CLIMATE:

Future of CO2 storage may be etched in German sandstone

BERLIN -- The Stuttgart Formation may just be the most important hunk of rock in the world.

Buried some 2,000 feet down 25 miles west of here, the saltwater-flush sandstone has seen its eons-old life disturbed recently by clanging seismic probes, electrical currents and drills as geologists examine its every pore and flaw.

It's so much attention for such a small amount of carbon dioxide.

For the past 22 months, an international team of scientists has injected CO2 into the formation, wedged beneath the hamlet of Ketzin. Though commercial projects have injected more CO2 under the ocean, Ketzin is Europe's first and most rigorous onshore storage experiment. Ton by ton of injected gas, the project wants to begin answering the many questions that cloud the future potential of underground CO2 storage.

The stakes couldn't be larger. CO2 sequestration is the linchpin of carbon capture and storage (CCS). Politicians and the coal industry have placed their hopes -- and their "clean coal" rhetoric -- on the notion that CO2, the gas driving climate change, can be contained underground and far from the atmosphere for a thousand years. If storage proves a failure, or carries so much uncertainty that the public rejects it, CCS crumbles.

And for CO2 emissions to be stored underground at any large scale, the CO2 will have to be injected into porous rocks like the Stuttgart Formation.

It is an endeavor that is not without precedent. For decades, governments have injected natural gas underground for seasonal storage, the gas trapped by its own buoyancy and a curved ceiling of impermeable rock.

But once the gas is stored, it's been a black box. Little has been done to follow its plume as it glides through a formation's vast internal structure.

With its array of underground geophones, electrical sensors and biochemical monitors, the Ketzin experiment, also known as CO2Sink, wants to definitely establish if CO2 can be continuously tracked underground, said Hilke Würdemann, the experiment's coordinator and a scientist at the German Research Center for Geosciences (GFZ), the country's leading geoscience lab.

"We have the most comprehensive monitoring effort in the world," she said.

From the surface, the project appears modest: several large gas tanks that resemble squat water towers and a box-like building to inject compressed CO2 underground. It's a former industrial site; natural gas was drilled for decades nearby. There is little hint of the action underground. Flat farm fields surround the facility and lead to Ketzin's tiny center, which does not even merit a train station on Germany's extensive rail network.

Most of Ketzin's heavy lifting is done remotely, in Potsdam, Germany, or Sweden or Holland. Using the site's data, scientists have begun to verify the complex models they have constructed of CO2 behavior, building off systems from the oil industry.

Ketzin and projects like it will need to correct these conjectures, which will be vital for Europe to use in its already-passed storage regulations, said Michael Kühn, the head of GFZ's Center for CO2 Storage, which oversees the Ketzin experiment.

"We want to be able to qualitatively and quantitatively monitor the CO2 migration," he said. "We always want to know where it is. If we can, we want to make predictions based on the observations we've made. We want to be able to look into the future."

Researchers from across Europe are collaborating on the project. And unlike the public, these scientists have no doubts about whether CO2 storage could work. It can. It's simple geology.

Instead, they ask finer questions: What happens to deep groundwater displaced by the gas? How much pressure can be added to a reservoir without cracking its sealing rock? How much CO2 can leak through faults or failed wells?

These questions, and all the risks they imply, are only beginning to be answered.

Olympic feats

It can seem an absurd notion, injecting CO2 into underground rock formations for storage that will last centuries. And it may speak to man's hubris. But while the thousand-year time frame remains a valid point of contention, nearly every German geologist points out that for decades, the government has stored another, far more dangerous gas -- natural gas -- beneath a sensitive location: Berlin.

"Most people don't know we do this, but there's a very large storage reservoir in Berlin," Kühn said. "It's under Olympic Stadium. ... For decades it has worked safely. And this is a dangerous gas to store. It's toxic, explosive, flammable. You don't have any of that with CO2."

Man-made natural gas storage, using formations similar to those in CO2 storage, is common throughout the world. There have been scattered accidents. Notably, in 2001, gas stored beneath Hutchinson, Kan., leaked through abandoned wells and exploded in spectacular geysers, killing two people. But despite these anecdotes, natural gas storage is considered a success. It has provided a starting point for CO2 storage, Kühn said.

The oil and gas industry is not omniscient, however. Often, less gas is extracted from underground reservoirs than has been injected. And it is not always known if the gas remains trapped in the formation, or if it has escaped on its own underground walkabout, seeping past fossilized shellfish and curling against salt deposits.

"If you look at the oil and gas industry, even after 30 years of production history, we don't know exactly how [gas] behaves" in underground storage, said Rob Arts, a Dutch geologist and leading expert on CO2 monitoring, who collaborates with Ketzin among other projects.

Still, gas storage in either variety works because sandstone and similar formations, for their sheer internal cavities, defy the picture most have when they think "rock." This is far from kitchen-countertop granite.

Take, for example, a core sample from the offshore Sleipner Project in the North Sea, where the Norwegian oil company Statoil has injected CO2 kilometers underground since 1996, the world's best-known CO2 storage project. More than 30 percent of the sandstone's volume is vacant. Excavated for study, it would easily crumble in the open air, said Arts, who also monitors Sleipner's plume.

"You could almost compare it to sand on the beach," he said. "They had to freeze this core to maintain it. You thaw it, and it falls apart."

The size of these pores compared to CO2 is a matter of magnitudes, added Christoph Heubeck, a petrogeologist at Berlin's Free University.

Each molecule is like a parishioner in a cathedral, he said. And like a cathedral, the pores have entrances of varied size, from towering front doors to attic windows. Some are easily torn apart by invading gas; others resist. And each pore leads to a new cathedral, an unending chain of geological piety.

Geologists will depend on several mechanisms in these pores to hold CO2, which, because of the depth and temperature at most storage sites, will be a supercritical liquid, an odd state that is halfway between liquid and gas. At Ketzin's shallow formation, however, CO2 is only gaseous.

Initially, the CO2 will rise above the water and be pinned against the caprock. Then, it will begin to dissolve. And finally, after centuries, it should begin to form new minerals.

The upshot: "With increasing time, the storage security also increases," said Holger Class, the head of the CO2 research at Stuttgart University and one of Germany's leading CO2 modelers.

Ears to the ground

At Ketzin, more than 30,000 tons of CO2 has been sent down and contained into its formation's pores, a modest amount compared to the million tons Statoil injects each year at Sleipner.

But Ketzin's small size and poor composition -- with its internal flaws and a fault nearby, it would never be used commercially -- make the project more rigorous, Kühn said.

"If we are able to successfully apply our monitoring techniques in Ketzin with only a little CO2 and under quite complicated constraints," he said, "it's promising for these technologies to be used in higher-quality reservoirs in the future."

Ketzin has several systems to monitor its CO2. Two monitoring wells are dug into the formation, outfitted with gas, pressure, temperature and electrical sensors along their length. (Salt water carries charge far better than CO2, allowing electricity to serve as a measure.) These sensors are supplemented with active seismic imaging, essentially terrestrial sonar that gauges the depths from afar through the reflection of sound.

Seismics, and particularly time-lapse imaging, will likely remain a primary method of measuring stored CO2. Compared to water-saturated rock, seismic waves flare off CO2 "like a contrast fluid," Arts said.

But seismics are far from perfect. They require interpretation, introduce human error and can miss small details.

Scientists at Sleipner and Ketzin missed internal features in their reservoirs -- clutches of mud or shale -- that caused mismatches with predicted flow patterns. In Ketzin's case, CO2 was delayed in its arrival to the second observation well by nine months, Würdemann said. The models will have to be reworked.

The data continue to get better, Arts said. Recently, his research group at TNO, the Dutch national applied sciences institute, installed seismic geophones 50 meters down into Ketzin's soil.

The phones are always on, he said, "listening to what the underground does."

Under pressure

Many lessons from Ketzin will be applied at any CO2 storage site in the future, though to a certain limit: Geology is stubborn in its refusal to create exact analogues. This is most acutely seen with Statoil's Sleipner Project, which many CCS proponents use as proof that CO2 storage is ready to go.

Certainly Sleipner is a success, with seismic monitors locating more than 10 million tons of CO2 trapped inside it and no leaks, Arts said. But the formation is so porous and vast -- about double the size of Maryland -- that it is "unrepresentatively good," as one researcher put it. At current rates, it could store CO2 for 100 years. This will not be the case for most storage sites.

"Sleipner is frequently taken as an example for aquifer storage," Arts said. "'Look how good it is!' But don't forget that it's so enormous that there's no pressure to build up. And you get pressure build up in a lot of other aquifers. So that's tricky."

Pressure is a primary challenge facing CO2 storage. Before gas is injected, rocks like the Stuttgart Formation are balanced, their pores naturally filled with salty water. When CO2 arrives, it rises and pushes through the brine toward the formation's arched caprock, which is typically hundreds of feet thick -- at Ketzin, 200 meters thick -- and made of impermeable salt or clay. The gas increases pressure on the entire formation. And too much pressure could lead to fracturing of the caprock.

Ketzin has seen a rise in pressure even with its low injection rate, but not large enough to get close to the limit set by mining authorities. However, larger sites could see a marked rise in pressure, a progression that must be carefully monitored, said Stuttgart's Class, the CO2 modeler.

"It's a serious problem," he said. "How far does the pressure spread?"

In one storage model, Class found that after 10 years of injection, pressure already extended far beyond the CO2 in the reservoir. But, he stressed, this pressure largely depends on each formation's borders, known as boundary conditions. If the water displaced by CO2 can flow out of the reservoir, pressure could remain low. But "if you have a completely closed system," he added, "then everything has to be compressed."

The displacement of salt water by CO2 is "a risk we need to take into account," Kühn said. It is possible, though unlikely, that the brine could move up along faults -- areas of fractured rock -- in remote areas of a storage formation. It will be up to larger sites to study the possibility, since Ketzin's displacement is low, he added.

"Geology is quite heterogeneous," Kühn said. "That means somewhere there might be a discontinuity in the caprock. Then the saltwater might be pushed so far upwards that it moves into the drinking water. This must be avoided."

Freshwater contamination will be a primary point of study over the next years, though this focus will be done as much out of duty as actual concern. All storage projects will be located so far below actual fresh water supplies that the spread of salt water or CO2 is unlikely without a catastrophic leak, Class said.

"What does geological storage have to do with drinking water?" he said. "My provocative answer is, nothing."

Multiple barriers

Beyond contamination, geologists will have other concerns if salt water does not flow out of the reservoir. If so, the added pressure could break the caprock and allow CO2 leaks.

Each caprock variety will need to be stressed and each formation probed for pressure tolerance. To that end, German scientists are gouging out specimens of caprock -- one Ketzin sample looked like several hockey pucks melded vertically -- and putting them under piston-induced pressure in jury-rigged lab equipment. Field experiments will have to verify these tests.

If caprock strength is misjudged and it fractures under its burden, CO2 will have found its primary method of escape. These cracks could conduct the gas upward much like a fault. Scientists remain uncertain how much gas could be lost in such a situation, and again Ketzin could shed some light on the question, since its storage is located near a fault, said Würdemann, the project's coordinator.

Under the worst-case scenario, up to 20 percent of the CO2 could escape the Stuttgart Formation, she said. But from there, it is far from a clear shot to the surface. The gas would have to avoid more layers of impermeable rock, what Würdemann calls the "multi-barrier" concept.

"Even if we have a very leaky fault, we expect quite small CO2 concentrations at the surface," she said.

Extremely small leaks can be expected, and most geologists are following a guideline that no more than 0.01 percent of the CO2 will be allowed to escape its formation each year, meaning that in 1,000 years, 90 percent of the CO2 would remain underground. This sounds more dangerous than it is, Kühn said.

"This sounds quite dangerous, but it's a question of the rate," he said. "How much actually escapes? I think it will be very small. But the question is, how small?"

With its monitoring, Ketzin has already provided valuable results, he added. Kühn hopes to continue injecting CO2 until 2013, up to 60,000 tons total. Soon after, he hopes, larger sites like those targeted by companies like Vattenfall and Shell can take over.

"We don't need to have answers for all of our questions by 2015," he said. "We should have answers as best we can. But the demonstration sites will answer many questions, as well. The demonstration sites are necessary to move on."

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