OREANDA-NEWS. Alaska’s Prudhoe Bay producers are using innovating production and drilling techniques—some invented or first applied on the North Slope—to boost ultimate expected recovery rates.

Producers had originally estimated a 40% recovery rate when Prudhoe was first discovered in 1968. But now they estimate a 60% recovery, or as much as 14.1 to 14.2 billion barrels.

Gas re-injection for pressure maintenance and a miscible injectant made with natural gas liquids, used in enhanced oil recovery, have also been major factors in improved recovery, along with a field waterflood, said Bruce Laughlin, BP’s manager for Alaska reservoir development. BP is the operator and part-owner of Prudhoe Bay. Other owners include ConocoPhillips and ExxonMobil.

As an oil field, Prudhoe is showing its age. The Prudhoe Oil Pool, the field’s largest oil reservoir, is now producing about 250,000 b/d, or one-sixth of its original rate of 1.5 million b/d. Its wells once produced 10,000 b/d or more but many are now at 1,000 b/d or less.

The underlying reservoir, however, is still the largest slope producer and one of the largest US producing fields. It is also the economic linchpin of the North Slope, producing half of the overall slope output of about 500,000 b/d. It would be economically difficult to operate the Trans Alaska Pipeline System without Prudhoe, Laughlin said.

Over the years Prudhoe has become a kind of laboratory for new technologies, mainly because the field is big enough, and the returns great enough, that the producers can afford to take risks.

New technologies that Prudhoe Bay has fostered include “multi-lateral” wells, or a single surface well with several underground producing legs, which are in effect several wells drilled below surface. As many as six underground wells, or “sidetracks” are now drilled off a single surface well. These were first done on the slope and are now a mainstay of shale drilling that enabled US shale production to blossom.

Another procedure is coiled-tubing drilling, a radical innovation where drilling and well completions are now done routinely with low-cost coiled-tubing units rather than large rotary drill rigs.

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.

So far, about half of Prudhoe’s production to date, about 12 billion barrels, can be credited to production enhancements like waterflood and the other half to the gas-cycling and vaporization and the injection of miscible gas fluids as EOR, Laughlin said.

BP has also tested two new EOR techniques at Prudhoe Bay and Endicott, another North Slope field, and is now applying them worldwide. One involved a polymer injected to aid conventional waterflood, called “Bright Star.” The second is “Low Sal,” which uses low-salinity water in the waterflood, and which has shown very good results

Prudhoe still has a long life ahead as an oil producer but the producers are now planning a new role for the field, as a gas producer.

In the long run, thanks to gas production, there will be more oil recovery from the Prudhoe Oil Pool 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 which will decline further as production drops.

It’s a synergistic relationship. Gas production needs oil to share the infrastructure costs, because the economics of gas are thin and needs costs to be shared. 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, as an example, 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 remaining in the Prudhoe reservoir after the expected 14 billion barrels are produced. That’s a lot of oil left in the rocks, and a big target for the future assuming continued improvements in technology.