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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

© NERC All rights reserved Fault plane cemented by gypsum

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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