oil recovery, up from the 9.6 billion bbl estimated at discovery. Digert reckons that half the recovery above the initial figure is due to increased gas cycling.

The 234-sq-mile reservoir is an anchor for 25 other ANS producing fields. BP operates Prudhoe for itself and other owners.

Field life conceivably could reach 100 years, said Digert, who came to BP from ARCO Oil & Gas Co. in 2000.

Prudhoe Bay today

Besides being an oil supergiant, Prudhoe has long been known to be a gas supergiant without a market.

Discovery well celebrations were muted because the wells penetrated the giant gas cap weeks before hitting oil (OGJ, Jan. 22, 1968, p. 40; Feb. 26, 1968, p. 55).

The field is nearly fully developed, Digert said. It has 2,500 penetrations, including 1,200 active wells. Almost all new wells are horizontals, including Alaska’s first stacked hexalateral with 29,198 ft of hole drilled in 2008.

Prudhoe and its core satellites have more than 1,000 producing wells, 30 gas injectors, and 140 water injectors.

Wells are drilled radially from 42 central pads. Well spacing is 160 acres broadly and 80 acres in some areas.

Six gathering stations handle produced oil and associated gas, sending oil to the Trans-Alaska Pipeline and gas for natural gas liquids separation and reinjection to maintain field pressure.

The field produces as much as 8.8 bcfd of gas in winter (5 bcfd in summer) that goes into an 8.5 bcfd gas processing plant, the world’s largest. Then 450 MMcfd goes for fuel, making BP Alaska’s largest gas user.

The plant extracts 70,000 b/d of NGL, of which 25,000 b/d goes to Kuparuk field for blending into miscible injectant. Some NGL is blended with produced gas to make miscible injectant for Prudhoe, and the rest goes down the oil pipeline.

Gas and water injection have slowed the field’s decline rate the past 2 years to 1-2%/year from the 7-8%/year averaged since 1988. And 450,000-600,000 b/d of water injected into the gas cap supports at least 10,000 b/d of current oil production and could lead to recovery of more than 150 million bbl of oil, Digert said.

Gas outlook

The length of Prudhoe’s future depends on continuing to produce oil while beginning to sell its other resource, gas.

Prudhoe, with an estimated 24 tcf of recoverable gas, and Point Thomson, 60 miles east with more than 8 tcf and 200 million bbl of condensate estimated recoverable, are the slope’s two chief gas reservoirs (OGJ Online, July 15, 2008). Other gas may yet be found.

Options being evaluated to move gas from the ANS range up to a 48-52-in., 4-5 bcfd pipeline (OGJ Online, July 6, 2009).

A 20 billion bbl resource of viscous and heavy oil extends from western Prudhoe across through Milne Point and Greater Kuparuk. With viscosity in the hundreds or thousands of centipoise, the deposit has not been proven economic. A research group is working on the problems.

The oil pipeline corridor’s next license extension date is 2075.

“At the very high oil prices we saw last year, there could be cases that extend that far out,” Digert said. “With gas and with the successful major development of heavy oil, those things look possible. When we look at it under today’s prices, we’re not going to make it out that far.”

Beside the 1 million hp gas processing and compression plants is a site where a gas pipeline consortium could build a 2-4 bcfd gas treatment plant. It would strip carbon dioxide, dry residue gas, set calorific values, and compress to 2,500 psi from 650 psi for shipment down a gas pipeline. Gas shipments would begin around 2018 at the earliest.

Digert said, “While Prudhoe is ready and capable of supporting a gas sales project in this timeframe, a project would also benefit from having other gas sources available at the start. This would allow us to optimize gas offtake from Prudhoe and maximize both oil and gas recovery.”