 "Photo: Maisie Crow")
Brace yourself. This subject matter is heavy; 7 billion tons of heavy. How do you begin to comprehend that weight? Seven billion tons is 1,995,000,000 Chevy Suburbans. It's 107,700,000 jet airplanes or 58,821 full oil tankers. Does that sound like a lot? Try to grasp this: 7 billion tons is also the amount of carbon dioxide released into the air every year by human activities.
UT researchers hope to reduce that number by locking the gas in a place where it can do no harm: deep underground.
Why is carbon dioxide a concern?
Carbon dioxide, along with methane, water vapor and other molecules, is a greenhouse gas, explains Jay Banner, a UT professor of geological sciences and the director of the Environmental Science Institute.
"The way that greenhouse gasses work is that they absorb the outgoing radiation from the surface of the Earth," said Banner.
When sunlight hits the Earth as ultraviolet radiation, the Earth absorbs it and emits it back as infrared radiation. Instead of escaping into space, this energy is absorbed by greenhouse gasses and reradiated into the atmosphere. This process warms the atmosphere, said Banner.
The extent of the impact human-made carbon dioxide has had on the global climate remains controversial. Yet Banner maintains that two related principles are established: Greenhouse gasses exist and warm the Earth's atmosphere, and the amount of carbon dioxide in the air is rising. While there is a consensus in the scientific community that the warming observed in the last half of the 20th century is most likely due to human activities, the Bush administration thinks that natural variation of Earth's temperature cannot be ruled out as the cause, Banner said.
Controversy aside, many nations last week began following the Kyoto Protocol, an agreement to reduce greenhouse gas emission drafted by the United Nations Framework Convention on Climate Change in 1997. While the United States hasn't ratified this pact, the Environmental Protection Agency has projected that by 2100, carbon-dioxide levels could be as much as 150 percent higher than they are now.
One way to reduce these emissions is to drill wells deep into the Earth's crust and pump pressurized carbon dioxide into salt water aquifers, where it is sequestered for geologic time scales. This might sound bizarre, but researchers feel it is a viable option. Norway is already utilizing this storage method at Sleipner oil field in the North Sea.
Storage underground
About 5,000 feet below the Earth's surface are brine aquifers - thick layers of porous rock that are submerged in salt water, says Steven Bryant, an assistant professor in the Department of Petroleum and Geosystems Engineering at UT. These aquifers are too deep to be used for drinking water, have large volumes and can potentially store gasses for thousands of years.
The only problem is knowing what will happen to the carbon dioxide once it is down there.
"It will cost a certain amount of money, and you'd like to make sure that it's going to stay there," Bryant says.
When carbon dioxide is injected into a brine aquifer, one of four things will happen to it, says Robin Ozah, a graduate student in Bryant's group. The gas could dissolve into the water; the gas could react with the rock and form a solid; it could float to the top of the aquifer as free gas; or it can be trapped in rock pores or in between rocks and held there by capillary forces.
Gas trapped through capillary forces is called "residual gas" and is more stable than "free gas," or gas that merely floats inside the aquifer. If the majority of the gas can be trapped in rocks, then it will stay underground for thousands of years. If there is a lot of free gas, the gas sits at the roof of the aquifer in much the same way that a helium balloon is trapped by a ceiling. As long as the aquifer roof is flat, and there are no cracks, the gas will stay underground. Without this geologic seal, the gas will eventually escape, said Bryant.
Bryant's group has focused on the conditions needed to reduce the amount of free gas while maximizing the amount of carbon dioxide the aquifer can store. Group members focus on trapping the gas as residual gas. Predicting how much gas can be stored and what will happen to it has been the focus of computer simulations run by Ozah and Justin Ferrell, another graduate student.
Additionally, Chris Irle, a geosystems engineering senior, has been running physical experiments with sand and oil to see how much material can be locked away as residual.
Ozah has shown that if 50 million standard cubic feet of carbon dioxide per day are pumped into a sequestration well for 50 years, it is possible to contain the gas so that in 10,000 years, no free gas will even approach the roof of the aquifer.
Besides tackling the physical questions of carbon dioxide sequestration, Bryant's group has also researched the economic implications of this plan.
"The cost is about $1.15 per ton of carbon dioxide, surface to ground," Ferrell said. "Surface to ground means the price to put the gas down there - any other significant cost is from the pipelines to get the carbon dioxide to the well."
As a result, the most ideal locations to use this technique are close to where large amounts of carbon dioxide are produced.
From theory to practice
Susan Hovorka of the Bureau of Economic Geology in the Jackson School of Geosciences, leads a team in East Texas that has already placed a large amount of carbon dioxide under the ground to see if it behaves according to models.
The Frio Brine Pilot, a test well for sequestering experiments, sits surrounded by oil wells in various states of decline, 30 miles northeast of Houston. Several refineries are close by, producing pure streams of carbon-dioxide gas as a by product.
"Seventeen percent of the United States carbon dioxide emissions are from the Gulf coast area," says Hovorka. Additionally, because of the presence of oil in the area, the ground formations are well-studied, making it an ideal location for a carbon dioxide sequestering experiment.
Last September Hovorka and fellow researchers deposited 1,600 tons of carbon dioxide 5,050 feet into the ground. They then monitored the area to see where the carbon dioxide went, how fast and in what state it traveled - as free gas, residual gas or gas dissolved within the salt water.
An obseration well 100 feet away verified that the models they were using were correct. It collected data in December and will collect more today, said Hovorka. If Bryant's model is correct, there will be a large amount of carbon dioxide shown as residual gas in the rock close to the well.
While this information will improve the understanding of underground carbon storage, research in different areas, such as fuel efficiency and alternative energy sources, is needed to combat the carbon dioxide problem as well. Yet, researchers hope that with the contribution of this research, the 7 billion tons can be brought down to a more fathomable number.
"I don't think that there is one magic bullet that will save us all. There has to be a combination of sequestration, more fuel efficient vehicles, reassessment our energy use and energy sources, and personal conservation," Banner said.
Finding out More
For more information about global warming and other environmental issues, log on to the Environmental Science Institute's main page at: www.esi.utexas.edu
You can also attend the Abrupt Climate Change Symposium on April 15. Morning and afternoon sessions will be held in ACES, with an evening session in WEL 2.224.
Be the first to comment on this article!