Scientists deepen confidence in technique to reduce greenhouse gas emissions

AUSTIN, Texas—Each year in the U.S., we burn enough fossil fuels to blanket the country a foot (30 centimeters) deep in carbon dioxide. That’s according to Sue Hovorka, a senior research scientist at the Bureau of Economic Geology at The University of Texas at Austin's Jackson School of Geosciences.

Fortunately for us, CO2 isn’t heavy enough to settle to the bottom of our atmosphere. “Otherwise, we’d have choked on our own emissions many years ago,” said Hovorka. Still, carbon dioxide is a powerful greenhouse gas. We may not be choking on it, but we are baking in it.

Hovorka is the lead scientist for the Frio 2 Brine Test, an experiment to test the viability of carbon sequestration—storing CO2 in the ground to reduce emissions to the atmosphere. For a week starting Sept. 25, she and her team pumped nearly 500 metric tons of CO2 a mile below ground in east Texas. Over the following year, she and other scientists will monitor how the CO2 moves through the subsurface.

Hovorka’s team predicted that it would only take several days for the plume of CO2 to stop expanding through the brine and porous sandstone in the subsurface. Early indications show this is just what happened. If the CO2 continues to remain in place over the coming year, it will bolster scientists’ confidence that carbon sequestration works in this particular geological setting. It also means they understand the physics of the subsurface well enough to predict how much CO2 other areas can store.

Animation: Simulation of CO2 flow in the Frio Blue sand (Visualization by Christine Doughty, Lawrence Berkeley National Lab. Petrophysical data collected by Jeff Kane and Mark Holtz. Simulation using TOUGHT2.) View animation.

Perhaps most importantly, the test has demonstrated the effectiveness of new tools and techniques for monitoring CO2 underground. These tools could become critical to companies wishing to buy or sell carbon credits in the future (see sidebar: A Market for Sequestration).

According to Ian Duncan, associate director of the Bureau of Economic Geology, many experts in the U.S. power industry believe that some form of carbon cap and trade system will eventually be enacted by the federal government. In fact, as states such as California begin to regulate carbon emissions, some industry insiders are calling for federal regulations so they can have a consistent set of guidelines to operate by.

“If a CO2 sequestration market is going to develop, then you have to have monitoring to make sure the CO2 is going to stay there,” Duncan said.

The Frio Brine Tests

The Frio 2 Brine Test, funded by the National Energy Technology Laboratory of the U.S. Department of Energy, is the second in a series conducted by the Bureau of Economic Geology to determine the long-term feasibility of carbon sequestration. The original test, conducted in 2004, was the first such test ever carried out in the U.S.

The "tubing conveyed seismic source" is designed to fit inside the injection well, between the injection pipe and the well casing.  See larger image.

Hovorka emphasizes the collaborative nature of the Frio tests. She is the lead scientist and the Bureau is the lead institution, but considerable work is also being done by Lawrence Berkeley National Laboratory, Oak Ridge National Laboratory, the U.S. Geological Survey and others.

The test site is in the South Liberty oil field near Dayton, Texas, 40 miles northeast of Houston. The spot sits above the Frio Formation, which stretches along the Gulf Coast from Alabama to Mexico and contains porous sandstone and brine.

Several times a day during injection, trucks hauling 20-ton tanks of cold liquified CO2 arrive at the test site, where it is transferred to two 70-ton storage tanks. The CO2, which comes from a natural reservoir near a Mississippi salt dome, is transported most of the way by train.

During injection, the liquid CO2 is pumped through a heat exchanger, which warms it up to 21 degrees C (70 degrees F), converting it to a gas. Then it is pumped through the injection well head and a mile down the well. The CO2 enters the porous sandstone and brine through perforations in the well casing and spreads out in a plume.

For the scientists, knowing just what’s going on a mile below their feet is part art, part science.

Tick, Tock

Tom Daley is a geophysicist from Lawrence Berkeley National Laboratory. He developed a novel seismic source that fits down in the injection well between the injection pipe and the well casing. There is about an inch-and-a-half of doughnut-shaped space between the two pipes in which to place what he calls the “tubing conveyed seismic source.”

The ticking sound of a seismic source travels from the injection well (left) to an array of 24 seismic sensors in the observation well (right). When CO2 crosses between the source and a sensor, the sound slows down. Scientists use this to track the spread of the CO2 plume. Illustration by Jonathan Ajo-Franklin. See larger image

Without this unique approach, researchers would have to stop the injection, remove equipment and replace it with the seismic source each time they wanted to take measurements. Instead, the researchers get a precise, continuous chronology of how the CO2 moves without disturbing the injection.

At an observation well about 100 feet (30 meters) away, an array of 24 seismic sensors situated at regular intervals down the well act like tiny microphones, picking up the regular ticking of the seismic source over in the injection well.

As the ticking sound travels through the ground, it travels at a certain speed. But once the spreading CO2 begins to cross between the source and a particular seismic sensor in the observation well, the tick arrives a little later. That’s because sound travels slower in CO2 than it does in rock or water. Daley collects the data in a small trailer on site. He tracks the advancing plume of CO2 by noting when the travel time changes for each sensor in the array. Unlike most seismic images, it doesn’t provide an image of the plume, but it gives a very good sense of how fast it’s spreading as the CO2 crosses the “trip wire” ray paths.

As predicted, he observed the plume spread across the space between the wells in just a few days.

Residual Saturation

A second line of research looks at how saturated the brine and sandstone become with CO2. In other words, how sticky are the materials?

Susan Hovorka demonstrates how CO2 gets trapped in the brine and sandstone pores of the Frio formation: "If you were an ant down where the CO2 is being injected, this is what you would see." She uses a jar of glass beads, water and oil (red) to show how residual saturation works. She turns the jar upside down and as the oil rises, some gets trapped between the beads. Photo by Marc Airhart.  See larger image.

One way to measure that is to use a “wire line log”—a device that is lowered into each well to measure chloride. CO2 pushes saltwater away during injection, resulting in falling chloride levels, allowing the wire log to indirectly measure how much CO2 is present. Another method looks for a series of tracers in the observation well, chemicals that travel along with the CO2 to track how much of the gas is dissolving into the brine.

“It’s like a person with a red shirt in a race, you can see how the group is traveling by looking at the person in red,” Hovorka said.

Before the first tests, the scientists had predicted that an effect called residual saturation, caused by capillary forces, would cause the brine-filled pores in the stone to trap and hold about 20 percent CO2. The other 80 percent moves on to the next set of pores, and as it moves, it’s continuously diminished. In other words, the plume smears out. Hovorka said the effect is intuitive.

“It’s the same reason you can’t get grease off the stove,” she said. “You can’t wash it loose with water, you have to use soap.”

The 2004 test confirmed this prediction and now initial results from the 2006 test seem to reconfirm it. “It means we got the physics right,” said Hovorka. It also means she and her colleagues can predict the CO2-trapping ability of other sites before injection begins, a powerful and necessary tool for carbon sequestration to become a common practice.

Tasting the Soup

A third line of research involves directly sampling gasses and fluids from the observation well to detect how the chemistry changes down below.

Injection well (foreground) with CO2 storage tanks and heat exchanger (background). Photo by Marc Airhart.  See larger image.

Samples are brought up using a U-shaped tube with an opening at the bottom. High pressure nitrogen is pumped in to one side of the so-called U-Tube, forcing gasses and fluids from the bottom up and out the other side. Researchers from Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory conduct chemical analyses on the gasses using a gas chromatograph and other tools. Researchers from the U.S. Geological Survey (USGS) analyze the fluids for acidity and salinity and the presence of metals, organic compounds and other substances.

Following the first Frio test, the USGS team announced that CO2 injected at the Frio site causes the brine at depth to become acidic. The acidic brine in turn dissolves some of the rock and minerals it comes into contact with, adding iron and other metals to the salty water. It can also allow the brine and CO2 mixture to open new paths through the rock.

When those results were published last July in the journal Geology, there were concerned media reports about the potential for the escape of salty brine and contamination of shallow water supplies.

“Above the Frio formation, you have several hundred feet of shale,” said Yousif Kharaka, head of the USGS team. “That’s a barrier. The CO2 might dissolve a little carbonate and create a path. But when you have several hundred feet of shale, it can’t escape. The Frio was and still is a good place to put a lot of CO2.“

Also, the leaching of iron and other metals might not be as severe as originally thought. Some of the metals contaminating the samples appear to come from the well casing, and not the brine.

Hovorka does not think acidification and mineral leaching will turn out to be a deal breaker. She is actually more concerned about displacement of salty water. As the CO2 is pumped in at high pressure, some of the salty water is displaced, potentially contaminating fresher water above which might be used for drinking. Or it might alter overlying ecosystems. This side effect might limit how much or how long carbon sequestration can be safely done in an area.

Several times a day during injection, trucks hauling 20-ton tanks of cold liquified CO2 arrive at the test site. The CO2, which comes from a natural reservoir near a Mississippi salt dome, is transported most of the way by train. Photo by Marc Airhart. See larger image.

More Work Ahead

There appears to be no shortage of places to store the world’s excess carbon. By some estimates, the sediments below the North Sea could store all the CO2 emitted by Europe’s power plants during the next 600 years. Researchers at the Massachusetts Institute of Technology, Harvard University and Columbia University estimate that the U.S. coast has enough storage capacity for thousands of years of anticipated CO2 emissions from the U.S.

“There appear to be no showstoppers,” said Howard Herzog, research engineer and expert in carbon sequestration at the Massachusetts Institute of Technology. “But there are many questions and uncertainties that need to be addressed. Projects like the Frio Brine will help us address those issues.”

Hovorka said so far, the Frio 2 test results are very promising. But there will be several more months of monitoring and analysis before definitive results will be published. Even after that, there will still be much more work to do.

“Science is based on duplication of results, changing the experimental conditions, testing different rock fluid systems,” she said. “The Frio results answer a few questions, but mostly they let us design better tests to ultimately reach durable conclusions.”

Tom Daley from Lawrence Berkeley National Laboratory in the "dog house," analyzing seismic data from the observation well. Photo by Marc Airhart. See larger image.

Hovorka is discussing with colleagues the possibility of more tests at the Frio site, as well as the Mississippi salt basin and other sites along the Gulf Coast. She is also participating in tests to evaluate CO2 storage at a site in the Permian Basin near Snyder, Texas, with a long history of Enhanced Oil Recovery—the practice of pumping CO2 into the ground to increase oil or gas production.

“To establish carbon sequestration as a viable CO2 mitigation option,” said Herzog, “we need both pilot projects, like Frio Brine, and large scale projects, about million-ton-per-year injections, to help resolve the uncertainties. The Frio Brine project is important because it is one of the first steps on this road to commercialization.”

Carbon sequestration tests are being conducted around the world, including one in the Norwegian North Sea and another in Canada. Since the Frio tests began in 2004, seven regional carbon sequestration partnerships have been created across the U.S., with funding from the Department of Energy. These partnerships are beginning to evaluate possible locations for carbon storage, as well as the technologies and public policies that might make the practice feasible.

Chart showing different methods for storing CO2 deep underground. By the U.N. Intergovernmental Panel on Climate Change. See larger image.

Many of the scientists involved with the Frio tests said they feel it is their ethical responsibility to try to find solutions to human-induced greenhouse warming.

“Carbon sequestration is not a silver bullet,” said Herzog, “but it has the potential to play a major role in controlling greenhouse gas emissions.”

Kharaka said we’ll continue to use fossil fuels for several more decades. “And to me, carbon sequestration is probably the answer and I think we could do it right so that we don’t lose too much of it or contaminate shallow water that we use for drinking or other purposes,” he said. “I think it can be done, I just think we need to be careful.”

For more information about the Jackson School contact J.B. Bird at jbird@jsg.utexas.edu, 512-232-9623.