Science - Written by on June 1, 2009

Sequestration puts Carbon Dioxide Underground

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Supercomputer simulations show where it will go

Supercomputing meets sequestration in an experiment that could determine if carbon sequestration, removing carbon dioxide from the air and storing it underground, is an effective way to keep this greenhouse gas out of the atmosphere.

A team led by Peter Lichtner of Los Alamos National Laboratory will attempt to understand long-term carbon sequestration using the National Center for Computational Science’s Cray supercomputer , Jaguar, at Oak Ridge National Laboratory (ORNL). The 12 million processor hours allotted by the Department of Energy for Lichtner’s project in 2009 will help shed light on the risks and rewards of carbon sequestration.

“We are looking at disposing of carbon dioxide deep underground and how i t will change over time,” Lichtner said. “By modeling the underground chamber and the surrounding rock, including abandoned wells, we can identify where any problems or leaks will be.”

Out of sight deep underground

Coal-burning power plants are the main source of carbon emissions in the United States, releasing more than 2 billion tons into the atmosphere every year, where they remain for up to 200 years. However, we can’t just quit using coal. It supplies more than 50 percent of the nation’s electricity.

Carbon sequestration could capture the carbon emissions of these power plants and put them underground so they never reach the air we breathe. Carbon dioxide is captured from coal-burning plants by placing a scrubber on the flue from which the gases would be released. The scrubber absorbs the carbon dioxide, which is then transported in pipes to carefully chosen—ideally nearby—sites and injected into storage containers.

Supercritical carbon acts as a gas, expanding to fill its container. But it also has the density of a liquid. In this supercritical phase, carbon dioxide is injected into the ground much like oil is pumped out of the ground, using wells drilled to great depths.

Lichtner’s project will focus on a saline aquifer as the storage container for carbon dioxide. Aquifers are underground water reservoirs and ideal for this sequestration because they commonly occur in nature, can hold a large volume of liquid, and have a layer of mineralized brine. Once injected into the saline aquifer, the supercritical carbon dioxide begins to absorb the brine and sink to the bottom of the chamber and away from the most likely leak points, which are the surface and nearby abandoned wells.

As the brine absorbs the supercritical carbon dioxide, the brine becomes heavier and more acidic and starts to sink, resulting in a convection current with “fingers” of sinking brine. In the simulation Lichtner will explore this poorly understood event, which is thought to speed up the dissolution of carbon dioxide and play a role in its ultimate fate.

Understanding the dissolution speed is important to estimating the risk of leakage, as well as how the increasing acidity of the brine affects the walls of the aquifer. The faster the carbon dioxide leaves the supercritical phase (dissolves) and sinks, the less chance of a dangerous leak that could pollute the groundwater flowing around the aquifer or escape into the atmosphere.

The magnitude of this study is such that researchers have had to wait for supercomputing to reach the power needed for their simulations. Looking at square-kilometer chunks of land is no small feat. Now the Cray XT Jaguar, the fastest supercomputer for open scientific research, can run 1.64 quadrillion calculations per second, enabling the research team, which consists of Lichtner, Glenn Hammond of Pacific Northwest National Laboratory, and Richard Mills of ORNL, to study carbon storage and capture in greater detail than ever.

Out of mind for a few millennia

Carbon sequestration is a new field of research, and no computer code exists that can predict exactly what will happen when carbon dioxide is injected into the ground. The encompassing goal of this project is to improve the PFLOTRAN code, which will eventually reveal the behavior of underground flow processes.

Until now the vast range of scales for both length and time has limited computer simulations of subsurface processes. Now, with more advance computer technology and PFLOTRAN, it is possible to explore lengths ranging from chemical reactions inside the aquifer that are less than one centimeter long up to the size of the aquifer itself—on the order of kilometers. Times range from relatively fast chemical reactions that take days up to thousands of years.

Lichtner’s team will focus its calculations on the Scurry Area Canyon Reef Operators Committee (SACROC) unit of the Permian Basin in western Texas for its calculations. This 8,000-cubic-kilometer aquifer is six to seven thousand feet below the earth’s surface and contains more than 200 oil wells. Carbon dioxide flooding operations began here in 1972, making SACROC the oldest demonstration of carbon sequestration in the United States. Since then 55 million tons of carbon dioxide have been sequestered.

Using the PFLOTRAN code, the team can simulate this huge geologic formation and how the carbon dioxide has changed since it was injected. The computer power required for this project is enormous given the range of scales and complex processes. And Jaguar now offers the power to model subsurface processes with unprecedented detail, enabling researchers to improve the PFLOTRAN code for future experiments.

This research brings the idea of carbon sequestration from strictly scientific study to near-future application as a means of keeping carbon dioxide out of the atmosphere. Trapped kilometers underground, carbon dioxide will do far less harm to the planet than if it was released to the atmosphere.

“Once we understand how supercritical carbon dioxide behaves underground, sequestration can definitely be a useful method for reducing greenhouse gases in the atmosphere,” Lichtner said. — by Elizabeth Storey

Oak Ridge National Laboratory is supported by the US Department of Energy’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.