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THE EXPLORERS 2007: BP ‘exploring’ in known fields Company uses technology to access vertical ‘string of pearls’ that lie above developed North Slope fields Kristen Nelson Petroleum News
BP Exploration (Alaska), no longer active in traditional exploration in Alaska, is focused on exploring for — and recovering — resources through technology.
BP is the operator at Prudhoe Bay, the largest oil field in North America, and at the Endicott, Milne Point and Northstar fields — all on Alaska’s North Slope, and is working on plans to develop its Liberty prospect through extended reach drilling from Endicott. BP is also a major working interest owner in the Kuparuk River field.
BP is a successful explorer worldwide, Scott Digert, the company’s resource manager for full-field waterflood in the Greater Prudhoe Bay business unit, told Petroleum News in an Oct. 9, 2007, interview.
The company has “done really, really well in remote basins,” areas like the deepwater Gulf of Mexico and in the deepwater off Angola, Digert said. But in Alaska, it wasn’t “seeing that same sort of success” and is now focusing on where it thinks the big remaining resources are in Alaska, “around the existing oil fields,” including undeveloped oil at Prudhoe Bay and heavy oil across the slope.
“We actually know where it is. The problem is getting it out,” he said.
Frank Paskvan, Prudhoe Bay western region subsurface development manager, said that at Prudhoe Bay, with 30 years of development, there have been “a series of projects, wells, reservoir techniques and facility expansions that have substantially improved our ultimate recovery.”
Seismic helps with recovery Original oil in place at Prudhoe Bay was 22.6 billion barrels of oil, Digert said, with an estimated recovery rate of 42 percent, or some 9.5 billion barrels. To date, more than 11 billion barrels have been produced and the belief now is that another 2 billion to 2.1 billion barrels, can be recovered.
“We know where the oil was to start — before we started moving things around — so now the question is, of the oil that’s left, where is it?”
One thing that helps with oil recovery is three-dimensional seismic with its “much finer vertical resolution.” Faults can be seen on a smaller scale, Digert said, probably around 20 feet vs. 60 feet originally.
Four-dimensional seismic, comparing seismic with that shot earlier over the same area, can show “changes in fluids or pressure from the injection that we’ve done,” he said, and “… helps you identify where are the pockets that you’re not sweeping out” either in the gravity drainage area or with water. “And now you can start targeting the sidetracks into these smaller and smaller remaining pockets of oil.”
And with new technologies — coiled tubing drilling and multilateral wells — “you can now start to envision how you can actually target these smaller and smaller pools, things that we couldn’t have even done two or three years ago.”
2,500 wells drilled Information on remaining oil also comes from “a pretty active appraisal and delineation effort within Prudhoe,” Paskvan said. Some 2,500 wells have been drilled, and starting in the mid-‘90s, BP did appraisal drilling “on what you might call initially discovered satellite reservoirs.”
This included delineating the western satellite pools and it also included the Put reservoir, “an accumulation that was included in the (Prudhoe Bay) initial participating area,” he said.
The Put was found in early field drilling and “we’ve gone back in the last half-dozen years, figured out where that hydrocarbon is gas and where that hydrocarbon is oil; where the reservoir quality is the best and where it’s not so good; where we should put in water injectors; and where we should put in oil producers,” Paskvan said.
“The Sag River formation falls in exactly the same category,” Digert said. “It’s something that we’ve known about; we’ve drilled through it.” The Sag is minor compared to the Ivishak, and sits right above it. It’s “much tighter. It’s still nice, light oil, but it’s a much more difficult reservoir to produce from. It’s thin and tends to be broken up by the faulting more; and it’s much less productive.” When Sag River is commingled with Prudhoe Bay Ivishak production, Sag may only be contributing 1 to 2 percent of the total.
Vertical development “The State of Alaska talks about a string of pearls,” Paskvan said. “… And I think they thought of it as … stringing pearls along the Barrow Arch,” and fields have been added laterally across the North Slope.
“But we’re stringing pearls vertically,” he said.
Referring to a schematic of stratigraphy in the western satellite area of Prudhoe Bay, Paskvan noted that the Schrader Bluff Orion accumulation lies above the lighter-oil Borealis pool — and both lie above a Sag-Ivishak accumulation. Above all is the Ugnu, the heaviest oil on the North Slope, which is not yet being developed.
Development started with the deeper, light oil. Deeper, lighter oils have a lower viscosity — they flow more readily. The Schrader Bluff-West Sak formation is heavier oil, but it can be waterflooded, and 100 million barrels have been produced to date.
The “transforming technology” for Schrader Bluff-West Sak production was horizontal drilling, Digert said, “drilling these long horizontal producing wells.” Production went from an initial 200 barrels per day with wells that dropped off quickly to 50 bpd with “wells that have come on above 1,000 barrels a day,” he said.
“They still decline pretty fast, but you’re starting from a much higher point so it’s been absolutely transformational in our ability to now drill this lighter heavy oil.”
Western region development Paskvan said the number of developed oil pools at Prudhoe has doubled in recent years.
Four of those accumulations — Aurora, Borealis, Orion and Polaris — are in the western region development area at Prudhoe Bay, the area west of the Kuparuk River.
“The western region is kind of a microcosm of the North Slope story, because you’ve got Ivishak development first (deeper, lighter oil), more than 250 million barrels produced to date from the western region,” from more than 220 wells producing some 50,000 barrels per day, Paskvan said.
“And we’re unlocking the heavy oil resource as we’re moving through … the reservoir,” he said.
Western region development started with “Eileen West End development in 1988, continued with the Borealis reservoir installation in 2001 of L and V pads,” he said.
Z pad is being expanded now and a new pad, I, is proposed for the far northwest corner with startup planned for 2011.
“We’re adding a gas partial processing plant on Z pad,” and will use gas-lift to get more oil out of the reservoir.
Paskvan said I pad appraisal wells were drilled in the winter and spring of 2006 and engineering is being done for the area today. Z pad expansion should be put in next year, he said, with startup in 2009; I pad would be installed in 2011.
This is the long-term forecast, he said. Full funding has not yet been approved.
“Western region development is … such a large program that we broke it into separate projects,” Paskvan said. Some elements are operating — which accounts for the 50,000 bpd from the area — other components have not been sanctioned, but are budgeted over the next five years.
$2 billion in future The Western region development is an ongoing five- to 10-year program with an estimated $2 billion in future investments.
At least half a billion has been spent recently on the western region, Paskvan said, excluding the original Eileen West End work.
The current project involves some 400 people.
Because Prudhoe is mature, Paskvan said the challenge is to process and reinject all of the gas that is produced with the oil and produce all of the seawater needed to supplement produced water for waterflood.
With equipment fully employed, there is no idle equipment for a project like western region development.
Because of that, sealift modules will be required for western region development, with three sealift modules planned for the project. That’s happening because “the drilling successes, the recovery successes, have created an opportunity to put in a new facility,” Paskvan said.
The target is a 2010 sealift, he said. Long-lead materials commitments have been made to preserve the option.
If that portion of the project is approved, the sealift modules would come up in summer 2010 and the startup target would be the fourth quarter of 2010.
Light oil technologies For lighter oil Prudhoe Bay development is underpinned by waterflood technologies and enhanced oil recovery through miscible gas injection.
Examples of new technologies used with lighter oils are low salinity waterflood and Nalco’s Bright Water™.
Talking about low-salinity waterflood, Digert said BP thinks that “by changing the chemistry of the water we inject and actually engineering that chemistry, we can increase recovery in zones that have already been flooded by mobilizing some of the oil that’s been left behind” by earlier waterflood. “And in some cases we see that as being as much as 10 percent of the oil that was originally in place,” and at BP’s Milne Point field, he said, the original oil in place is about a billion barrels.
Bright Water™ is useful in areas that are under waterflood, where the oil has been displaced. Water can move through the area where oil has been displaced, following a route through portions of the formation where the oil has already been swept.
Bright Water™ redirects the waterflood.
Particles are pumped into the injector well. The particles are so small they move into the sandstone. Heat in the reservoir expands the particles, which mesh up with other particles, filling the pores in the sandstone through which water had been moving, creating “a deep diverting block” which prevents water movement, Paskvan said, causing injected water to move into new areas where it can sweep remaining oil to producing wells.
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