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Plunging into Carbon Sequestration Research

Sometime in 2001, probably at a one-hectare site on a depleted oil reservoir in Texas or California, a slug of carbon dioxide (CO₂) will be injected some 600 m (2000 ft) into the ground. But it won’t be pure CO₂. It may be tagged with helium, argon, and other noble-gas tracers introduced into the injected stream at various known concentrations in a regular pattern, providing a "chemical wave" signature. It may also contain intentionally introduced isotopes of carbon, hydrogen, nitrogen, and oxygen whose ratios could shed light on the effects of the injected CO₂ on the site's geochemistry and on the ability of the underground formation to trap the CO₂.

Gerry Moline, a hydrologist in ORNL's Environmental Sciences Division (ESD) who developed the concept of tagging CO₂ with a chemical wave, and David Cole, a geochemist in ORNL's Chemical and Analytical Sciences Division, will use noble gas and isotopic tracers in injected CO₂ to help determine whether and how long the site will securely store, or sequester, the CO₂. The tagged CO₂ will be introduced at injection wells and sampled as it migrates through return wells, a hundred or so meters away. Moline will measure the chemical wave signal in the sampled gas using a portable gas chromatograph. Cole will measure isotope ratios in gas and fluid samples using mass spectrometry.

These measurements should help answer these questions about injected CO₂: Where did it go? How fast does it move? How long will it stay? Could it leak back into the atmosphere, thereby contributing to greenhouse gas levels?

Schematic showing both terrestrial and geological sequestration of carbon dioxide emissions from a coal-fired plant. Rendering by LeJean Hardin and Jamie Payne.

The ORNL results should also help address these questions about the selected site for CO₂ injection: Is this a suitable or unsuitable site for carbon sequestration? What are the optimal depths, temperatures, and pressures for carbon sequestration in this reservoir? How much injected CO₂ is dissolved in underground fluid, trapped in pores, or adsorbed on rock? How much CO₂ can be stored at this site?

Moline says that the noble-gas tracers should provide useful information because they are inert, so they do not interact with subsurface rock and fluids in the same way as the isotopic tracers do. She will be looking for an attenuation of the chemical wave signature when she samples for CO₂ and noble gases. A diminished signal for CO₂ relative to the noble gases will indicate how much of the injected CO₂ has been "lost" during transport—that is, sequestered.

Cole says that analysis of the isotope tracers should help scientists better understand the interactions between the injected CO₂ and the site geochemistry. "When a bubble of carbon gas is injected into the formation," he says, "it may displace the water there. Some of the CO₂ will be dissolved in the fluid, and the CO₂-bearing fluid will interact with the underground rock. Our tracer studies may show whether minerals are precipitated from the CO₂-bearing fluid to the rock, whether minerals in the rock are dissolved in the fluid, and whether the fluid affects the permeability or porosity of the rock in the formation."

These ORNL scientists are participating in the geological sequestration (GEO-SEQ) project sponsored by DOE's Office of Fossil Energy. They are helping to study the effects of injecting CO₂ into depleted oil reservoirs, brine formations, and coal beds. Participants in this three-year project on geologic sequestration of CO₂ include Lawrence Berkeley, Lawrence Livermore, and Oak Ridge national laboratories, in cooperation with Chevron, Texaco, Pan Canadian Resources, Shell CO₂ Co., BP-Amoco, Statoil, the Alberta Research Council Consortium, Stanford University, and the Texas Bureau of Economic Geology. One goal of the work is to determine whether CO₂ injection can result in economic benefits—enhanced oil recovery and production of methane from coal beds—as well as sequestration.

In these studies, we will be studying whether the injection of CO₂ changes the formation," Cole says. "For example, CO₂ interacting with brine, which is mineral-rich saltwater, may lead to changes in the fluid chemistry that could either increase or decrease the porosity-permeability characteristics of the saline aquifer, depending on its mineral composition."

ORNL's role in GEO-SEQ is to evaluate and demonstrate monitoring technologies, with a goal of selecting ones that are safe and that work best in verifying that carbon has been sequestered. GEO-SEQ's other tasks are to (1) develop methods to sequester CO₂ in enhanced oil recovery, sites, depleted gas reservoirs, and saline aquifers; (2) enhance and compare computer simulation models for predicting, assessing, and optimizing geologic sequestration in brine, oil, gas, and coal-bed methane formations; and (3) improve the methodology and information available to assess the sequestration capacity of different sites.

Another approach to slowing the buildup of greenhouse gases in the atmosphere is to minimize losses of carbon and nitrogen from disturbed lands. ORNL and Pacific Northwest National Laboratory have joined Ohio State University and Virginia Polytechnic Institute and State University in a two-year project funded by DOE's Office of Fossil Energy to find ways to improve the natural carbon uptake of lands disturbed by mining, highway construction, or poor management practices. For this purpose, they are studying the use of soil enhancers made from the wastes of coal-fired power plants and sewage treatment facilities. The research is being done at sites in Kentucky, Ohio, and West Virginia. Tony Palumbo, John McCarthy, Gary Jacobs, Jizhong Zhou, Jeff Amthor, and Patrick Mulholland, all of ESD, will study the impact on soil carbon of reclaiming disturbed lands by planting them with grasses and trees (e.g., pines and poplars) and fertilizing them with coal fly ash and sewage waste.

This strip mine site is a source of acid mine drainage (note the amber color) to the environment.

"Because adding fly ash to clay soil makes it friendlier for plant growth, we hope that more carbon will be built up in the soil," Palumbo says. "We will measure soil organic carbon before and after each site is planted. We may plant grass first because it will remove boron and other toxins from the fly ash so they don't adversely affect tree growth. On the other hand, we may find that toxic fly-ash metals will get tied up with organic compounds in the sewage sludge, making them less of a threat to the trees."

Reclamation in progress is turning this degraded land into a site that could reduce soil erosion and sequester more carbon.

If carbon tax credits are granted in the United States, landowners will have more of an economic incentive to exercise stewardship over lands they must reclaim to comply with state and federal laws. They will want their reclaimed sites to receive credit for sequestering carbon.

"We will also measure nitrogen releases from the soil to determine whether ammonia in sewage sludge is being converted to nitrous oxide, a greenhouse gas," Palumbo says. "We will study ways to manipulate the soil, perhaps by wetting it and introducing specific microorganisms, to minimize nitrous oxide emissions."

The project's findings as to what works and what doesn't in reclaiming land to increase carbon storage will be transferred to mine reclamation firms and other interested industrial companies. This effort is part of the activities of DOE's Center for Research on Enhancing Carbon Sequestration in Terrestrial Ecosystems (CSiTE), which is co-managed by Jacobs.

In a project funded by DOE's Office of Fossil Energy, Tommy Phelps, an ESD microbiologist, proposes to build a football-field-sized pond 30 m (100 ft) deep next to a coal-fired power plant. The object of his research is to see if carbon captured from the coal plant and injected into the pond will be successfully sequestered. To sequester carbon in the pond, Phelps plans to introduce the TOR-39 bacteria that he discovered in 1993 in Virginia. The pond will also receive iron leached from coal fly-ash waste.

"These bacteria feed on carbon and respire iron, converting it to magnetite," Phelps says. "They also do not need oxygen or light, so they thrive anywhere in the pond."

Experiments at ESD show that TOR-39 bacteria in water can also make micron-sized particles of iron carbonate (siderite) from iron hydroxide and carbon dioxide introduced into the test tube. "We believe that these bacteria can combine carbon dioxide from the coal plant with iron in the water to produce iron carbonate," Phelps says. "The iron carbonate will sink into the pond sediments, sequestering the carbon."

ORNL researchers are sinking their talents into interesting approaches to carbon sequestration in the hope of delaying climate change.

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Related Web sites

The DOE Center for Research on Enhancing Carbon Sequestion in Terrestrial Ecosystems
ORNL's Environmental Sciences Division