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).
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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.
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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.
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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.
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"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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