Background

Geological storage of CO₂ as a means to mitigate global warming would entail capture of CO₂ from a range of point source industrial emissions. Capture technology represents the major cost element of the CCS chain and the required purity of CO₂ can have a major bearing on actual capture costs.  Impurities in the CO₂ stream have the potential to affect the efficiency and safety of transport and storage systems, for example through increased risks associated with corrosion, or changes in the phase behaviour of the CO₂ stream.

The presence of impurities is likely to have a significant effect on the phase behaviour or CO₂ streams, with implications for the design and operation of pipelines and injection wells. The presence of impurities could also pose a significant threat of increased corrosion of pipeline and well materials.

The presence of impurities in the CO₂ stream may have an effect on all types of geological storage scenarios, especially in terms of changes in storage capacity and injectivity due to changes in phase behaviour with respect to pure CO₂. In addition, impurities could have a significant effect on injectivity through geochemical reactions in the vicinity of injection wells. Geochemical effects such as dissolution of CO₂ and reactions with minerals may determine the long term fate of injected CO₂, especially CO₂ injected into deep saline formations; and thus the effects of impurities on geochemistry may affect the risk profile of storage sites, as geochemical reactions are widely seen as a key mechanism for the stabilisation of pressure and brine displacement. Geochemical reactions also have the potential to affect the integrity of caprock sequences above storage complexes.

IEA GHG Report PH/4-32 (2004) looked at the maximum level of impurities which might be expected in captured CO₂ from a wide range of fuels and from a selection of the contending capture processes; there has been a considerable advance since in the understanding of purity in CO₂ captured using oxy-combustion processes. It is now unlikely that SO₂ and NO_x would be co-captured with CO₂ for transport and storage at the levels previously suggested of 0.5 – 3%.

Natural Resources Canada, were commissioned by IEAGHG and GCCSI in 2009 to undertake a study, aiming to provide an overview of the effects of impurities on storage, with key issues and limiting factors highlighted.  The study will also aim to identify the current state of knowledge and/or gaps and recommend further research priorities on these topics.

Conclusions

In the studied scenarios of impure CO₂ streams where O₂, N₂ and Ar have the highest levels, the greatest impact of impurities is physical, i.e., reducing storage efficiency and injectivity.

The most significant effect is the reduction of storage capacity. It has been shown that the non-condensable impurities cause reduction of CO₂ capacity by a degree greater than their molar fractions when the temperature is not high above the critical temperature of CO₂. Particularly, there is a maximum reduction of the storage capacity in a certain pressure range, where the capacity can drop to below 40 % for the 15% light impurities case compared with that of pure CO₂. The injectivity of impure CO₂ streams reduces as a result of lower density. However, due to the compensation by increased viscosity the reduction of injectivity is smaller than that of storage capacity. In the studied scenarios using the 15% light impurities case, the average reduction of injectivity is 6%, with the largest reduction 26%. The higher buoyancy of impure CO₂ streams will reduce the efficiency of CO₂ dissolution in formation water and CO₂ trapping in rock pores, and thus has the potential reduce the security of CO₂ storage in the near to medium terms.

With regard to chemical effects on rocks, the most significant species are SO_x, NO_x and H₂S. NO_x can catalyze the oxidation of SO₂ to sulphuric acid. The impact of SO₂ on reduction of rock porosity and injectivity appears much smaller than previously thought, because its contact with water is limited with the development of the dry-out zone. NO_x will also promote dissolution of minerals, but will not cause precipitation and therefore reduction of rock porosity. H₂S on its own has not been found to reduce the injectivity in acid gas injection operations and computer simulations. However, if H₂S and SO₂ are co-injected, deposition of elemental sulphur, in the pores over the whole injection period can be a serious concern.

For evaluation of chemical effects on caprock integrity, thermochemical calculations show that SO_x and NO_x increase dissolution of carbonate rocks and aluminosilicate rocks. However, for CO₂ streams considered in this study where SO_x and NO_x concentrations are within 200 ppm, the impact on the dissolution of the rocks is insignificant.

Corrosion of injection well materials may not be serious when the CO₂ stream is dry, due to desiccation of the well zone in the injection period. However, after termination of injection and return of water, corrosion by the acidic impurities could be an issue of concern.

The results of the study have implications on site selection for different CO₂ streams, i.e. a relatively shallow storage site may be appropriate for a low impurity stream, whereas a high impurity stream may reduce the storage capacity by a significant amount. There are also implications for designing pipeline systems, for multiple CO₂ streams and mixing of the streams.

Recommendations

The literature on the effects of impurities on the geological storage of CO₂ is currently limited and the some of the theoretical effects examined in the study cannot yet be verified by experimental data. It is important that IEAGHG keeps updated on further experimental work which would be able to verify the effects.

The outcome of this study may have implications on site selection for different impure streams of CO₂ and injection of different streams into the same storage site.

IEAGHG should ensure that adequate attention is paid to these topics through future storage network meetings and by the study programme.