Page 28 - Synthetic Fuels Handbook

16 CHAPTER ONE

1.2.5 Gas Hydrates
Gas hydrates are crystalline solids in which a hydrocarbon, usually methane is trapped in
a lattice of ice. They occur in the pore spaces of sediments, and may form cements, nodes,
or layers. Gas hydrates are found in naturally-occurring deposits under ocean sediments or
within continental sedimentary rock formations. 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.
Methane trapped in marine sediments as a hydrate represents such an immense car-
bon 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.
Hydrates store immense amounts of methane, with major implications for energy resources
and climate, but the natural controls on hydrates and their impacts on the environment are
very poorly understood.
Gas hydrates occur abundantly in nature, both in Arctic regions and in marine sedi-
ments. 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 ice. Methane hydrate is
stable in ocean floor sediments at water depths greater than 300 m, and where it occurs, it
is known to cement loose sediments in a surface layer several hundred meters thick.
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. The immense volumes of gas
and the richness of the deposits may make methane hydrates a strong candidate for devel-
opment 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.
Seafloor slopes of 5° 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 land sliding 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.

1.2.6 Biomass
Biomass, in the present context, refers to living and recently dead biologic material which
can be used as fuel or for industrial production. For example, manure, garden waste, and
crop residues are all sources of biomass. Biomass is a renewable energy source, unlike
other resources such as petroleum, natural gas, tar sand, coal, and oil shale. But like coal
and petroleum, biomass is a form of stored solar energy.