Geologist Martin Cassidy, who co-authored the new study, samples a gas well at Bravo Dome, the world's largest natural CO2 reservoir.

Geologist Martin Cassidy, who co-authored the new study, samples a gas well at Bravo Dome, the world's largest natural CO2 reservoir.

Marc Hesse

Storing greenhouse gas underground—for a million years

When Canada switched on its Boundary Dam power plant earlier this month, it signaled a new front in the war against climate change. The commercial turbine burns coal, the dirtiest of fossil fuels, but it traps nearly all the resulting carbon dioxide underground before it reaches the atmosphere. Part of this greenhouse gas is pumped into porous, water-bearing underground rock layers. Now, a new study provides the first field evidence that CO2 can be stored safely for a million years in these saline aquifers, assuaging worries that the gas might escape back into the atmosphere.

“It's a very comprehensive piece of work,” says geochemist Stuart Gilfillan of the University of Edinburgh in the United Kingdom, who was not involved in the study. “The approach is very novel.”

There have been several attempts to capture the carbon dioxide released by the world’s 7000-plus coal-fired plants. Pilot projects in Algeria, Japan, and Norway indicate that CO2 can be stored in underground geologic formations such as depleted oil and gas reservoirs, deep coal seams, and saline aquifers. In the United States, saline aquifers are believed to have the largest capacity for CO2 storage, with potential sites spread out across the country, and several in western states such as Colorado also host large coal power plants. CO2 pumped into these formations are sealed under impermeable cap rocks, where it gradually dissolves into the salty water and mineralizes. Some researchers suggest the aquifers have enough capacity to store a century’s worth of emissions from America’s coal-fired plants, but others worry the gas can leak back into the air through fractures too small to detect.

To resolve the dilemma, geoscientists need to know how long it takes for the trapped CO2 to dissolve. The faster the CO2 dissolves and mineralizes, the less risk that it would leak back into the atmosphere. But determining the rate of dissolution is no easy feat. Lab simulations suggest that the sealed gas could completely dissolve over 10,000 years, a process too slow to be tested empirically.

So computational geoscientist Marc Hesse of the University of Texas, Austin, and colleagues turned to a natural lab: the Bravo Dome gas field in New Mexico, one of the world's largest natural CO2 reservoirs. Ancient volcanic activities there have pumped the gas into a saline aquifer 700 meters underground. Since the 1980s, oil companies have drilled hundreds of wells there to extract the gas for enhanced oil recovery, leaving a wealth of data on the site’s geology and CO2 storage.

To find out how fast CO2 dissolves in the aquifers, the researchers needed to know two things: the total amount of gas dissolved at the reservoir and how long it has been there. Because the gas is volcanic in origin, the researchers reasoned that it must have arrived at Bravo Dome steaming hot—enough to warm up the surrounding rocks. So they examined the buildup of radiogenic elements in the mineral apatite. These elements accumulate at low temperatures, but are released if the mineral is heated above 75°C, allowing the researchers to determine when the mineral was last heated above such a high temperature. The team estimated that the CO2 was pumped into the reservoir about 1.2 million years ago.

Then the scientists calculated the amount of gas dissolved over the millennia, using the helium-3 isotope as a tracer. Like CO2, helium-3 is released during volcanic eruptions, and it is rather insoluble in saline water. By studying how the ratio of helium-3 to CO2 changes across the reservoir, the researchers found that out of the 1.6 gigatons of gas trapped underground at the reservoir, only a fifth has dissolved over 1.2 million years. That’s the equivalent of 75 years of emissions from a single 500-megawatt coal power plant, they report online this week in the Proceedings of the National Academy of Sciences.

More intriguingly, the analysis also provided the first field evidence of how CO2 dissolves after it is pumped into the aquifers. In theory, the CO2 dissolves through diffusion, which takes place when the gas comes into contact with the water surface. But the process could move faster if convection—in which water saturated with CO2 sinks and fresh water flows into its place to absorb more gas—were also at work. Analysis revealed that at Bravo Dome, 10% of the total gas at the reservoir dissolved after the initial emplacement. Diffusion alone cannot account for that amount, the researchers argue, as the gas accumulating at the top of the reservoir would have quickly saturated still water. Instead, convection most likely occurred.

Hesse says constraints on convection might explain why CO2 dissolves much more slowly in saline aquifers at Bravo Dome than previously estimated, at a rate of 0.1 gram per square meter per year. The culprit would be the relatively impermeable Brava Dome rocks, which limit water flow and thus the rate of convective CO2 dissolution. At storage sites with more porous rocks, the gas could dissolve much faster and mineralize earlier, he says.

Even so, the fact that CO2 stayed locked up underground for so long at Bravo Dome despite ongoing industrial drilling should allay concerns about potential leakage, Hesse says. Carbon capture and storage “can work, if you do it in the right place,” he says. “[This is] an enormous amount of CO2 that has sat there, for all we can tell, very peacefully for more than a million years.”