Monitoring Carbon Dioxide Sequestration in Deep Geological Formations for Inventory Verification and Carbon Credits

Abstract

Abstract Large scale implementation of CO2 Capture and Storage is under serious consideration by governments and industry around the world. The pressing need to find solutions to the CO2 problem has spurred significant research and development in both CO2 capture and storage technologies. Early technical success with the three existing CO2 storage projects and over 30 years experience with CO2-EOR have provided confidence that long term storage is possible in appropriately selected geological storage reservoirs. Monitoring is one of the key enabling technologies for CO2 storage. It is expected to serve a number of purposes - from providing information about safety and environmental concerns, to inventory verification for national accounting of greenhouse gas emissions and carbon credit trading. This paper addresses a number of issues related specifically to monitoring for the purpose of inventory accounting and trading carbon credits. First, what information would be needed for the purpose of inventory verification and carbon trading credits? With what precision and detection levels should this information be provided? Second, what monitoring methods and approaches are available? Third, do the instruments and monitoring approaches available today have sufficient resolution and detection levels to meet these needs? Theoretical calculations and field measurements of CO2 in both the subsurface and atmosphere are used to support the discussions presented here. Finally, outstanding issues and opportunities for improvement are identified. Introduction Geological storage of carbon dioxide, as a method to avoid atmospheric CO2 emissions to the atmosphere, has been underway for more than a decade, beginning in 1996 with the Sleipner Project in Norway1. Today, more than 3 million tonnes of CO2 are injected for the purpose of sequestration annually1,2,3,4. Another 30 million tonnes are injected for CO2 enhanced oil recovery5. Many more sequestration projects are under development, with several new projects announced each year6,7,8. Growing interest in geological sequestration for avoiding or offsetting CO2 emissions has stimulated the need to develop monitoring approaches for assuring that geological sequestration is safe and containment is effective4,9,10,11. Each of the three existing geological sequestration projects uses a different combination of monitoring techniques, depending on the questions that the monitoring program is trying to address, ease of access, and geological attributes of the site. For example, at Sleipner, a combination of time-lapse 2-D and 3-D imaging has been used to track migration of the injected CO2 in the Utsira brine formation with great success12. Recently, gravity measurements were used to estimate the in situ density of CO2 at Sleipner13. At Weyburn, a comprehensive program that included time-lapse 3-D seismic imaging, geochemical sampling and soil gas surveys was used as a multifaceted approach to demonstrate effective containment14. The In Salah Project plans to install a permanent 3-D seismic monitoring array, sample soil gases and introduce tracers for tracking CO2 breakthrough into the gas reservoir15. In addition to these commercial-scale projects, monitoring methods have been tested on a smaller scale at pilot test sites16,17,18. Surface to borehole seismic imaging (VSP), cross-well seismic, cross-well EM, well logs (e.g. RST, resistivity), pressure transients, natural and introduced tracers, brine and gas composition sampling and analysis, flux accumulation chambers, soil gas sampling, and groundwater sampling have been used to monitor the fate and migration of CO2 in the subsurface16,17, 18, 19, 20. Theoretical studies have also been carried out to identify additional monitoring technologies. Borehole gravity, surface EM, and self-potential have been evaluated for application to a Schraeder Bluff-like setting21. Pressure transients below secondary seals have been calculated22. Eddy covariance methods for monitoring surface fluxes have been assessed23. Open-path and plane- or satellite-based optical methods, including the potential for isotopic analysis, are also being developed24,25.