Buried at Sea

Norway’s largest producer of oil and gas is injecting 1 million metric tons of carbon dioxide below the North Sea every year in one of the world's largest attempts to lock away the greenhouse gas.  Scripps Institution of Oceanography at UC San Diego researchers are monitoring the CO₂ trapped beneath the ocean floor to determine if it is truly remaining buried at sea.

The Scandinavian country is scattered with eco-friendly projects. The blustery island of Utsira, located roughly 18 kilometers (11 miles) off the coast in the North Sea, uses wind turbines to power the small island community. The oil and gas companies are even going green, mainly to avoid a federally imposed environmental tax.

In an effort to become carbon neutral by 2030, the Norwegian government imposed an ambitious carbon tax on companies emitting greenhouse gases into the atmosphere.

Energy production is a big business in Norway but it is also one of the biggest environmental polluters. Beginning when the fossil fuels are extracted from the earth’s crust, oil and gas carry tons of unwanted carbon dioxide, which is inevitably released into the atmosphere.

Norway’s largest energy company, Statoil, responded quickly to the government policy by identifying alternatives that were cheaper than the fine imposed by the CO₂ tax. In 1996, Statoil began capturing the CO₂ it separated out from the natural gas and pumping it back into an underground chamber, which is surrounded by impermeable rocks and lies just below their Sleipner Field oil platform in the North Sea.

While Scripps researchers Mark Zumberge and Glenn Sasagawa were conducting gravity studies in the region to help energy companies uncover oil and gas deposits, Statoil solicited the help of the geophysicists to monitor the CO₂ sequestration experiment. The U.S. Department of Energy also became interested and from 2002 to 2005 partially funded the research team in part to see if the lessons Statoil was learning from the monitoring effort were applicable closer to home.

Interest in CO₂ sequestration is growing as the U.S. and countries around the world look for ways to reduce their carbon footprint.  But before trapping greenhouse gas underground becomes common practice, its effectiveness must be established. To that end, Scripps researchers are using the force of gravity to monitor its subterranean movements.

Investigating what’s inside the earth requires scientists to use novel approaches. Geophysicists like Zumberge and Sasagawa work with Earth’s gravitational force, which produces subtle changes in elevation of the seafloor, to infer what is happening beneath the surface.

As carbon dioxide accumulates inside the earth, it increases the density of the chamber into which it is pumped, causing the gravitational force above to decrease. By knowing the density of the gas inside, researchers will be able to measure the exact amount that is contained inside the reservoir.

Earth’s gravitational force “actually varies from place to place on Earth and depends on the density of rocks at a particular location,” said Zumberge, who is monitoring the changes in gravity above the CO₂ sequestration experiment.

The researchers first designed and built marine gravimeters to measure seafloor gravity adjacent to volcanoes and fault zones on the Juan de Fuca Plate off the coast of the U.S. Pacific Northwest.

By modifying the inner components of a readily available gravimeter, the research team developed the remotely operated vehicle deep-sea gravimeter (ROVdog) to take the gravity measurements on the seafloor required to monitor the greenhouse gas Statoil pumps underground.  

The carbon dioxide is injected through a thick layer of impermeable rocks and released inside a dome-shaped reservoir consisting of water and sand. The gas is more buoyant than the surrounding water and rises to the top of the dome-shaped tank. This density difference between the water and the gas is what researchers are using to detect the presence of the greenhouse gas and determine if any has leaked out. 

“The hope is that you pump it down there and it never comes out,” said Zumberge.

The team’s gravity measurements are being used in conjunction with seismic surveys that create a picture of how the trapped carbon dioxide is spreading across the massive sand aquifer, known as the Utsira formation. The scientists are using the gravity measurements to confirm what models based on the seismic surveys predict.

The Scripps research team conducted monitoring missions in Norway in 2003 and 2005, choosing summertime and the calm waters it brings to the North Sea to deploy sensitive research equipment to the seafloor.

Launching instruments from a large ship, the team used remotely operated vehicles (ROV) to drop three instruments at a time onto the seafloor 100 meters (328 feet) below. Technicians spent long days maneuvering the vehicles’ arms to gently place the instruments on 30 observation points along a 26-square-kilometer (10-square-mile) stretch of seafloor.

The researchers spent several weeks during a single expedition collecting measurements to create a snapshot of the gravity at various points in time. They anticipate that 10 years of monitoring will provide enough evidence to map the gravity changes at the site over time, a direct indication of the amount of CO₂ trapped in the reservoir.  

In the first five years of the research project, scientists have been able to detect small changes confirming that gas is trapped inside. How much or if any has leaked out is not yet known. The research team anticipates the next round of monitoring in the summer of 2009 will be able to detect if any of the gas is escaping.

“We will have a much better picture of what is there the next time we monitor,” said Zumberge.

The controversy surrounding CO₂ sequestration is whether the gas will remain contained underground. A small leak or massive rupture from a nearby drilling site or fault could send tons of carbon dioxide rushing toward the surface, disturbing the marine environment.

“It is important to determine if this is a safe and effective way to put CO₂ away so it stays there forever,” said Zumberge. “If the gas is leaking out, we want to know about it.

The natural gas extracted from the fossil fuel-rich North Sea contains up to 9 percent carbon dioxide, which is now being sequestered in a vast sub-seafloor cavern that can potentially hold endless amounts of gas, never reaching the atmosphere.

Over roughly 100,000 years, the carbon dioxide in the saline aquifer would gradually diffuse into the water and dissolve into the surrounding rock, becoming part of a solid material.

However, in the short-term, the concern is that a catastrophic release caused by an earthquake or even a slow but steady leak would damage the fertile environment of the North Sea, which is home to highly productive fisheries including herring, cod, and mackerel, which are the target of major commercial operations.

“A leak would slowly change the composition of the surrounding seawater, making it less suitable for the various creatures living there,” said Andrew Dickson, a marine chemist at Scripps who studies the effects of carbon dioxide in seawater but is not involved with the study. “A significant rupture might immediately kill off everything in the sediments and some of the organisms in the overlying waters.”

The life within the deep-sea sediments, from the bacteria and marine worms to clams and flatfish, are at greatest risk. As a steady stream of carbon dioxide percolates through the rocks, it reaches the sediments first, shocking life in the sediments with the highest concentrations of the gas.

The gas would dissipate as it rises through the water column, but over time it could pose a threat even in diffuse form.

In recent years, scientists have observed an increase in ocean acidity levels triggered by an infusion of large amounts of atmospheric carbon dioxide. Ocean acidification is a growing concern to marine scientists because it is known to inhibit the growth of organisms with calclum carbonate shells, such as corals, plankton, and crustaceans. In ways only now being understood by scientists, it has the potential to cause major disruptions to the marine food web.

As CO₂ sequestration on land and in the ocean becomes a widespread practice, more studies like Zumberge’s will be necessary to ensure its long-term viability as an answer to combat global warming.

“This is the kind of thing we have to start doing if we want to sequester CO₂ in a responsible way,” said Zumberge.

By Annie Reisewitz