Jaguar makes possible the largest groundwater simulations.
As the American scientific community develops a long-term plan to address climate change, not every response focuses on new energy sources. Even as we develop promising new carbon-free technologies for solar power, biofuels and nuclear energy, the economy faces the prospect of being tethered for some time to the old energy sources—primarily fossil fuels such as coal and oil.
Climate change represents a collision of economic and environmental concerns. Coal is very abundant and critical to much of America's economic base. With coal power, however, comes a variety of serious environmental problems, including the discharge of carbon dioxide, or CO2, into the air by coal-fired power plants. Carbon dioxide is the most worrisome of the greenhouse gases. According to the Intergovernmental Panel on Climate Change, levels of CO2 in the atmosphere are 35% higher than they were before the Industrial Revolution and are in fact higher than at any time in the last 650,000 years. Climate scientists believe it is no coincidence that the planet is experiencing a string of the warmest years since the taking of measurements began more than 150 years ago.
One proposal for mitigating the effect of coal power on the earth's climate involves separating CO2 from power plant emissions and pumping the gas deep underground, where it could remain indefinitely dissolved in the groundwater or converted into a solid form of carbonate minerals.
A team of researchers led by Peter Lichtner of Los Alamos National Laboratory is using Oak Ridge National Laboratory's Jaguar supercomputer to simulate this process, known as carbon sequestration, and is searching for ways to maximize its benefits and avoid its potential drawbacks. Indeed, many view carbon sequestration as an absolutely critical component of worldwide efforts to lower substantially the volume of carbon emissions. Using Jaguar, the team has been able to conduct the largest groundwater simulations to date, pursuing the research with an application known as PFLOTRAN.
The process being simulated by Lichtner's team involves taking CO2 that has been separated from a power plant's emissions and injecting it nearby into a deep saline aquifer 1 to 2 kilometers below the surface. If all goes according to plan, the CO2 would disperse under a layer of impermeable rock with the opportunity to dissolve into the surrounding brine.
When pumped into the ground, the CO2 would be in a state known as a supercritical phase, which is present when the gas is kept above 50°C (120°F) and more than one hundred times atmospheric pressure. The plan assumes that the CO2 would be kept in the supercritical phase by the heat and pressure naturally present deep underground. According to Lichtner, CO2 in this phase is in some ways like a liquid and in some ways like a gas. The primary benefit is the avoidance of the rapid expansion that would accompany changes between the two phases.
Lichtner's team is investigating a process known as "fingering," that speeds the rate at which the CO2 dissolves. Fingering grows out of the fact Labothat while CO2 in the supercritical phase is lighter than the surrounding brine, the brine in which CO2 has been dissolved is actually heavier than unsaturated brine. The result is a convection current, with "fingers" of the heavier, saturated brine sinking. This fingering in turn increases the surface area between the CO2 and the brine and speeds the dissolution of the supercritical CO2 into the brine.
The rate of dissolution is critical to the success of carbon sequestration. When first injected in the ground, the CO2 pushes the brine out of place. Once the CO2 dissolves, however, little is added to the volume of the brine, which can then move back into place.
"The problem is that we are injecting huge amounts of CO2 by volume," Lichtner explained. "If we were injecting it into a deep saline aquifer, for example, we would initially have to displace the brine that was present. The question then would be, ‘Where does that go?' If we inadvertently pushed the brine up into the overlying aquifers we might contaminate, say, the drinking water for the whole Chicago metropolitan area. The dissipation of CO2 is literally a race against time."
Other hazards must be better understood before large volumes of CO2 can be pumped underground. If the CO2 were to rise to the surface, another substantial hazard would be created. The process of dissolving CO2 into groundwater is, in fact, known as carbonation. The unintended rise of CO2 to the surface could rapidly turn the groundwater into seltzer water.
"There are natural occurrences of CO2 shooting out of the ground," Lichtner noted. "As the pressure and temperature are artificially lowered, the bubble of injected CO2 starts approaching the surface, with a change of phase from supercritical to liquid to gas. The result could suddenly occupy a much larger volume, forming a geyser like those in Yellowstone National Park.
"So long as the supercritical phase exists, the possibility that the CO2 could escape through fractures, abandoned boreholes, or boreholes that leak presents a hazard to people living in the vicinity. Therefore, understanding the rate of dissipation is important to knowing how rapidly we can move from the supercritical phase."
A final issue that must be studied focuses not so much on the rate at which CO2 dissolves, but rather on the changes the process brings to the aquifer itself. As Lichtner explained, CO2 produces carbonic acid, which in turn lowers the pH of the brine. This could speed the reaction between the newly acidic brine and surrounding minerals and potentially release contaminants into the environment that otherwise would not be present.
Lichtner's team is focusing its simulations on the Illinois Basin, a 60,000-square-mile area ranging across most of Illinois as well as eastern Indiana and Kentucky. The area relies heavily on coal power, but the size of the region also provides a daunting task to anyone who wants to model it computationally.
The team is simulating carbon sequestration in the Illinois Basin using an application known as PFLOTRAN, which is built on the PETSc parallel libraries, developed by a team led by Barry Smith at Argonne National Laboratory. Chuan Lu of the University of Utah developed the supercritical CO2 implementation in PFLOTRAN while working with Lichtner as a postdoctoral researcher at LANL. Lichtner and his team have demonstrated that PFLOTRAN can handle grids on the order of a billion cells—an unprecedentedly large number for a groundwater simulation. Nevertheless, each cell in such a simulation will be nearly 100 square meters, too large to analyze with confidence the fingering process that takes place at the scale of tens of centimeters to tens of meters, depending on the properties of the aquifer.
Lichtner noted that his team is working both to improve the performance of PFLOTRAN and to prepare for the arrival of even more powerful supercomputers. To make PFLOTRAN more effective, for example, the team is working to evolve from the use of a structured grid, in which a quarter of the cells give no useful information, to an unstructured grid that can redistribute those cells where they will be of most value.
The team looks forward to using ORNL's new Jaguar supercomputer, capable of speeds greater than 1,000 trillion calculations a second, or a petaflop. Jaguar will make possible the simulations needed to address questions on the scale of the Illinois Basin posed by carbon sequestration.
Meanwhile, researchers realize that the scale and complexity of the climate change challenge, equaled only by the scale of the stakes involved, will require scientific tools such as Jaguar that are equal to the task.—Leo Williams
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