How to handle carbon dioxide? Lock it in rock

Bryn Nelson Columnist

Escalating carbon dioxide levels tied to global warming have increasingly forced world leaders to grapple with environmental costs of delaying reduction efforts versus economic costs of implementing tighter regulations. But therein lies a key dilemma: How can future power plants meet a projected spike in energy demand yet keep the atmosphere from amassing even more of the heat-trapping gas?

About two months from now, three narrow wells will plunge thousands of feet through the industrial scrubland of southeastern Washington state, reaching for a solution to the expected crisis through a natural volcanic formation created in the distant past. Within that thick layer-cake of basalt rock, liquefied carbon dioxide — which would otherwise accumulate as a major greenhouse gas in the atmosphere — will begin taking the place of brackish water. And if all goes well, that pressurized carbon will gradually mineralize into limestone, trapping itself forever within the vast underground prison and assuming a major role in the fight to ward off a future environmental catastrophe.

In the last few years, growing ranks of researchers have suggested capturing carbon dioxide emitted from fossil fuel-burning plants and then sequestering it away as one strategy for keeping it out of the atmosphere. But the challenges have proven daunting, with some deep-ocean storage plans shot down over fears of the carbon escaping en masse or of other environmental damage emerging as a byproduct.

Underground storage
When Peter McGrail first heard about a proposal to instead store carbon dioxide underground, he was dubious. “I said, you’ve got to be nuts, this is crazy,” he said. Now, the senior scientist at the Pacific Northwest National Laboratory in Richland, Wash., expects to spend the rest of his career working on it. In fact, McGrail believes deep geological sequestration presents the most economically attractive solution — and also one of the largest storage sinks. Without a comparable strategy, he said, the economics of future carbon dioxide mitigation efforts are likely to be “kind of ugly.”

Between 6 million and 17 million years ago, as many as 300 lava flows covered a huge swath of what is now southern Idaho, northeastern Oregon and southeastern Washington. Within these flows, extending thousands of feet below the surface, successive layers of permeable and porous basalt formations have trapped water beneath less permeable rock caps. In all, McGrail said, the multilayered deposits have the capacity to store 50 to 100 gigatons of carbon dioxide — equivalent to 20 to 50 years worth of emissions from every coal-fired power plant in the United States.

The potential solution, he said, “is a story about fate and transport.” Because carbon dioxide floats, any engineering strategy would have to seal its fate by cutting off escape routes to keep it from migrating back up to the surface. Computer simulations suggest that once pumped through the well, liquid carbon dioxide will remain stuck between layers of the massive basalt formation.

High-pressure confines
For their pilot project, McGrail and his collaborators plan to inject between 3,000 and 5,000 tons of liquid carbon dioxide over a two- to three-week period. Based on the region’s geography, McGrail said he expects the carbon dioxide to be stored at a depth of between 3,000 and 4,000 feet.

Within its high-pressure underground confines, laboratory tests support the idea that the compound’s ultimate destiny will be to mineralize into a calcium carbonate coating — the main component of limestone and “the most safe and secure storage you could have,” McGrail said. “Essentially, the CO₂ is converted back to rock.” The process would require no additional energy input, whereas options such as trapping carbon dioxide under sandstone layers would lack the same mineralization and require leaving an impermeable caprock in place for a century or more.

Modeling experiments suggested that even if injected into fractured basalt formations, carbon dioxide would migrate upwards only about half the length of a football field over a period of 17 years. “Assuming that the CO₂ stays in place, the chemistry is pretty straightforward,” McGrail said. “The mineralization is really just a bonus. You can’t have that unless the carbon dioxide is staying put for a while.” In the lab, exposing samples of the area’s basalt to carbon dioxide-saturated water yielded calcium carbonate mineral formation in just four to six weeks and extensive mineralization within eight months.

Based on current designs, one test well will act as the pilot bore hole to reveal underground rock properties in advance and enable temperature and pressure monitoring after the injection. A second well will serve as the injection site for the carbon dioxide. And a third will be used for gathering seismic measurements and collecting water chemistry samples after the injection, the latter of which should reveal the rate of the mineralization process. The entire pilot project, slated at more than $10 million, has been funded primarily by the U.S. Department of Energy, with assistance from power companies and industrial firms belonging to the Big Sky Carbon Sequestration Partnership.

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