8.7 Conclusions
The security of carbon dioxide storage in geological formations first and foremost depends on careful storage site selection followed by characterization of the selected site in terms of geology, hydrogeology, geochemistry and geomechanics (structural geology and deformation in response to stress changes). The Utsira Formation is well characterized with respect to porosity and permeability (good storage capacity and injectivity), mineralogy, bedding, depth, pressure and temperature. It is a very large aquifer with a thick and extensive claystone top seal. Available geological information shows absence of major tectonic events after the deposition of the Utsira formation. This means that the geological environment is tectonically stable which implies that the site is suitable for carbon dioxide storage. Microseismic studies suggest the injection of CO2 in sands of the Utsira Formation has not trigged any measurable microseismicity. This further builds the confidence in geological security of carbon dioxide storage at Sleipner. Moreover, evidence from ten years experience of carbon dioxide storage shows no leakages.
The Sleipner project is a commercial CO2 injection project and proved that CO2 capture and storage is a technically feasible and effective method for greenhouse mitigation. It further demonstrates that CO2 storage is both safe and has a low environmental impact. Monitoring is needed for a wide variety of purposes. Specifically, to ensure and document the injection process, verify the quantity of injected CO2 that has been stored by various mechanisms, 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. Finally monitoring is required to detect leakage and provide an early warning of any seepage or leakage that might require mitigating action and to assess environmental effects. The work that has been undertaken at Sleipner Gas Field has shown that the injected CO2 can be monitored within a geological storage reservoir, using seismic surveying. The geochemical and reservoir simulation work have laid the foundations to show how the CO2 has reacted and what its long term fate in the reservoir will be. The results of the simulations indicate that most of the CO2 accumulates in a stack of accumulations under thin clay layers interbedded in the sand unit few years after the injection is turned off. The CO2 plume spreads laterally on top of the brine column and the migration is controlled by the interbedded thin clay layers within the sand unit. In the long term (> 50 years) the phase behaviour (solubility and density dependence of composition) will become the controlling fluid parameters at Sleipner. The solubility trapping has the effect of eliminating the buoyant forces that drive CO2 upwards and through time can lead to mineral trapping, which is the most permanent and secure form of geological storage.
The recent studies at Sleipner area reveal the integrity of the cap rock (efficient sealing capacity). The injected CO2 will potentially be trapped geochemically and the regional groundwater flow having an effect on the distribution of CO2 with the potential of pressure build up as a result of CO2 injection is unlikely to occur. Monitoring techniques (both Time- lapse Gravity and Seismic methods) proved to be key tools in understanding the whole-
reservoir performance. Overall, the recent studies at Sleipner area demonstrate further the geological security of carbon dioxide storage and the monitoring tools strengthen verification of safe injection of CO2 in the Utsira formation. Subsequent work in the following years is necessary to reinforce these findings further that CO2 storage is safe through monitoring and verification procedures that will be able to detect potential leaks.