Effects of Impurities on Geological Storage of CO₂

Publication Overview

Study on evaluation of the effects of impurities on CO₂ transport, injection and storage, sponsored by the International Energy Agency Greenhouse Gas R&D Programme (IEA GHG). The IEA GHG’s objectives of this study are:

• To provide a review of existing information and published research on the potential impact of CO₂ stream purity on storage reservoir and caprock performance and associated engineering costs;
• To provide a high level overview of available knowledge. The focus is on storage of impure CO₂ in deep saline formations, since this scenario has the largest theoretical storage capacity and the most significant potential for complex geochemical reactions, although depleted gas fields and CO₂-EOR are also relevant.

Particular aspects to be considered include:

• The potential effects of impurities on phase behaviour and storage capacity calculations;
•  Effects on the rates of geochemical reactions with both formation and caprocks, and the impacts on injectivity, reservoir permeability and caprock integrity.
•  Effects on buoyancy forces and trapping mechanisms;
• Potential for corrosion of well components and estimated impact on system reliability if not mitigated.

Publication Summary

1. The measure of storage capacity for impure CO₂ and the impact of the impurities have been established. With a simple formula proposed in this work, normalized CO₂ storage capacity can be calculated for any CO₂ mixtures, and the impact of the impurities can be clearly seen.
2. Non-condensable impurities which cannot be liquefied at ambient temperature, such as N₂, O₂ and Ar, result in significant decrease in CO₂ storage capacity. The degree of decrease in the capacity is a function of pressure, temperature, and composition.
3. For all mixtures of supercritical CO₂ and non-condensable gases, there is a maximum decrease of the storage capacity at given temperature. This pressure corresponding to the maximum decrease changes with temperature.
4. In contrast to non-condensable gases, impurities which are more easily condensable than CO₂, such as SO₂, could increase the storage capacity, and there is a maximum increase at a certain
5. For a supercritical CO₂ stream with high levels of N₂, O₂ and Ar, which are common impurities from oxyfuel combustion, the volume and density may be determined by the use of Ar to represent all impurities, computationally and/or experimentally.
6. The change of density caused by non-condensable gas impurities results in lower injectivity of impure CO₂ into geological formations. Above a threshold pressure range the injectivity could reach the level of pure CO₂ due to lowered viscosity. However, more condensable impurities like SO₂ may have an effect of increasing the injectivity, through increasing density of the CO₂
7. Non-condensable gas impurities increase the buoyancy of the CO₂ This would decrease the sweep efficiency of injected CO₂. As a result, the efficiency of solubility trapping and residual trapping of CO₂ would decrease.
8. A formula has been developed to enable quick determination of the effect of SO_x on CO₂ injectivity through precipitation of sulphates, which reduces rock porosity, an issue of substantial concern. With this formula, the degree of the porosity reduction could be directly predicted from the SO_x content of the injected CO₂.
9. Among reactive impurity species, SO_x, H₂S and NO_x would have the greatest chemical effects on the rocks. Based on our analysis, the effect of SO_x on reduction of rock porosity and injectivity would be much smaller than commonly thought. However, if H₂S and SO_x are co-injected, such as in the case where CO₂ streams from pre-combustion capture and oxyfuel combustion are merged, deposition of elemental sulphur in the pore space could be a concern for pore plugging and hence injectivity reduction.  
10. The potential of corrosion of injection well materials has been assessed. The corrosion would be lower if the CO₂ stream is dry, due to desiccation of the well zone during the injection period. However, after termination of injection the acidic impurities would promote the corrosion of cements in the presence of water. CO₂ containing SO_x, NO_x and O₂ impurities would be more corrosive to cements and steels than pure CO₂ or CO₂/H₂S mixtures.