1. © NERC All rights reserved CCS main geological issues Storage capacity Injectivity Containment

2. © NERC All rights reserved Exposure of biosphere to the CO 2 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

3. © NERC All rights reserved Processes that enhance leakage potential Pore fluid pressure increase in the storage reservoir Might induce or open fractures and faults Transport of CO 2 and formation brine due to pressure gradients in the reservoir Transport of CO 2 due to its buoyancy Dissolution and transport of CO 2 in the formation water by natural fluid flow Chemical reactions caused by acid (CO 2 -saturated) formation water corrosion of steel, cement and rock matrix

4. © NERC All rights reserved 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

5. © NERC All rights reserved Fault plane cemented by gypsum

6. © NERC All rights reserved Damage zone Fault plane cemented Fractures caused by fault movement only partially cemented

7. © NERC All rights reserved 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

8. © NERC All rights reserved 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

9. © NERC All rights reserved Caprock - capillary entry An injected (non-wetting) fluid like CO 2 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 CO 2 will not escape from the reservoir CO 2 brine CO 2 residual brine overpressure hydrostatic pressure

10. © NERC All rights reserved Geochemical issues Corrosion of elements of the rock matrix by CO 2 /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