Carbon sequestration

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Carbon sequestration
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The carbon cycle
Credit U.S. Geological Survey
Natural and man-made processes
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Atmospheric levels of CO2 have risen from
pre-industrial levels of 280 parts per million
(ppm) to present levels of 375 ppm. Evidence
suggests that the rise in carbon emissions is
closely linked to our increased usage of fossil
fuels.
Credit U.S. Geological Survey
Credit U.S. Geological Survey
Credit U.S. Geological Survey
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There is a clear link between the rise in CO2
levels and the rise of global temperatures
although this is not yet fully understood.
Scientists are still working on it!
Data from Vostok ice core
Data from NASA
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• Predictions of global energy use in the next
  century suggest a continued increase in carbon
  emissions and rising concentrations of CO2 in the
  atmosphere unless major changes are made in the
  way we produce and use energy - in particular,
  how we manage carbon.

Credit U.S. Geological Survey
Credit U.S. Geological Survey
Credit U.S. Geological Survey
One way to manage carbon is to use energy more
efficiently to reduce our need for a major energy
and carbon sourcefossil fuel combustion.
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Another way is to increase our use of low-carbon
and carbon -free fuels and technologies (nuclear
power and renewable sources such as solar energy,
wind power, and biomass fuels).
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Images 1, 2, 3 Credit U.S. Geological Survey
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The newest way to manage carbon is
through. carbon sequestration
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Carbon sequestration
Credit U.S. Geological Survey
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Carbon sequestration refers to the provision of
long-term storage of carbon in the terrestrial
biosphere, underground, or in the oceans so that
the build up of carbon dioxide (the principal
greenhouse gas) concentration in the atmosphere
will reduce or slow.
In some cases, this is accomplished by
maintaining or enhancing natural processes in
other cases, novel techniques are developed to
dispose of carbon.
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Types of carbon sequestration
Credit U.S. Geological Survey
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• There are different types of geological
  formations in which CO2 can be stored, and each
  has different opportunities and challenges.
• Suitable formations are found in three main
  geological situations
• Depleted oil and gas reservoirs
• Unmineable coal beds
• Saline formations

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1. Depleted oil and gas reservoirs as possible
CO2 repositories. Locations considered for CO2
storage are layers of permeable and porous rock
deep underground that are capped by a layer or
multiple layers of non-porous/permeable rock
above them.  ie. These are the same places where
oil and gas are found !!!!!!
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Depleted oil and gas reservoirs that hold crude
oil and natural gas over long geological time
frames are ideal. In general, these involve
layers of permeable/porous rock with layers of
impermeable/non-porous rock above such that they
form a dome.  It is the dome shape that traps the
hydrocarbons.  This same dome offers great
potential to trap CO2 and makes these formations
excellent for sequestration.
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Q. Which situation (s) would be most suitable for
carbon sequestration and why ?
All of them are suitable as long as the gas is
injected into the right place !
Answer
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The technique Sequestration involves drilling
a well down into the reservoir rock and injecting
pressurized CO2 into it.  Under high pressure,
CO2 turns to liquid and can move through a
formation as a fluid.  Once injected, the liquid
CO2 tends to be buoyant and will flow upward
until it encounters a barrier of impermeable
rock, which can trap the CO2 and prevent further
upward migration.
There are other mechanisms for CO2 trapping as
well  CO2 molecules can dissolve in brine, react
with minerals to form solid carbonates, or adsorb
in the pores of the porous rock.  
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The degree to which a specific underground
formation is amenable to CO2 storage can be
difficult to discern.  Research is aimed at
developing the ability to characterise a
formation before CO2-injection to be able to
predict its CO2 storage capacity.   Another area
of research is the development of CO2 injection
techniques that achieve broad dispersion of CO2
throughout the formation, overcome low diffusion
rates, and avoid fracturing the cap rock.  
Site characterisation and injection techniques
are inter-related because improved formation
characterisation will help determine the best
injection procedure.
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In these operations, CO2 is separated from the
fuel and captured either before or after the
combustion of coal. It is then compressed to a
super critical liquid, transported by pipeline to
an injection well and then pumped underground to
depths sufficient to maintain critical
temperatures and pressures. The CO2 seeps into
the pore spaces in the surrounding rock and its
escape to the surface is blocked by a caprock, or
overlaying impermeable layer.
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As a value-added benefit, CO2 injected into a
depleting oil reservoir can enable recovery of
additional oil known as Enhanced Oil
Recovery - EOR When injected into a depleted
oil bearing formation, the CO2 dissolves in the
trapped oil and reduces its viscosity.  This
frees more of the oil by improving its ability
to move through the pores in the rock and flow
with a pressure differential toward a recovery
well.  Typically, primary oil recovery and
secondary recovery via a water flood produce
3040 of a reservoir's original oil.   A CO2
flood enables recovery of an additional 1015 of
the oil. 
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2. Unmineable coal seams.     Unmineable coal
seams are too deep or too thin to be mined
economically.  All coals have varying amounts
of methane adsorbed onto pore surfaces, and wells
can be drilled into unmineable coal beds to
recover this coal bed methane (CBM).  Initial
CBM recovery methods, dewatering and
depressurisation, leave a fair amount of CBM in
the reservoir.  Additional CBM recovery can be
achieved by sweeping the coal bed with nitrogen. 
CO2 offers an alternative to nitrogen.  It
preferentially adsorbs onto the surface of the
coal, releasing the methane.  Two or three
molecules of CO2 are adsorbed for each molecule
of methane released, thereby providing an
excellent storage sink for CO2. 
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3. Saline formations.    Saline formations are
layers of porous rock that are saturated with
brine. They are much more commonplace than coal
seams or oil and gas bearing rocks, and represent
an enormous potential for CO2 storage capacity. 
Saline formations tend to have a lower
permeability than hydrocarbon-bearing formations,
and work is directed at hydraulic fracturing and
other field practices to increase the potential
injection.  Saline formations contain minerals
that could react with injected CO2 to form solid
carbonates. The carbonate reactions have the
potential to be both a positive and a negative.   
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4. Other geological formations a) Shale.  
Shale, the most common type of sedimentary rock,
is characterized by thin horizontal layers of
rock with very low permeability in the vertical
direction.  Many types of shale contain 15
percent organic material, and this hydrocarbon
material provides an adsorption substrate for CO2
storage, similar to where CO2 can be stored in
coal seams.  Given the generally low permeability
of shale, research is focused on achieving
economically viable CO2 injection rates.
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b) Basalt formations.   Basalts are of solidified
lava.  They have a unique chemical makeup that
could potentially convert all of the injected CO2
to a solid mineral form, thus permanently
isolating it from the atmosphere.  Research is
currently being focused on enhancing and
utilizing the mineralisation reactions and
increasing CO2 flow within a basalt formation. 
Research is in its infancy, but these formations
may, in the future, prove to be optimal storage
sites for stranded CO2 emissions.
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5) Other options a) Terrestrial and Marine
Ecosystems Terrestrial sequestration is the
enhancement of CO2 uptake by plants that grow on
land and in freshwater and, importantly, the
enhancement of carbon storage in soils where it
may remain more permanently stored.  Terrestrial
sequestration provides an opportunity for
low-cost CO2 emissions offsets.  Early efforts
include tree-plantings, no-till farming, and
forest preservation.  More advanced research is
being conducted to develop fast-growing trees and
grasses, in deciphering the genomes of
carbon-storing soil microbes and in nutrient
enrichment to enhance algal growth in the oceans.
All of these are potential carbon stores of the
future.
Credit U.S. Geological Survey
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b) Carbon Capture Technologies A new coal-based
generation technology known as Integrated
Gasification Combined Cycle Process offers
promise as a pathway to capture CO2 before
combustion at coal plants and sequester it
downstream. IGCC plants are able to capture
emissions more cost-effectively than methods
currently used at more conventional plantssuch
as supercritical pulverized coalbecause they do
not rely on direct combustion and instead convert
coal feedstocks using gasification. The current
carbon capture rate for IGCC plants is believed
to be around 85 percent.
Efforts are underway to develop capture
technologies for traditional pulverized coal
power plants. At these plants, CO2 would need to
be captured from flue gases after combustion
through a chilled ammonia or amine stripping
process. CO2 capture at conventional plants is
likely to be more costly than at IGCC plants but
has advantages, particularly in the re-fit of
existing plants.