Gas (Methane) Hydrates -- A New Frontier
Gas (Methane) Hydrates -- A New Frontier
Methane trapped in marine sediments as a hydrate represents such an immense carbon reservoir that it
must be considered a dominant factor in estimating unconventional energy resources; the role of
methane as a 'greenhouse' gas also must be carefully assessed.
Gas hydrates occur abundantly in nature, both in Arctic regions and in marine sediments. Gas hydrate is a crystalline solid consisting of gas molecules, usually methane, each surrounded by a cage of water molecules. It looks very much like water ice. Methane hydrate is stable in ocean floor sediments at water depths greater than 300 meters, and where it occurs, it is known to cement loose sediments in a surface layer several hundred meters thick.
The worldwide amounts of carbon bound in gas hydrates is conservatively estimated to total twice the amount of carbon to be found in all known fossil fuels on Earth.
This estimate is made with minimal information from U.S. Geological Survey (USGS) and other studies. Extraction of methane from hydrates could provide an enormous energy and petroleum feedstock resource. Additionally, conventional gas resources appear to be trapped beneath methane hydrate layers in ocean sediments.
Recent mapping conducted by the USGS off North Carolina and South Carolina shows large accumulations of methane hydrates.
A pair of relatively small areas, each about the size of the State of Rhode Island, shows intense concentrations of gas hydrates. USGS scientists estimate that these areas contain more than 1,300 trillion cubic feet of methane gas, an amount representing more than 70 times the 1989 gas consumption of the United States. Some of the gas was formed by bacteria in the sediments, but some may be derived from deep strata of the Carolina Trough. The Carolina Trough is a significant offshore oil and gas frontier area where no wells have been drilled. It is a very large basin, about the size of the State of South Carolina, that has accumulated a great thickness of sediment, perhaps more than 13 kilometers. Salt diapirs, reefs, and faults, in addition to hydrate gas, may provide greater potential for conventional oil and gas traps than is present in other east coast basins.
The immense volumes of gas and the richness of the deposits may make methane hydrates a strong candidate for development as an energy resource.
Because the gas is held in a crystal structure, gas molecules are more densely packed than in conventional or other unconventional gas traps. Gas-hydrate-cemented strata also act as seals for trapped free gas. These traps provide potential resources, but they can also represent hazards to drilling, and therefore must be well understood. Production of gas from hydrate-sealed traps may be an easy way to extract hydrate gas because the reduction of pressure caused by production can initiate a breakdown of hydrates and a recharging of the trap with gas.
USGS investigations indicate that gas hydrates may cause landslides on the continental slope.
Seafloor slopes of 5 degrees and less should be stable on the Atlantic continental margin, yet many landslide scars are present. The depth of the top of these scars is near the top of the hydrate zone, and seismic profiles indicate less hydrate in the sediment beneath slide scars. Evidence available suggests a link between hydrate instability and occurrence of landslides on the continental margin. A likely mechanism for initiation of landsliding involves a breakdown of hydrates at the base of the hydrate layer. The effect would be a change from a semi-cemented zone to one that is gas-charged and has little strength, thus facilitating sliding. The cause of the breakdown might be a reduction in pressure on the hydrates due to a sea-level drop, such as occurred during glacial periods when ocean water became isolated on land in great ice sheets.
Methane, a "greenhouse" gas, is 10 times more effective than carbon dioxide in causing climate warming.
Methane bound in hydrates amounts to approximately 3,000 times the volume of methane in the atmosphere. There is insufficient information to judge what geological processes might most affect the stability of hydrates in sediments and the possible release of methane into the atmosphere. Methane released as a result of landslides caused by a sea-level fall would warm the Earth, as would methane released from gas hydrates in Arctic sediments as they become warmed during a sea-level rise. This global warming might counteract cooling trends and thereby stabilize climatic fluctuation, or it could exacerbate climatic warming and thereby destabilize the climate.
Results of USGS investigations indicate that methane hydrates possess unique acoustic properties.
The velocity of sound in hydrate is very high, and therefore the velocity of sound in the surface layer of hydrate-cemented sediments also is high. Specific acoustic characteristics of hydrate-cemented sediments are not well known and require further study. Such information has significant implications in the use of sonar devices for defense, seismic exploration, and research.
Realizing the importance of methane hydrates in marine sediments, the USGS has focused work on selected areas where hydrates are known to be common, and where the influences of hydrates on energy resources, climate, and seafloor stability can be analyzed.
At this stage, it is important for USGS scientists to learn how the hydrates form, evolve, and break down, how they affect sediments, and what factors control their concentration at certain locations, as well as to explore for new hydrate accumulations. Cooperation with other Federal agencies, such as the National Oceanic and Atmospheric Administration for bathymetry studies, the Department of Energy for application of hydrate gas extraction technology, and the U.S. Navy for acoustic studies, will enhance the success of future work.