Bury It Deep

By Gregg Kleiner

Despite tree planting campaigns, solar energy installations and “no car” days, carbon dioxide continues an inexorable rise in the atmosphere. And as the signs of global warming mount — rising seas, extreme storms, melting ice sheets, longer fire seasons — world leaders struggle to find solutions.

Instead of looking up at the sky for a fix, an Oregon State University engineering professor is focused underground. She hopes to buy time for a transition to renewable energy sources and other approaches to the climate crisis.

Dorthe Wildenschild leads a study of efforts to capture the carbon dioxide (CO₂) released from the burning of fossil fuels and inject it a mile or so deep into the Earth, where it would remain locked away. Ironically, the process stems from methods used by the fossil-fuel industry to extract harder-to-obtain oil trapped in small pores of geological formations near existing wells. Companies pump CO₂ into oil fields where it mixes with and releases the black liquid from the pores in the formation, so the oil flows more freely toward the production wells. The process is known as CO₂ Enhanced Oil Recovery.

Researchers worldwide are exploring ways to use this same process to sequester CO₂ deep in the Earth, so the greenhouse gas can’t contribute to global warming. CO₂ sequestration is sometimes called carbon dioxide capture and storage, or CCS.

“Many of the scientists working on this CCS technology are the same people who developed the technology for the extraction of fossil fuels,” says Wildenschild, a professor of environmental engineering at Oregon State. “Many of us used to work on getting oil out of the ground more effectively, both for oil recovery and to clean up groundwater reservoirs, but now we work on putting CO₂ back where it came from.”

Because carbon dioxide is a gas, however, pumping it back into the Earth’s crust presents several challenges. CO₂ can migrate to the surface through geologic fault lines or well boreholes and escape into the atmosphere. It can also react with other subsurface deposits and contaminate groundwater.

Trapping CO2

Wildenschild is an international expert in this subterranean world, where the vagaries of geology affect the movement of the water and oil that underlie, literally, much of the modern economy. Named a Henry Darcy Distinguished Lecturer in Groundwater Science in 2014 (she gave 48 talks around the world that year), Wildenschild specializes in what scientists call “capillary trapping.”

“It’s like holding water in a straw by placing your finger over one end of the straw,” she says. “The capillary trapping technique we’re working on increases the safety of sequestration dramatically, because it holds the CO₂ in place by capillary forces so it can’t migrate to the surface.”

When injected underground, CO₂ displaces fluids occupying pore spaces that fill the rock like holes in a sponge. When injection stops, capillary action holds the CO₂ in place.

Before researchers inject CO₂ into the subsurface, they concentrate and pressurize the gas to transform it into what scientists call a supercritical state. In this form, CO₂ has the properties of both a liquid and a gas. The transformed CO₂ can then flow through rock like a gas and mix with other subsurface fluids like a liquid. And because of the temperature and pressure at the depth where it is injected, it will match the state of CO₂ that is already present underground.

Several other methods can be used to sequester CO₂ underground. By injecting it below layers of impermeable rock, the gas can be physically trapped. It can also be dissolved in a brine and sequestered by another process called mineral trapping, which gradually transforms the CO₂ into rock. However, both these methods are slow and carry risks that make them less desirable than capillary trapping.

Seeing Inside Rocks

In the Advanced Imaging Facility in Johnson Hall, the recently opened home of Oregon State’s School of Chemical, Biological, and Environmental Engineering, Wildenschild is using high-tech tools to find ways to increase the amount of CO₂ that can be trapped by capillary action.

Taking center stage in her lab is a new, custom-made, $800,000 imaging system capable of “seeing inside” and “flying through” opaque materials. Wildenschild and her students can study form, character and function at the scale of one thousandth of a millimeter, aka a micron. How small is that? It is about the width of a strand of spider web silk. A droplet of fog, mist or cloud water is about 10 microns across.

The imaging system is made possible by a $1.2 million grant from the National Science Foundation. When installation is complete and the equipment is fully functional, it will be the most advanced system in the Pacific Northwest, Wildenschild says. The data generated, she adds, will be among the most accurate from any such facility in the nation.

“Using the latest imaging techniques allows us to not only measure how good we are at trapping the CO₂ but also to better determine what variables we can adjust to attain even higher efficiency,” she explains. Some of the factors that affect trapping capacity include injection techniques that break up the CO₂ and variance in the injection rates.

“We’ve found that if we turn the injection process on and off, what’s called cyclic injection, we get much higher levels of trapping,” she says.

A Bridge to Renewables

Wildenschild admits that CO₂ sequestration will not solve climate change, but it could slow it down and provide an important bridge as the world transitions from fossil fuels to renewables for energy generation. What’s needed most, she says, is national leadership and funding to speed the research.

“If there were political interest and acknowledgement from Washington, D.C., that climate change is a critical issue, you could totally do this, because the techniques and research are there, and the basic process has been used for years,” says Wildenschild, who is originally from Denmark and has been at OSU since 2006.

Although the U.S. has some test sites, major CO₂ sequestration operations are underway in Canada, Algeria and Norway.

“There is a huge program in Norway, because the Norwegians realized the dangers of climate change very early on and instituted a carbon tax, which has forced the oil industry to deal with CO₂ emissions,” Wildenschild says. “The cost for flaring off the CO₂ was so high, due to this tax, that it drove them to figure out how to deal with the CO₂.”

Wildenschild is providing high-resolution imaging data to a Norwegian collaborator, Johan Olav Helland, a senior scientist with the International Research Institute of Stavanger (IRIS). He is working to assess the potential for simultaneous CO₂ storage and oil recovery in mature oil fields.

“Dorthe Wildenschild and her research group at OSU are making a very important contribution to the project,” says Helland. “For the first time ever, we will be directly comparing 3D images of three-phase fluid distributions from experiments and simulations at the pore scale in porous media.”

First in Her Family 

Wildenschild grew up on a farm in Denmark, the first person in her family to attend college. “There was no history of academics in my family,” she says. “I didn’t even know what a Ph.D. was until I was working on my master’s degree in Copenhagen. But my parents were very supportive of me going to school.”

In 2014, the year she was named a Darcy Lecturer by the National Groundwater Association, Wildenschild became the first woman to be promoted to full professor in the School of Chemical, Biological, and Environmental Engineering.

Wildenschild’s work on multiphase flow in porous media has applications beyond carbon sequestration — from agriculture and groundwater remediation to fuel cells. In agriculture, where water is quickly becoming a limited resource, increasingly complex computer models of subsurface water movement, combined with soil sensors, are helping farmers avoid over irrigation.

Wildenschild’s research contributes to understanding how water and oil interact below the surface during oil spills and how that affects groundwater. Similar principles are at work in fuel cells, which involve the interaction of multiple fluids in a porous medium.

“But funding is critical,” says Wildenschild, who adds that research support from both the U.S. Department of Energy and the National Science Foundation for this type of work has declined during the past few years. “The best thing that could happen would be to better fund this type of research, because when it comes to slowing climate change, we are running out of time.”

Gregg Kleiner is a freelance writer in Corvallis. 

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Watch X-Rays and Tricycles, a video at the Advanced Photon Source by former OSU graduate student Anna Herring, now at the Australian National University in Canberra.