Chemical & Materials Sciences Division
Research Highlights

May 2012

Plump Up the Clay

Carbon dioxide moves into and expands a common mineral in carbon sequestration caprocks

Producing power with fossil fuels accounts for just over half of the planet's human-made CO₂ emissions. Storing these emissions underground could ease the impact of these emissions. Enlarge Image

Results: For the first time, scientists have direct evidence that high-pressure carbon dioxide or CO₂ migrates into the clay montmorillonite causing it to expand, according to scientists at Pacific Northwest National Laboratory. Montmorillonite is found in the rocks used to cap carbon sequestration sites, and scientists previously thought that only water could make it expand.  Caprocks spend thousands of years halting the escape of injected CO₂. To learn how these rocks respond to CO₂, the researchers studied the material under realistic sequestration conditions.

"Our goal is to eventually be able to predict how a caprock will respond to CO₂ containing variable amounts of water," Dr. John Loring, a PNNL chemist and materials scientist who led the study.

Why It Matters: Producing power with fossil fuels accounts for just over half of the planet's human-made CO₂ emissions, and that number is rising. These emissions are altering the climate, changing weather patterns, and affecting cities and crops. Reducing emissions by storing them in underground locations could ease these changes. By understanding the fundamental reactions, including those of caprocks, scientists can inform industry and the public about the cost and safety of different options.

Like a confused dieter, the common clay montmorillonite would plump up, but nobody knew why. Scientists at Pacific Northwest National Laboratory showed that the clay grows because carbon dioxide molecules from the supercritical CO₂ work their way into the mineral, making it expand.

Methods: While the concept of a carbon sequestration caprock is relatively simple, the chemistry provides intriguing "what ifs." Scientists are answering these scenarios by studying how different aspects of the rock respond to different situations. These situations revolve around supercritical CO₂, which behaves a bit like a liquid and a bit like a gas. Carbon dioxide will exist in this supercritical form at the underground depths where it is to be stored.

One question that arises is: what happens when the rock encounters the buoyant supercritical CO₂ that was pumped into the well? Supercritical CO₂ can be "wet" or "dry." The wet version contains water that has co-mingled with the pollutant. The dry version contains no water. The team at PNNL tackled what happened when both versions contacted montmorillonite.

Found in veins crisscrossing some caprocks, montmorillonite is well known to expand when it encounters water. In carbon reservoir caprocks, veins of the material could swell shut, possibly preventing the escape of CO₂.

However, the team was intrigued by an unusual result. Todd Schaef and his team put the clay in a small sample cell, pressurized the sample with CO₂ to reservoir conditions, and interrogated it with x-rays. Using the x-rays, they measured the distance between atoms in the clay. They found that clay puffed up, and this time, water was not the culprit.

"We believed right away that the CO₂ was going into the structure," said Schaef. "And, we knew this type of mineral and CO₂ interaction was not being accounted for in modeling scenarios for carbon sequestration."

The team turned to two techniques designed to confirm that CO₂ was expanding the clay from the molecule's point of view: magic angle spinning nuclear magnetic resonance spectroscopy and attenuated total reflection infrared spectroscopy. These instruments describe the reactions between the supercritical CO₂ and the clay at the warm temperatures and higher pressures found in a carbon repository, nearly a mile underground.  

"We wanted direct molecular evidence from the point of view of the CO₂ itself," said Loring.

The team got their evidence. The data, gathered in record time, showed that CO₂ molecules move into the clay. This behavior was not seen in the control clay, which did not expand when it encountered the supercritical CO₂.

What's Next? The team will measure the amount of CO₂ that can be held by the montmorillonite and determine how the CO₂ gets in. They will measure the update with varying amounts of water, getting the details needed to allow accurate predictions of caprock performance. They'll also be looking at four other components of caprocks.

"This is the beginning of our research efforts in caprock performance," said Schaef. "We are looking at the fundamental processes that will allow us to retain CO₂ for several thousand years."

Acknowledgments:

Sponsor: This work was supported the U.S. Department of Energy's Office of Basic Energy Sciences, Geosciences Program through a Single Investigator Small Group Research grant. Instrument development was made possible by the Carbon Sequestration Initiative, a Laboratory Directed Research and Development program at PNNL.

User Facility: EMSL

Research Team: John S. Loring, Herbert T. Schaef, Romulus V. F. Turcu, Christopher J. Thompson, Quin R. S. Miller, Paul F. Martin, Jian Zhi Hu, David W. Hoyt, Odeta Qafoku, Eugene S. Ilton, Andrew R. Felmy, and Kevin M. Rosso.

Reference: Loring JS, HT Schaef, RVF Turcu, CJ Thompson, QRS Miller, PF Martin, J Hu, DW Hoyt, O Qafoku, ES Ilton, AR Felmy, and KM Rosso. 2012. "In Situ Molecular Spectroscopic Evidence for CO₂ Intercalation into Montmorillonite in Supercritical Carbon Dioxide." Langmuir 28(18):7125-7128. DOI:10.1021/la301136w.