National Center for Computational Sciences

Groundwater simulation addresses the challenges of carbon sequestration

Not every response to global warming focuses on new energy sources. Even as we develop promising technologies such as solar power, biofuels, and nuclear energy, we face the prospect of being tethered for some time to the old energy sources—primarily fossil fuels such as coal and oil.

One proposal for mitigating the effect of coal power on the earth’s climate involves separating carbon dioxide—or CO2—from power plant emissions and pumping it deep underground, where it can 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 (LANL) is using the National Center for Computational Sciences’ Jaguar supercomputer to simulate this process, known as carbon sequestration, searching for ways to maximize the benefits and avoid potential drawbacks. Using Jaguar, the team has been able to conduct the largest groundwater simulations ever seen, pursuing its research with a 2008 DOE/Office of Advanced Scientific Computing Research Joule metric code known as PFLOTRAN.

Coal is very abundant in the United States, but coal power carries a variety of serious problems, one of which is that coal-fired power plants spew CO2 into the air. 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 one to two kilometers below the surface. If all goes according to plan, it would spread out under a layer of impermeable rock and get an opportunity to dissolve into the surrounding brine.

The CO2 would be pumped in a state known as a supercritical phase, which is present when it is kept above 50 degrees centigrade—120 degrees Fahrenheit—and over 100 times atmospheric pressure; it would be kept in that state 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 like a gas, but the primary benefit is that it avoids the rapid expansion that would go along with 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 that while CO2 in the supercritical phase is lighter than the surrounding brine, 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 it is first injected in the ground, the CO2 pushes the brine out of place. Once the CO2 dissolves, however, it adds little to the volume of the brine, which can then move back into place.

“The problem is that we’re talking about injecting huge amounts of CO2 by volume,” Lichtner explained. “If you were injecting it into a deep saline aquifer, for example, you would initially have to displace the brine that was present, and then the question is, ‘Where does that go?’ It’s a race against time how rapidly this CO2 will dissipate.”

There are other hazards as well that must be thoroughly understood before large volumes of CO2 can be pumped underground. If the CO2 were to rise to the surface, that would create another substantial hazard. The process of dissolving CO2 into groundwater is, in fact, known as carbonation; CO2 rising rapidly to the surface could literally turn the groundwater into seltzer water.

“As long as the supercritical phase still exists, it presents a hazard to people living on the surface,” Lichtner said, “because it could escape through fractures, abandoned boreholes, or boreholes that leak. And so the rate of dissipation is important to understand to know how rapidly you get rid of this supercritical phase.”

A final issue that must be studied focuses not so much on the rate that CO2 dissolves, but rather on the changes this 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 would not be present otherwise.

Lichtner’s team is simulating carbon sequestration 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 shown 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 comfortably analyze the fingering process, which 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.

Beyond that, he said, the team is anticipating a new generation of supercomputers capable of speeds greater than 1,000 trillion calculations a second, or a petaflop. At that scale, Lichtner’s team will approach the resources it needs to guide the process of carbon sequestration, enabling us to remove a substantial chunk of CO2 from the atmosphere.