Surprising release of metals from CO2 storage
Injecting CO2 into saline sandstones to slow global warming can dissolve minerals in the sandstones.
Injecting CO2 deep within the earth remains one of the more promising methods of getting rid of the greenhouse gas. So far, rocks, composed of porous sandstone filled with salty water many hundreds of meters below the surface, are the main storage-site candidates. Multiyear projects in such settings, from the North Sea to Canada, are generating encouraging results.
But preliminary geochemical data from the first U.S. project to inject CO2 into such a formation, which was located near Houston, Texas, offer a cautionary note on CO2 storage. They indicate that because CO2 makes the deep groundwater more acidic, metals in the sandstone get released. The results were reported in the journal Geology in July by the project’s geochemist, Yousif Kharaka of the U.S. Geological Survey, and his colleagues.
“We observed rapid dissolution of calcite and mobilization of large amounts of iron and other metals as a result of [a] major drop of pH from 6.4 to 3,” Kharaka says. “These data are not a snag for CO2 storage in continental sedimentary basins,” he contends. However, the data do strongly suggest that CO2 injection wells should use acid-resistant cements and that abandoned wells should be avoided or monitored carefully.
“These former oil and gas wells were never engineered to last for a long time,” says Susan Hovorka, a geologist at the Texas Bureau of Economic Geology and the project’s principal investigator. She adds that another reason to avoid old wells is that most are shallow, because deeper is generally considered better for CO2 sequestration.
The release of metals identified by Kharaka represents “a new element of risk, because it shows the potential for reactive chemistry that could be of concern,” says geologist Julio Friedmann at the Lawrence Livermore National Laboratory. “But these new results are not likely to present a substantial complication to underground storage,” he adds. This is because metal-bearing oxides and hydroxides usually make up <1% of saline aquifers and such aquifers have kept saline brines isolated over 100-million-year geological timescales.
Compared with the Weyburn project in Canada and the Sleipner West project that has been on-line since 1996 in the Norwegian North Sea, the U.S. project is a relative newcomer. Initiated in 2004, the U.S. Department of Energy-funded project is known as Frio, after the rock formation where the gas is stored. Researchers compressed the first batch of 1600 t of CO2 into a supercritical fluid, heated it to ~16 °C, and pumped it into a 24-m-thick sandstone layer ~1 mi below the surface.
Scientists already have enough information to be confident in the ability of some rock formations to hold CO2 for a long time, says Hovorka. “But we need clear guidelines to tell a good place from a bad place,” she says. “We want some indicators, and the data from Frio are part of that process.”
Reactions that dissolve and crystallize minerals are a mixed blessing, says geochemist Ernie Perkins with the Alberta Research Council (Canada). He notes that observations at Weyburn and elsewhere document other, slower reactions that eventually lead to the crystallization of new minerals. Initial dissolution of carbonates in the formation makes it easier to inject CO2, he says, by clearing pathways through the rock. Later crystallization reactions can retain CO2 by enclosing it in new minerals and reducing pore space in the rock, thus preventing the gas from percolating up. But if the crystallization reactions occur while CO2 is still being pumped into the formation, then they could interfere with the injection by blocking pathways. “The geochemical reactivity of the formation is another criterion in the selection of an appropriate storage site,” he says.
Hovorka hopes that the U.S. government will give larger-scale CO2 storage projects a green light soon. On July 25, the U.S. government–industry alliance known as FutureGen, which aims to build a $1 billion coal-gasification and CO2-sequestration project by 2012, short-listed 2 sites in Illinois and 2 in Texas for its final location.
On August 21, the scientists at Frio started the next stage of their investigation. They will seek to quantify how much CO2 is dissolved in the water underground and how much is trapped as gas.
