New techniques boost Prudhoe crude oil recovery

Wesley LoyPetroleum News

BP is on track to recover an impressive 60 percent of the crude oil within Alaska's giant Prudhoe Bay field, according to an article in the latest issue of the company's international magazine.

A key factor is BP's intensive use of enhanced oil recovery, or EOR, techniques.

EOR works to push or sweep more oil from porous rock formations. The techniques depend on a ready supply of natural gas, which Prudhoe is blessed to have in great abundance.

The article in BP Magazine says the industry has three main ways to meet the world's growing demand for oil: making new discoveries, developing unconventional resources such as shale and oil sands, and enhancing recovery from existing fields.

EOR might lack the sex appeal of new discoveries or a shale oil boom. But it can make a huge contribution, the article says.

The article quotes a former president of the Society of Petroleum Engineers, Alain Labastie, who said the ultimate average recovery factor for oil fields worldwide is about 35 percent, meaning 65 percent remains stuck in the ground.

"If you drill a well into a reservoir and rely on natural pressure to force the oil to the surface, you will typically recover around 10 percent of the available volume in place," the article says. "Unless other forces act on the oil, pressure in the reservoir will naturally fall as it empties, until, eventually, there isn't enough to force up oil."

Producers long ago discovered they could maintain reservoir pressure by injecting water or gas underground. Water injection, or water flooding, is almost as old as the industry itself, and very important for BP's production, the article says.

"Injecting gas to maintain reservoir pressure is not as common as water injection, both because gas is a valuable -- and marketable -- commodity in its own right, and because more sophisticated machinery is needed to inject it. But it, too, is a technique that BP has used at large scale since the 1980s."

EOR generally refers to techniques that go beyond the simple injection of water or gas to maintain pressure, the article says.

One of the most widely used EOR methods is using steam to heat up viscous (thick) oil, making it easier to recover.

BP operates Prudhoe Bay on behalf of itself and other owners including ConocoPhillips and ExxonMobil. The field has produced since 1977, yielding about 12.4 billion barrels of crude thus far.

BP processes natural gas at Prudhoe for EOR uses.

The article quotes Bharat Jhaveri, BP senior adviser for gas EOR:

"We take the enormous quantities of gas produced from the field, almost as much gas as Britain uses every day, and put it through an enormous refrigeration plant -- effectively a $1 billion fridge -- which takes it to -40°C (-40°F). This enables us to isolate components in the gas, such as propane and butane. Then, we create one stream of 'lean' gas that's virtually all methane, and another stream of what's called miscible gas -- because it mixes with the oil, unlike water -- which includes propane, butane and carbon dioxide."

The miscible gas is injected into wells in the oil zone to boost crude recovery.

The gas often is alternated with injections of water to improve the sweep through the rock, the article says.

BP says its hydrocarbon miscible gas project in Alaska is the largest in the world.

The lean gas at Prudhoe Bay is used in another way.

Jhaveri explains that the whole Prudhoe structure originally was full of oil, but the field tipped and a gas cap formed atop the reservoir in space that previously held oil. As the gas moved in, the oil drained down over the sand grains and some of it got stuck.

"You end up with what's called 'relic oil' coating sand grains in the gas cap," Jhaveri says. "Only 8 percent of the oil sticks like this, so who cares? Well, we do actually, because there are 1 billion barrels of it at Prudhoe Bay."

The relic oil is patchy, lacking continuity, and can't flow. And pumping water through the well won't dislodge it.

"But, when you inject methane gas, components in the oil transfer to the gas," Jhaveri says. "It's analogous to having moisture on a surface and blowing warm air over it -- the moisture vaporizes. Then, we put the gas through the processing facilities, followed by the fridge again, which separates the vaporized oil, before recycling the gas to pick up some more oil. We've been doing this since 1987 and, eventually, it will help us to recover several hundred million barrels of extra oil."

Gas injection is great for Prudhoe Bay, but not so desirable at fields where gas would have to be imported, the magazine article says.

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