8.5 Geological security
Geological security of carbon dioxide storage depends on a number of factors. The first and foremost prerequisite is a careful storage site selection. The storage site and its surroundings need to be characterized in terms of geology, hydrogeology, geochemistry and geomechanics (structural geology and deformation in response to stress changes). The greatest emphasis should be placed on the reservoir and its sealing horizons to avoid leakages through the seal and/or faults. At Sleipner, characterisation of the reservoir and caprock was carried out at a range of scales. Available geological information show that extensive rifting and normal faulting occurred in the North Sea and the Norwegian Sea before and during early Cenozoic (Paleogene period, 65-23 million years). The Utsira formation was deposited in late Middle Miocene (ca.20 million years) to Early Pliocene (~13 million years). Recent geological structures are associated with mud volcanoes and intraformational faults and are more likely to affect the underlying Oligocene (ca. 36 million years) sediments (Fabriol 2001). Microseismic studies show that the injection of CO2 in sands of the Utsira Formation should not trigger any measurable microseismicity except in impermeable or semi-permeable shale lenses that block the rise of the CO2 toward the top of the formation. Absence of major tectonic events after the deposition of the Utsira formation coupled with the evidence from microseismic studies further builds the confidence in geological security of carbon dioxide storage at Sleipner. Moreover, evidence (e.g. reservoir flow modelling and seismic monitoring of the injected CO2) from ten years experience shows no leakages of carbon dioxide from storage site.
Monitoring is needed primarily to build our confidence in geological security of CO2 storage. Specifically, to detect leakage and provide an early warning of any seepage or leakage that might require mitigating action. Also to ensure and document the injection process, verify the quantity of injected CO2 that has been stored by various mechanisms and finally to demonstrate with appropriate monitoring techniques that CO2 remains contained in the intended storage formation(s). This is currently the principal method for assuring that the CO2 remains stored and that performance predictions can be verified and requires some combination of models and monitoring. At Sleipner the CO2 injection process was monitored using seismic methods and this provided insights into the geometrical distribution of the injected CO2. It also allowed increase understanding of the CO2 migration within the reservoir.
The effectiveness of geological storage also depends on a combination of physical and geochemical trapping mechanisms (Section 3.2). The most effective storage sites are those where CO2 is immobile because it is trapped permanently under a thick, low-permeability seal or is converted to solid minerals or through a combination of physical and chemical trapping mechanisms. Reservoir simulations were carried out successfully at both local and regional- scale models followed by a calibration of the local reservoir model to verify the seismic and geological interpretations and to predict the long-term fate of the stored CO2. The results of the simulations show that most of the CO2 accumulates in one bubble under the cap seal of the formation a few years after the injection is turned off. The CO2 bubble spreads laterally on top of the brine column and the migration is controlled by the topography of the cap seal only. Thus preliminary results suggest that in the long term (> 50 years) the phase behaviour (solubility and density dependence of composition) will become the controlling fluid parameters at Sleipner. The primary benefit of solubility trapping is that once CO2 is dissolved, it no longer exists as a separate phase, thereby eliminating the buoyant forces that
drive it upwards. Next, it will form ionic species as the rock dissolves, accompanied by a rise in the pH. Finally, some fraction may be converted to stable carbonate minerals (mineral trapping), the most permanent and secure form of geological storage. The recent studies at Sleipner area (Section 8.4) strengthens further the geological security of carbon dioxide storage in the Utsira formation.
Evidence from oil and gas fields indicates that hydrocarbons and other gases and fluids including CO2 can remain trapped for millions of years (Magoon and Dow, 1994; Bradshaw et al., 2005). Carbon dioxide has a tendency to remain in the subsurface (relative to hydrocarbons) via its many physicochemical immobilization mechanisms. World-class petroleum provinces have storage times for oil and gas of 5–100 million years, others for 350 million years, while some minor petroleum accumulations have been stored for up to 1400 million years. However, some natural traps do leak, which reinforces the need for careful site selection, characterization and injection practices.
8.5 Geological security
Geological security of carbon dioxide storage depends on a number of factors. The first and foremost prerequisite is a careful storage site selection. The storage site and its surroundings need to be characterized in terms of geology, hydrogeology, geochemistry and geomechanics (structural geology and deformation in response to stress changes). The greatest emphasis should be placed on the reservoir and its sealing horizons to avoid leakages through the seal and/or faults. At Sleipner, characterisation of the reservoir and caprock was carried out at a range of scales. Available geological information show that extensive rifting and normal faulting occurred in the North Sea and the Norwegian Sea before and during early Cenozoic (Paleogene period, 65-23 million years). The Utsira formation was deposited in late Middle Miocene (ca.20 million years) to Early Pliocene (~13 million years). Recent geological structures are associated with mud volcanoes and intraformational faults and are more likely to affect the underlying Oligocene (ca. 36 million years) sediments (Fabriol 2001). Microseismic studies show that the injection of CO2 in sands of the Utsira Formation should not trigger any measurable microseismicity except in impermeable or semi-permeable shale lenses that block the rise of the CO2 toward the top of the formation. Absence of major tectonic events after the deposition of the Utsira formation coupled with the evidence from microseismic studies further builds the confidence in geological security of carbon dioxide storage at Sleipner. Moreover, evidence (e.g. reservoir flow modelling and seismic monitoring of the injected CO2) from ten years experience shows no leakages of carbon dioxide from storage site.
Monitoring is needed primarily to build our confidence in geological security of CO2 storage. Specifically, to detect leakage and provide an early warning of any seepage or leakage that might require mitigating action. Also to ensure and document the injection process, verify the quantity of injected CO2 that has been stored by various mechanisms and finally to demonstrate with appropriate monitoring techniques that CO2 remains contained in the intended storage formation(s). This is currently the principal method for assuring that the CO2 remains stored and that performance predictions can be verified and requires some combination of models and monitoring. At Sleipner the CO2 injection process was monitored using seismic methods and this provided insights into the geometrical distribution of the injected CO2. It also allowed increase understanding of the CO2 migration within the reservoir.
The effectiveness of geological storage also depends on a combination of physical and geochemical trapping mechanisms (Section 3.2). The most effective storage sites are those where CO2 is immobile because it is trapped permanently under a thick, low-permeability seal or is converted to solid minerals or through a combination of physical and chemical trapping mechanisms. Reservoir simulations were carried out successfully at both local and regional- scale models followed by a calibration of the local reservoir model to verify the seismic and geological interpretations and to predict the long-term fate of the stored CO2. The results of the simulations show that most of the CO2 accumulates in one bubble under the cap seal of the formation a few years after the injection is turned off. The CO2 bubble spreads laterally on top of the brine column and the migration is controlled by the topography of the cap seal only. Thus preliminary results suggest that in the long term (> 50 years) the phase behaviour (solubility and density dependence of composition) will become the controlling fluid parameters at Sleipner. The primary benefit of solubility trapping is that once CO2 is dissolved, it no longer exists as a separate phase, thereby eliminating the buoyant forces that
drive it upwards. Next, it will form ionic species as the rock dissolves, accompanied by a rise in the pH. Finally, some fraction may be converted to stable carbonate minerals (mineral trapping), the most permanent and secure form of geological storage. The recent studies at Sleipner area (Section 8.4) strengthens further the geological security of carbon dioxide storage in the Utsira formation.
Evidence from oil and gas fields indicates that hydrocarbons and other gases and fluids including CO2 can remain trapped for millions of years (Magoon and Dow, 1994; Bradshaw et al., 2005). Carbon dioxide has a tendency to remain in the subsurface (relative to hydrocarbons) via its many physicochemical immobilization mechanisms. World-class petroleum provinces have storage times for oil and gas of 5–100 million years, others for 350 million years, while some minor petroleum accumulations have been stored for up to 1400 million years. However, some natural traps do leak, which reinforces the need for careful site selection, characterization and injection practices.
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