Geological sequestration of carbon dioxide concepts and potential impacts

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Geological sequestration of carbon dioxide-
concepts and potential impacts

• Sam Holloway

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Why carbon sequestration?

• The fundamental tenet of carbon sequestration is
  that it should result in a net environmental
  benefit
• Weigh potential for negative local/global
  environmental impact against potential positive
  impact on global atmosphere

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Time frame for CO2 storage

• Must be until well after the end of the fossil
  fuel era, the assumption being that CO2 levels in
  the atmosphere will subsequently begin to decline
• say 1000 to 10,000 years?
• that said, even significantly delayed leakage may
  have some peak shaving value

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Aquifer concept
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Natural analogues for storage

• Many oil fields, gas fields and natural CO2
  fields have existed underground for millions,
  indeed tens of millions, of years. This gives
  confidence that under favourable circumstances
  CO2 can be stored underground until any
  greenhouse crisis has long since passed
• As well as natural analogues for storage, there
  are also plenty of examples of natural leakage of
  carbon dioxide

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How varying leakage rates affect CO2 storage
0.01
0.0322
0.1
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See Hepple Benson 2003 for better exposition of
this
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Typical seals

• Mudstones and silty mudstones
• Have some intergranular permeability so fluids
  can pass through them, albeit very slowly, if
  there is a large enough pressure gradient across
  them
• May contain cracks, faults etc.
• Beds of rock salt (halite)
• Almost impermeable but may contain cracks

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Leakage mechanisms

• Capillary leakage
• Requires stored gas in reservoir to overcome
  capillary entry pressure of cap rock
• Seal will not leak unless this occurs
• Mechanical failure of seal (pre-existing cracks,
  faults, fissures)
• Human intervention (wells - pre-existing,
  injection wells, wells drilled after injection)
• Diffusion infinitesimally slow

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The Sleipner project

• Example of CO2 storage for environmental reasons
• Closely monitored by SACS project using repeated
  seismic surveys
• Seismic is an echo-sounding technique that
  enables us to image the rock layers beneath the
  ground or sea, and sometimes the fluids that
  occur in reservoir rocks
• Seismic reflections are a function of the
  impedance contrast between substances (density
  and sonic velocity)

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• Utsira caprock summary
• About 700 m of dominantly argillaceous strata
• Little evidence of faulting
• Lower Seal
• Grey silty mudstones
• Estimated capillary entrance pressure 2 5.5 MPa
  (Krushin 1997)
• Type A or type B seals (Sneider et al. 1997)
• Core analysis ongoing

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Monitoring

• Monitoring using repeat 3D seismic surveys at
  Sleipner has been extremely successful in imaging
  CO2 in the pore spaces of the reservoir rock
• The CO2 has displaced the water that was
  originally in the pore spaces and thus lowers the
  density and sonic velocity of the rock there is
  an impedance contrast between water-filled and
  CO2-filled reservoir rock

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Remediation

• If a well leaks it may be possible to plug it -
  this has certainly been achieved in an
  underground blowout in the North Sea
• If an unidentified natural leakage pathway is
  present it may be more difficult to seal

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Examples of natural leakage of CO2 from
underground

• Built environment Ciampino, nr Rome, Italy
• Ski resort Mammoth mountain, California
• Volcanic crater lake Lake Nyos, Cameroon
• The CO2 is thought to be of volcanic origin in
  all these examples

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Leakage and the built environment

• The EU Energie programme project NASCENT
  (Natural Analogues for CO2 Storage and Leakage)
  has shown that in Italy there is housing in areas
  of natural CO2 leakage
• Further info on this project from Jonathan
  Pearce, jmpe_at_bgs.ac.uk

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Conclusions - leakage in suburban environment

• Natural CO2 is tolerated but precautions must be
  taken
• e.g. pumps in basements
• Would it be tolerated if the leakage was man
  made?
• almost certainly not - mega lawsuits would result

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Tree kill, Mammoth Mountain
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Mammoth Mountain

• Significant flux through ground but only
  dangerous to man when builds up in snow-covered
  cabins etc.
• Kills trees by acidifying the groundwater around
  their roots?
• CO2 disperses rapidly into the atmosphere

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Lake Nyos
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Lake Nyos disaster

• Slow leak of volcanic CO2 into base of deep
  crater lake
• Saturates deep cold layer of lake water
• Stratified lake water overturns, triggered
  probably by landslide caused by minor earth
  tremor
• Immediate release of large quantity of cold CO2,
  confined by crater walls
• Dense cloud of nearly pure CO2 moves into valley
  via lake spillway
• Asphyxiates 1700 people as they sleep

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Lake Nyos event

• Requires special circumstances
• Lake at valley head
• Lake must be stratified (no seasonal overturn)
• May require crater walls to confine CO2
• Stratified lakes can be monitored
• Lakes can be degassed

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Degassing Lake Nyos
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Conclusions

• The fundamental issue for any geological CO2
  storage project is Will it leak?
• This must be considered on a site-by-site basis
  because the subsurface is an extremely variable
  natural system
• The background knowledge and infrastructure to do
  this is most likely to be available in oil and
  gas provinces
• Very low leakage rates (?c. 0.01) may be
  acceptable

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Storage in coal seams
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Coal seams are low permeability reservoirs
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Coal seams

• Difficulty is injecting it (swelling, slow
  injection rates
• Adsorption should stay there once adsorbed
• May displace methane from sorption sites (ECBM
  opportunity)
• Must capture all displaced methane GWP c. 23
  times greater than CO2 on mass basis

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Coal seams

• Energy mineral that someone may want to mine or
  gasify undergound
• Trials conducted in San Juan basin (exceptional
  permeability)
• Needs much further research
• Ultimate potential huge if low permeability seams
  can be accessed

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Coalbed methane well, Airth, Scotland