A well is a hole in the ground. Sometimes it has water in it. Sometimes it has oil in it. But increasingly, in southeastern Saskatchewan, it has carbon dioxide (CO2) in it.
Apache Canada Ltd. recently began injecting CO2 into its oilfield in the Midale area. EnCana has been injecting CO2 into the Weyburn oilfield since 2000.
They’re not doing it just for the fun of it, of course. Pumping CO2 into those oil fields also pumps new life into them, by allowing more oil to be pumped out of them.
The Weyburn oilfield covers more than 180 square kilometers, and the 1.4 billion barrels of oil it’s estimated to have contained when discovered in 1954 makes it one of the largest medium-sour crude oil reservoirs in Canada.
Getting all the oil out of any oilfield is a challenge. Some can be readily pumped out just by drilling a hole in the ground—but an oil well’s peak production usually lasts only one to three months, then slows. You can drill new wells for a while, but eventually, production from the entire field tapers off.
The next step is usually waterflood injection: water is pumped into the reservoir to increase its pressure and push oil to existing wells. Weyburn went on waterflood in 1964.
In the Weyburn field, oil is retained in two very different layers of rock: the Marly zone, made of a chalky dolomite, and, underneath it, the Vuggy zone, made of highly fractured (and thus highly permeable) limestone. Water did a fine job of flushing oil from the Vuggy zone, but didn’t make much headway in the much tighter Marly zone.
CO2 miscible flood technology injects not water, but almost pure, highly pressurized carbon dioxide. It’s “miscible” because the CO2 actually mixes with the oil. In effect, it become carbonated—and just like any other carbonated liquid, it expands. (Think of a what happens when you open a shaken can of pop.) This forces it through the tight openings in the rock and to the waiting extraction wells. The process is also helped by the fact that the oil/CO2 mixture is less viscous than the oil by itself, and by the fact that the mixture acts as a solvent, dissolving some of the rock and thus making it more porous.
The results are impressive. With waterflood technology, the field is thought to have given up about 30 percent of its overall contents. CO2 miscible flood technology should boost that to 46 percent—or, to put it another way, an additional 130 million barrels of oil should come out of the Weyburn field over the next 30 years, with production reaching 30,000 barrels of oil per day by 2008, compared to just 10,000 barrels of oil per day without CO2 injection.
Apache’s forecasts for its Midale unit are similar. The Midale oil pool was discovered in 1953; Apache’s Midale Unit was formed in 1962 as a waterflood project. To the end of 2004, 131 million barrels of an original 515 million barrels in the unit had been recovered; CO2 injection is expected to result in another 45 to 46 million barrels of recoverable oil.
The CO2 comes from the Great Plains Synfuels Plant, located near Beulah, North Dakota. Built during the 1970s energy crisis, it is the only active plant in the U.S. that produces synthetic natural gas from coal. The CO2, derived from one of the plant’s products previously sold as a low-value fuel, is delivered through a 325-kilometre pipeline.
Five thousand tonnes of CO2, which would otherwise have gone into the atmosphere and contributed to global warming, is injected into the Weyburn field every day. Over the 30-year lifetime of the project 20 million tonnes will be sequestered; another 8.75 million tonnes will be sequestered in the Midale reservoir. Since, on average, each Canadian’s activities results in the release of 4.5 tonnes of CO2 a year into the atmosphere, and Saskatchewan’s population is around one million, you could say the two projects will together lock away the equivalent of more than six year’s worth of the entire province’s carbon dioxide emissions.
And according to a study conducted by Regina’s Petroleum Technology Research Centre—the first large-scale study in the world on the geological storage of CO2 in a partially depleted oil field—only 0.2 percent of that CO2 will find its way into the atmosphere in the next 5,000 years.
More energy, less pollution: not bad for a bunch of holes in the ground.
