Storing greenhouse gas underground—for a million yearsWhen 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.
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.”
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