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ORNL is conducting research on methane hydrates, a huge source of an energy-rich greenhouse gas.
Methane illustration (jpg, 13K)

Methane Hydrates: A Carbon Management Challenge

An enormous natural gas resource locked in ice lies untapped in ocean sediments and the Arctic permafrost. If this resource could be harvested safely and economically by the United States, we could possibly enjoy long-term energy security. Known as methane hydrates, this resource also may have important implications for climate change. When released to the air, methane is a greenhouse gas that traps 20 times more heat than carbon dioxide (another greenhouse gas). When burned, methane releases up to 25% less carbon dioxide than the combustion of the same mass of coal and does not emit the nitrogen and sulfur oxides known to damage the environment.

Methane hydrates contain methane in a highly concentrated form. Hydrates are a type of ice in which water molecules form cages (clathrates) around properly sized guest molecules. Gas hydrates form when water and gas (e.g., methane, ethane, and propane) come together at the right temperatures and pressures.

Thanks to the recent passage of the authorization bill, The Methane Hydrate Research and Development Act of 1999, the Department of Energy's Office of Fossil Energy is planning a national research and development (R&D) program on methane hydrates. ORNL researchers are doing research in this area using internal funding from the Laboratory Directed R&D (LDRD) Program and are proposing projects for DOE funding.

"The driver of DOE's gas hydrates program is the need for a new, abundant source of relatively clean energy, yet concerns about climate change are being addressed, considering that methane is a greenhouse gas," says Lorie Langley, leader of ORNL's Natural Gas Infrastructure, Methane Hydrates, and Carbon Dioxide Sequestration programs. "Methane can be used as an inexpensive source of hydrogen, a carbon-free fuel that could help slow climate change, providing that methods are developed to sequester the carbon dioxide that results from hydrogen production."

Among the questions the DOE program will address are these: How much natural gas actually is present in the world's methane hydrates? (Estimates range as high as 700,000 trillion cubic feet, many times the estimated total of worldwide conventional resources of natural gas and oil.) Are the hydrates stable enough to sequester carbon dioxide injected into them? Which production methods could safely harvest methane from the hydrates?

What are the risks of recovering methane from ocean hydrates? Could the release of methane make the sediments unstable enough to cause the collapse of seafloor foundations for conventional oil and gas drilling rigs? Could the melting, or dissociation, of methane hydrate ice lead to releases of large volumes of methane to the atmosphere, raising greenhouse gas levels and exacerbating global warming?

To help answer questions about the formation and dissociation of methane hydrates in ocean sediments, ORNL is operating a new seafloor process simulator (SPS), which is the largest, most highly instrumented pressure vessel in the world for methane hydrate studies. This 72-liter vessel, which is more than 30 times larger than the typical vessel used for methane hydrate research, is the product of an LDRD project led by Gary Jacobs and Tommy Phelps, both of ORNL's Environmental Sciences Division (ESD).

Pressure vessel for methane hydrate production experiments (jpg, 49K)
Libby West, who is in charge of day-to-day operations of the seafloor process simulator, and David Peters prepare the pressure vessel for new methane hydrate production experiments. The device, which was designed by Jack Heck, an engineer at the Oak Ridge Y-12 Plant, is operated at 4°C in the cold room in the background.

In the SPS, methane is bubbled into the seawater-containing vessel. The fluid is cooled to ~4°C and pressurized between 50 and 100 atmospheres to form methane hydrates. Methane hydrate samples are produced for analysis by instruments at numerous ports around the vessel, and their formation is captured by a video camera.

"Because of the size of our vessel, we have found a way to make methane hydrates easily and predictably," Phelps says. "Our large pressure vessel is also more suitable for research on the interactions between heterogeneous sediments and hydrates during their formation and dissociation. We can mimic actual heterogeneous conditions such as ocean water and sediments mixed with microorganisms, organic matter, carbonate particles, sand, silt, clay, and sulfides. Our data will be used to test and verify computer models of heterogeneous hydrate formation."

The dissociation of methane hydrates is a major concern for oil companies, Phelps says, noting that five oil firms have expressed interest in conducting research at SPS. "When the temperature rises or the pressure drops, one cubic foot of methane hydrate ice can release 160 cubic feet of gas," he explains. "Forces from methane hydrate dissociation have been blamed for a damaging shift in a drilling rig's foundation, causing a loss of $100 million. Oil and gas drilling companies are more interested in protecting their drilling equipment than harvesting the hydrates as an energy resource, at least for the next 10 years."

At the SPS, hydrates could be grown in intact sediment cores filled with particles of controlled size to determine the effects of decomposing hydrates on sediment structure. Experiments at the SPS might also help determine which conditions could lead to a "burp" of methane from ocean hydrates that might enter the atmosphere and cause climate change. Some evidence suggests that a catastrophic release of frozen methane from the ocean 55 million years ago was responsible for an abrupt warming of the earth. As a result, ocean temperatures rose by 7 to 14 degrees over 1000 years, causing the die-off of more than half of some deep-sea species.

"Eventually, we could do dynamic production simulations at the SPS," Phelps says. "We may test ideas for harvesting methane hydrates, such as depressurization, stimulating them with sound waves to melt them gently, or injecting solvents to extract the methane into gas recovery wells."

What other research is being done at ORNL on methane hydrates? In 1999, Bill Doll, an ESD geophysicist, in collaboration with scientists from Kansas and Canada, used high-resolution seismic reflection methods developed for solving environmental problems to obtain very sharp images of hydrate-bearing zones 1000 m deep and of an overlying permafrost zone. The work was conducted in Canada's MacKenzie Delta, along the Arctic Ocean.

Line of geophones on Arctic snow (jpg, 51K) Pattern of velocity differences (jpg, 74K)
This line of geophones on the Arctic snow (left) receives sound waves produced by a vibrating truck (inset) and reflected back from boundaries marking a change in the type of sediment or rock or in the material filling rock pores. The pattern of velocity differences (right) provides images of the sand and silt layers and the effects of permafrost (high velocities in the top 400 m) and the underlying methane hydrate layer.

"We are developing tools to precisely locate methane hydrate layers, assess whether the hydrate is distributed uniformly or in pockets within the sediments, and ultimately determine how much methane is there," Doll says. "Our high-resolution measurements have impressed oil exploration companies."

In a collaboration with the U.S. Geological Survey (USGS), David Reister and N. S. V. Rao, both of ORNL's Computer Science and Mathematics Division, have been developing an improved method to determine how much methane is present in gas hydrates on the ocean floor. "Hydrates occupy pores of rocks," Reister says. "To determine how much methane is present in the ocean, we must accurately estimate porosity and hydrate concentration in the pores for all ocean sediments."

They are developing mathematical models based on rock physics to predict the locations and concentrations of methane hydrates in oceans and Arctic permafrost in the MacKenzie Delta. They use well log data obtained by oil and gas drilling companies, which provide a variety of measurements, including density, velocity, and electrical resistivity of sediments and the contents of their pores.

Peter T. Cummings, an ORNL-University of Tennessee (UT) Distinguished Scientist, Ariel A. Chialvo, an ORNL-UT collaborating scientist, and Mohammed Houssa of UT are using sophisticated models of methane, carbon dioxide, and water to better understand methane hydrates. "We are doing molecular simulations of methane hydrates at different temperatures, such as 270 K and 170 K," says Cummings. "Methane doesn't like water, so it pushes the surrounding water molecules away in clathrates, forcing them into a structure that is more stable than the normal arrangement of water molecules."

The scientists' goal is to predict the stability of methane hydrates in the real environment. Methane hydrates are trapped in pores of sandstone sediments that are contaminated with bacteria, algae, sand, and ions from saltwater. "We will eventually simulate the effects of impurities on the stability of methane hydrates," Cummings says. "Our models may show that confinement in pores enhances methane hydrate stability."

Claudia Rawn of the Metals and Ceramics Division, Bryan Chakoumakos of the Solid State Division, and Simon Marshall of ESD are interested in using neutrons to measure the effects of temperature and pressure changes on methane hydrate stability. "We measured the expansion of a unit cell of a USGS methane hydrate sample as temperature rises," Rawn says. They hope to determine the effects of different gases on hydrate stability and compare the movements of water molecules and the strengths of hydrogen bonds in hydrates and normal ice.

The DOE National Methane Hydrate Program Plan has four research goals: resource characterization, production technology, global climate change, and safety and seafloor stability. "ORNL has the opportunity and capability to contribute to all of these goals," says Langley.

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Related Web sites

ORNL's Computer Science and Mathematics Division
ORNL's Environmental Sciences Division
National Methane Hydrate R&D Program

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