Methane Hydrates Jake Ross and Yuliana Proenza

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Methane HydratesJake Ross and Yuliana Proenza

• The Three Questions
• What is a Gas Hydrate?
• What is their potential as an energy resource?
• What role do they play in global climate change?

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What is a Gas Hydrate?

• A gas hydrate is a crystalline solid its
  building blocks consist of a gas molecule
  surrounded by a cage of water molecules.
• It is similar to ice, except that the crystalline
  structure is stabilized by the guest gas molecule
  within the cage of water molecules.
• Suitable gases are carbon dioxide, hydrogen
  sulfide, and several low-carbon-number
  hydrocarbons. Most gas hydrates , however are
  Methane Hydrates.

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Hydrate Samples
Gas hydrates in sea-floor mounds Here methane
gas is actively dissociating from a hydrate mound.
Gas hydrate can occur as nodules, laminae, or
veins within sediment.
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CH4 Hydrate Stability
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Where are Methane Hydrates located?

• Found in 4 major location types
• Subduction zones (e.g., Nankai Trough Japan,
  Cascadia Basin)
• Passive Margins (e.g., Blake Ridge on the
  southeast cost of the US)
• Off-shore hydrocarbon (e.g., Gulf of Mexico,
  North Slope Alaska)
• On-shore Arctic Permafrost (e.g., Mackenzie
  Delta, Arctic Russia, Arctic Alaska)

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Where are Methane Hydrates located?

• Methane hydrate occurs in a zone referred to as
  the hydrate stability zone.
• The zone lies roughly parallel to the land or
  seafloor surface.
• Permafrost regions,
• depths about 150 - 2000 m below the surface.
• In oceanic sediment
• ocean is at least 300 m deep,
• depths of 0 - 1,100 m below the seafloor.

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Where are Methane Hydrates located?

• Hydrate concentration occurs at depocenters
• Where there is a rapid accumulation of organic
  detritus (from which bacteria
  generate methane). Carbon isotope analyses
  indicate most of the methane in hydrates is
  microbial, however thermogenic sources have been
  identified in the Gulf of Mexico
• Where there is a rapid accumulation of sediments
  (which protect detritus from oxidation).

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What is the potential of CH4 Hydrates as an
energy resource
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• The estimated conventional gas resources and
  reserves in the United States alone are 1,400
  trillion cubic feet.
• If it could be safely and economically recovered,
  one 50 by 150 kilometer area off the coast of
  North and South Carolina is estimated to hold
  enough methane to supply the needs of the United
  States for over 70 years

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Conceptual Drawing of Blake Ridge
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Why are CH4 Hydrates a good energy resource

• The gas is held in a crystal structure, therefore
  gas molecules are more densely packed than in
  conventional or other unconventional gas traps.
• Hydrate forms as cement in the pore spaces of
  sediment and has the capacity to fill sediment
  pore space and reduce permeability. CH4 -
  hydrate-cemented strata thereby act as seals for
  trapped free gas
• 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

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A Proposed Method

• For the gas production from hydrates and the
  seabed stability after the production, we
  proposed a new concept. The figure illustrates
  the molecular mining method by means of CO2
  injection in order to extract CH4 from gas
  hydrate reservoirs. The concept is composed of
  three steps as follows 1) injection of hot sea
  water into the hydrate layer to dissociate the
  hydrates, 2) produce gas from the hydrate, 3)
  inject CO2 to form carbon dioxide hydrate with
  residual water to hold the sea bed stable

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CH4 Hydrates and Climate Change

• Methane is a very effective greenhouse gas. It
  is ten times more potent than carbon dioxide.
• There is increasing evidence that points to the
  periodic massive release of methane into the
  atmosphere over geological timescales. Are these
  enormous releases of methane a cause or an effect
  of global climate change?

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• Global warming may cause hydrate destabilization
  through a rise in ocean bottom water
  temperatures. The increased methane content in
  the atmosphere in turn would be expected to
  accelerate warming, causing further dissociation,
  potentially resulting in run away global warming.
  
• Sea level rise, however, during warm periods may
  act to stabilize hydrates by increasing
  hydrostatic pressure, thereby acting as a check
  on warming.
• Hydrate dissociation may act as a check on
  glaciations, whereby reduced sea levels may cause
  seafloor hydrate dissociation, releasing methane
  and warming the climate.

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This diagram illustrates the affect sea level
change has on the stability of hydrates.
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The Past

• A prominent negative shift in d 13C has been
  recorded in Late Paleocene sediments worldwide.
• The late Paleocene-early Eocene interval (55.5
  mya) was a thermal maximum
• Ocean bottom waters warmed rapidly by as much 4
  degrees C, along with a concurrent rapid shift in
  d 13C values of all the carbon reservoirs in the
  global carbon cycle
• Data from sediments cores suggest that the
  isotopic shift occurring within no more than a
  few thousand years
• Only a catastrophic infusion of d 12C-enriched
  carbon from methane hydrates could cause such a
  rapid shift.

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CH4 Hydrates and the Atmosphere

• An important aspect of methane hydrates and their
  affect on climate change is their potential to
  enter the atmosphere
• Methane concentration in seawater is observed to
  decrease by 98 between a depth of 300m and the
  sea surface as a result of microbial oxidation.
• The flux of methane into the atmosphere is thus
  lowered 50-fold (Mienert et al., 1998)
• However during catastrophic events such as
  largescale sediment slumping much higher
  proportions of methane would be released.

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The Future of Methane Hydrates

• Worldwide gas production in the next 30-50 years
• Areas with unique economic and/or political
  motivations could see substantial production
  within 5-10 years
• We need to better understand the mechanisms of
  hydrate disassociation and its role in global
  warming, either as an accelerator or and
  inhibitor