Study urges caution on underground CO2 storage
Carbon dioxide leaks from deep underground wells could make otherwise good water undrinkable, a study by two scientists at the Duke University Center on Global Change found in a study released earlier this year. Opponents of a “clean” coal project in Taylorville say the study helps make their case against the project. But a scientist behind the study says his findings are less damning than coal critics imply.
“The point of the study was not to provide ammunition for people who say we should never do CCS [carbon capture and storage],” says Dr. Robert Jackson who, along with Dr. Mark Little, co-authored the study. “The point of the study was to try to predict where problems might occur and also to provide early warning of problems were carbon dioxide to leak.”
A member of the STOP (Stop Tenaska’s Overpriced Power) Coalition last month cited the study as a secondary reason not to back the coal-to-gas Taylorville Energy Center, which might bury its CO2 emissions deep underground. The coalition’s primary concern is the project’s cost to consumers. The energy center is located about 30 minutes away from Decatur manufacturer ADM, where the state’s first deep underground carbon capture and storage [CCS] project is already under construction and set to go online in 2012. The Department of Energy is also considering Taylorville and several other central Illinois towns as potential storage sites for CO2 captured by FutureGen, a project that would retrofit an existing coal-fired power plant in Meredosia. Site selection is expected to take place in early 2011.
For the study, Jackson and Little sampled water from four freshwater aquifers, including the Mahomet Aquifer located in Illinois north of Sangamon and most of Macon County, that lie above geologic formations suitable for CO2 sequestration. They then exposed the samples over the course of about one year to a steady stream of CO2 which, as expected, caused the water to become more acidic. That change caused certain contaminant levels to increase, in some cases beyond EPA-approved levels. While the reactions varied from sample to sample based on each aquifer’s unique circumstances, each one showed similar increases in manganese, iron and calcium.
Geologist Rob Finley with the Illinois State Geological Survey, which is working with ADM on its storage well, says the study shows the importance of site selection. “The objective, of course, is not to let the CO2 get to the groundwater in the first place,” Finley says. “And to only store the CO2 in locations where you can manage it, where you’ve done enough investigation that you have confidence that the CO2 will not reach the groundwater.”
Finley says the ADM site, which should be nearly geologically identical to the proposed Taylorville site, is ideal for deep underground storage because of three thick, impervious layers of shale that lie above a 6,500-foot-deep, 1,500-foot-thick saline aquifer, the Mt. Simon Sandstone, that would trap the CO2. The area also hosts no known geologic faults, where earthquakes could disturb the storage area. Above the three shale layers, all of which serve as seals above the trapped CO2, lays the lowermost available source of drinking water, which is only a couple hundred feet below the earth’s surface. That water source, as well as undrinkable saline water found between two layers of shale, will be monitored throughout the well’s operation, Finley says, adding that undisturbed nearby oil wells and underground natural gas storage sites have proved the area’s geologic stability.
Still, nothing is impossible. “It’s naive to suggest that we won’t have any leaks at all,” Jackson says. “If we implement CCS broadly it’s likely we will have some small leaks. It’s an issue of probability.”
“We need every tool in our toolbox to combat climate change, but we also want to be smart about how we site these facilities,” Jackson says, adding that he hopes any potential problems will be curbed by new rules from the U.S. Environmental Protection Agency. The rules will establish a new class of wells especially for deep CO2 storage.
In 2008 the Union of Concerned Scientists published a report, “Coal power in a warming world,” that outlined concerns regarding storing CO2 underground. One of the authors of that report, Barbara Freese, says she hasn’t had a chance to review the EPA’s new rules. In general, UCS believes several large-scale demonstration sites need to go online before the technology is implemented broadly, Freese says, adding that UCS has concerns, but “we don’t see them as so unmanageable that we should rule this technology out. We need a lot more data.”
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