Greenhouse gas emission sources generally produce mixed gases. Previous studies of CO2 capture and storage have typically examined only sequestration of pure CO2. This paper analyzes the cost of separating a gas mixture from a power station flue gas stream and injecting it into an offshore subsurface reservoir. The costs of separating and storing various gas mixtures were analyzed at two extremes. One extreme in which the entire flue gas stream containing both CO2 and N2 is stored. The other extreme in which as much CO2 is separated as is technically possible using gas membrane capture coupled with chemical absorption. The results indicate that for the gases investigated, using a gas membrane capture system, the lowest sequestration cost per tonne of CO2 avoided occurs when a mixed gas with a CO2 content of about 60% is sequestered. Lower costs and higher tonnages of CO2 avoided can be achieved using an amine based absorption capture system. At the lowest cost point, and for most of the range of cases studied, the cost of capture is significantly greater than the cost of storage. However, this depends on the source of the CO2, the distance between the source and the injection site, and the reservoir into which CO2 is injected.

Monitoring is a regulatory requirement for all carbon dioxide capture and geological storage (CCS) projects to verify containment of injected carbon dioxide (CO 2 ) within a licensed geological storage complex. Carbon markets require CO 2 storage to be verified. The public wants assurances CCS projects will not cause any harm to themselves, the environment or other natural resources. In the unlikely event that CO 2 leaks from a storage complex, and into groundwater, to the surface, atmosphere or ocean, then monitoring methods will be required to locate, assess and quantify the leak, and to inform the community about the risks and impacts on health, safety and the environment. This paper considers strategies to improve the efficiency of monitoring the large surface area overlying onshore storage complexes. We provide a synthesis of findings from monitoring for CO 2 leakage at geological storage sites both natural and engineered, and from monitoring controlled releases of CO 2 at four shallow release facilities -ZERT (USA), Ginninderra (Australia), Ressacada (Brazil) and CO 2 field lab (Norway).

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