Transcript A re-evaluation of the CO2 storage capacity of the UK

CCS main geological issues
• Storage capacity
• Injectivity
• Containment
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Exposure of biosphere to the CO2 stream
and entrained substances: leakage
pathways
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Natural system – the geology
• Permeable cap rocks
• Fractures – faults, joints, etc.
• Corrosion of the rock matrix
• Lateral transport to a point where there is no cap rock
• Diffusion
Engineered system
• Wells
• Subsidence
• Mines
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Processes that enhance leakage
potential
•
Pore fluid pressure increase in the storage reservoir
• Might induce or open fractures and faults
• Transport of CO2 and formation brine due to pressure gradients in
the reservoir
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Transport of CO2 due to its buoyancy
Dissolution and transport of CO2 in the formation
water by natural fluid flow
Chemical reactions caused by acid (CO2-saturated)
formation water
• corrosion of steel, cement and rock matrix
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Faults
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Can be fluid conduits or barriers to fluid movement
Role can change through geological time, depending
on stress regime
• High pore fluid pressures could induce fluid
movement through faults – need geomechanical
modelling
• Consider damage zone as well as actual fault
plane
Many faults in the North Sea are sealing and prevent
the migration of oil and gas
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Fault plane cemented by gypsum
Damage zone
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Fault plane cemented
Fractures caused by fault
movement only partially
cemented
Fault seals
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Depend on:
• Cementation
• Which rocks are juxtaposed
• Fault smear (mudstones may be smeared along the fault
plane)
• Lithology
• Pore fluid pressure in the reservoir
Geomechanical modelling can give some indication of the
reservoir pore fluid pressures that might induce fault movement
Empirical knowledge helpful
• Fisher & Knipe for North Sea
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Fluid movement through
caprocks
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Many caprocks (shales and mudstones) consist of
tightly packed very small grains
They can have quite high porosity but they have very
low permeability
Nevertheless, fluids can move through the connected
pore spaces, especially if the reservoir and cap rock
are saturated with a single fluid
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Caprock - capillary entry
•
hydrostatic pressure
brine
overpressure
•
An injected (non-wetting)
fluid like CO2 has to
overcome the capillary
forces in the pore throats in
order to enter and eventually
pass through a cap rock – it
must exceed the capillary
entry pressure which can be
measured
Otherwise CO2 will not
escape from the reservoir
CO2
CO2
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residual brine
Geochemical issues
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Corrosion of elements of the
rock matrix by CO2/water
mixtures
• Carbonates dissolve early,
basic aluminosilicates very
slowly
• Requires flux of acid
formation water
Precipitation of minerals in the
pore spaces of the reservoir
rock
• Stores carbon
• ?injection problems unlikely
– slow kinetics
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