NERC All rights reserved CCS main geological issues • Storage capacity • Injectivity • Containment
Dec 18, 2015
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CCS main geological issues
• Storage capacity
• Injectivity
• Containment
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Exposure of biosphere to the CO2 stream and entrained substances: leakage
pathways• 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
• 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
• 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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Damage zone
• Fault plane cemented
• Fractures caused by fault movement only partially cemented
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Fault seals
• 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
• 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
• 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
brine
CO2
residual brine
overpressurehydrostatic
pressure
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Geochemical issues• 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