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CCS main geological issues

CCS main geological issues. Storage capacity Injectivity Containment. 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

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CCS main geological issues

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  1. CCS main geological issues • Storage capacity • Injectivity • Containment

  2. 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

  3. 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

  4. 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. Fault plane cemented by gypsum

  6. Damage zone • Fault plane cemented • Fractures caused by fault movement only partially cemented

  7. 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. 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. 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 hydrostatic pressure brine overpressure CO2 CO2 residual brine

  10. 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

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