With Undersecretary of Energy Steven Chu calling for “widespread, affordable deployment” of carbon capture and storage technology within 8 to 10 years, the future looks bright for carbon sequestration researchers. But sequestration will only be viable if scientists can assure the public CO2 won’t leak back to the surface, or worse, into their drinking water and cellars. And that requires a deep understanding of where the CO2 goes and how it interacts with its environment.
Most of what is known about CO2 trapping mechanisms comes from computer models. Marc Hesse, who has done pioneering work with these kinds of simulations, warns the computer models can only get you so far.
He spent the last two summers at the University of Cambridge running experiments in a physical model of the subsurface and comparing them to simulations. The physical model, developed with colleagues from his days as a graduate student there, is a clear plastic box about the size of a board game box packed with water and clear glass beads. To mimic what happens when CO2 is injected deep underground, the researchers add a blue fluid designed to act like CO2.
The researchers determined the rate at which the CO2 dissolves into water, which is thought to be the main trapping mechanism in many reservoirs. As the CO2 dissolves into the water its density increases and the CO2 saturated water begins to sink, forming long fingers. Fresh water rises up to take its place through convection, allowing more CO2 to dissolve. This convective process has the potential to speed up the rate of dissolution, trapping the CO2 much more quickly and increasing the storage security.
Early computer models missed these plumes of CO2 because they’re small, perhaps only a meter across once you scale up from the tabletop model to the real world. Yet most computer models of the subsurface, to minimize computational demands, carve up the world into three dimensional blocks 10 or 100 meters to a side, far too large to capture these features. That means CO2 may dissolve into the water much faster than predicted by computer models, significantly reducing the distance it travels.
“It’s dangerous because one should never trust numerical simulations that have not been benchmarked against experiments with similar dynamics,” he says. “There is almost no data in the literature that allows quantitative comparison, in other words, one that goes beyond ‘Well it looks sort of the same.’ We are producing that data, and the theory that is necessary to draw any general conclusions from it.”
Hesse plans to harness the computational infrastructure of The University of Texas at Austin’s Center for Computational Geosciences & Optimization and the Texas Advanced Computing Center to resolve the small scale convective currents and allow more accurate predictions of the long-term migration of injected CO2.
Downhill Run
Hesse grew up in Germany near the Alps.
“I guess being close to the Alps subliminally influenced my decision to become a geologist,” he says. “I enjoy skiing, snowboarding, sleds, telemarks, and anything else that slides downhill.”
Like a cross country skier, he zigged and zagged through his education. He sailed from geological engineering — answering questions like, “What size building will this soil support?”–to classical geology–studying things like how rocks form — to flow in porous media–the study of how fluids flow through and interact with rocks in many geological processes. He realized that flow in porous media is a fundamental process with applications across the Earth sciences and that theoretical advances in this field have the potential to solve a wide range of problems.
One such process fundamental to the evolution of Earth is melt migration or magma dynamics, essentially the study of how material deep underground melts, moves to Earth’s surface, and changes along the way. Another is carbon sequestration.
He slalomed through a master’s degree in oceanography melting rocks in high-pressure lab experiments to study melt migration at mid ocean ridges. He sliced through a second master’s modeling fluid dynamics on a computer. He slid to a graceful finale with a Ph.D. at Stanford in petroleum engineering, where he combined theoretical work with numerical models to investigate how far an injected plume of CO2 would spread before becoming trapped. That work laid the foundation for his recent physical modeling work at Cambridge University. In California, the boy from the mountains learned to ride the waves and became the president of the Stanford Windsurfing club.
In a postdoctoral appointment at Brown University, he revived his interest in melt migration. At UT Austin he is currently pursuing research in both CO2 storage and in melt migration.
Texas and the Carbon Boom
Hesse came to UT Austin in 2009 in part because of strong carbon sequestration programs in the Bureau of Economic Geology’s Gulf Coast Carbon Center, the Department of Petroleum & Geosystems Engineering (PGE), and the Institute for Computational Engineering and Sciences. He works with colleagues and students in all three units.
“UT is the largest center for CO2 storage research in the world,” he says.
Hesse organized a Gordon Research Conference this past July focused on the simulation of geological CO2 storage.
“The idea is you take the world’s top people in a subject area and sequester them in a prep school in Maine for a week,” he says. “There are some lectures, but mostly a lot of open discussions. People present cutting edge results and it is understood that what is said at the conference stays there.”
This confidentiality promotes a direct and free exchange of ideas. The conferences are designed to stimulate new directions for research by creating a third channel for communication beyond publications and large scientific meetings.
Things should stay busy for some time to come. Carbon sequestration, a barely existent research area a decade ago, is now undergoing tremendous growth, aided in part by federal stimulus funding through the Department of Energy and increasing interest from the coal and power generation industries.
“You shouldn’t let fashions decide your research area,” says Hesse. “I chose it because it was an exciting and active field and currently provides the motivation to rethink many old assumptions in porous media flows. Still, it has turned out to be a smart area because of the growth.”
By Marc Airhart
For more information about the Jackson School contact J.B. Bird at jbird@jsg.utexas.edu, 512-232-9623.
