Producers still squeezing barrels out of middle-aged Prudhoe Bay
It has been almost 50 years since it was discovered and nearly 40 years since it started producing, but today the Prudhoe Bay oil field is still the engine that drives Alaska’s North Slope oil industry. The field produces about 250,000 barrels per day — one sixth of what it produced in the late 1970s — but that’s still about half of the state’s total production.
Without it, it’s unlikely the Trans-Alaska Pipeline System could operate economically. Even in its maturity Prudhoe is still the largest oil field in North America.
Prudhoe’s producing companies now say they now expect to ultimately recover approximately 14.1 to 14.2 billion barrels of oil from the giant North Slope field, about 60 percent of the estimated oil-in-place in the reservoir rock.
That’s according to Bruce Laughlin, BP’s reservoir management team leader. BP operates the Prudhoe Bay field on behalf of itself, as a co-owner, and partners ConocoPhillips and ExxonMobil.
When natural gas is produced commercially from Prudhoe — which is expected in about 2025 if a planned North Slope gas pipeline and liquefied natural gas export project is built — the equivalent of another 3.5 billion barrels of oil will be produced in the form of natural gas.
Originally, the field was expected to produce approximately 9.6 billion barrels of oil, or 40 percent of the oil-in-place (the oil physically in the reservoir rock), when it was discovered in 1968, but over the years the field operators employed innovative new technologies including gas re-injection and miscible injectant for enhanced oil recovery, some of them invented or first applied on the Slope, to push the oil recovery rate much higher, Laughlin said.
State Natural Resources Commissioner Mark Myers said the companies have done an excellent job of squeezing more oil out of the Prudhoe reservoir.
“It’s a good quality reservoir and they are employing very advanced technologies. We’re fortunate to have large, experienced companies working on the North Slope,” he said.
The Prudhoe Oil Pool (the largest oil reservoir in the Prudhoe Bay field) is now producing about 250,000 barrels per day and its wells once produced 10,000 barrels per day, or more. Many are now at 1,000 barrels per day or less. The field is in its middle age, but it is still the largest oil field in North America and it ranks among the 20 largest fields of the world, according to the Alaska Oil and Gas Conservation Commission, a state regulatory agency.
Because of its size, however, Prudhoe has also become a laboratory for new production and drilling technologies because the expected return, the amount of additional oil recovery, is well worth the risk of experimentation.
New technologies Prudhoe Bay has fostered over the years include “multi-lateral” wells, or a single well from surface with several underground producing legs, in effect several wells, were first done on the Slope and now as many as six underground wells are drilled off a single surface well.
There is also coiled-tubing drilling, a radical innovation where drilling and well completions — not just well repair jobs — are now done routinely with low-cost coiled-tubing units rather than large rotary drill rigs. This was also first done on the North Slope
Extended-reach and horizontal producing wells were developed elsewhere but applied on a scale at Prudhoe Bay that had not been done before. Prudhoe Bay producers are also employing water injection into the Prudhoe Oil Pool reservoir “gas cap,” the layer of gas overlying oil, to boost pressure in the reservoir. That has not been done elsewhere at the scale Prudhoe Bay producers are doing it, Laughlin said.
The gas-cap water injection has resulted in 100 million to 120 million barrels of additional Prudhoe Bay oil recovery so far and another 170 million to 200 million barrels of added production are estimated, he said
Also, there is now one of the world’s largest gas cycling operations at Prudhoe Bay, with approximately eight billion cubic feet of gas produced daily, the gas coming up the wells along with oil, and water. The bulk of the gas is injected back underground after removal of gas liquids, which are injected into TAPS to mix with crude oil, or used as miscible injectant for enhanced oil recovery.
The injected gas, now “lean” with liquids having been removed, circulates back through the reservoir rocks, soaking up crude oil molecules as it moves back toward producing wells. When the gas comes, again, up the producing wells, this process known as vaporization brings more oil with it.
So far, about half of Prudhoe’s recovery to date, about 12 billion barrels, can be credited to production enhancements like waterflood and the other half by the gas-cycling and vaporization and the injection of miscible gas fluids, Laughlin said.
Assuming continued availability of gas liquids for miscible injectant, another 200 million barrels of crude oil is expected from all reservoirs in the Prudhoe Bay Unit by 2024, he said.
In producing and injecting about 8 billion cubic feet of gas daily, Prudhoe now has the world’s largest gas handling and injection plants.
BP has also tested two new enhanced oil recovery, or EOR, techniques at Prudhoe Bay and is now applying them worldwide. One involved a polymer injected to aid conventional waterflood, called “Bright Star.”
Prudhoe still has a long life ahead as an oil producer, but the producers are planning a new role as a gas producer. Prudhoe will be the lynchpin for a $50-billion-plus gas pipeline and LNG project now planned by the companies with the State of Alaska as a partner.
Prudhoe holds about 23 trillion cubic feet of gas and would supply about 75 percent of the 3.5 billion cubic feet of gas needed daily for the planned Alaska LNG Project (the remaining 25 percent of gas will come from Point Thomson, a new gas field being developed east of Prudhoe Bay).
Production of Prudhoe’s gas will result in an additional 3.5 to 3.6 billion barrels of oil equivalent (i.e. gas expressed in terms of equivalent energy as crude oil).
Thanks to gas production there will be more oil recovery from the Prudhoe Oil Pool in the long run because infrastructure maintenance and operations will be shared between gas and oil production. Those costs are now being spread across a diminished number of oil barrels being produced that will decline further as production drops.
After 2025, when the gas project is to start up, gas will help shoulder the costs, which will extend Prudhoe’s life as an oil producer.
It’s a synergistic relationship, too. Gas production needs oil to share the infrastructure costs, because the economics of gas are thin and needs costs to be shared by oil. Conversely, sharing of the costs will extend Prudhoe’s life for decades, resulting in more oil recovery as operators figure out ways to tap bypassed pockets of oil.
One pocket of bypassed oil is in the lower parts of the Prudhoe Oil Pool reservoir. There is an estimated 1.2 billion barrels in a tar layer at the bottom, and while not much of this is likely to be produced, there are accumulations of oil of higher quality just above it that might be economic to tap, Laughlin said.
Overall, there’s about 10 billion barrels of oil-in-place remaining after the expected 14 billion barrels of recovery.
“That a big target for future, assuming continued improvements in technology,” he said.