Conclusions

Further knowledge on CO2 reactivity with reservoir rocks and cap rocks has been gained through studies of three CO2 storage sites, Sleipner and Weyburn ('industrial' sites), and Montmiral (a natural site).

Modelling of long timescale upward diffusion of dissolved CO2 through the thick clay cap rock at Sleipner shows that diffusion is a very slow process, and is potentially retarded further by geochemical reactions. The overall net result is that only the bottom few meters of the cap rock adjacent to the reservoir is exposed to chemical reactions. As a consequence, the overall performance of the caprock seal is likely to be good. That said, however, migration of CO2 may still be possible through pre-existing or newly formed fractures, or via the well bore. Indeed, for the latter, there is still a question regarding the chemical reactivity of CO2 with borehole completions and the consequent impact on sealing.

Laboratory experiments, modelling and field monitoring at Weyburn show that the carbonate reservoir undergoes mainly dissolution reactions and an increase of porosity, especially close to the injection zone. However, mineral trapping could play a significant role in the long term due to the predicted precipitation of dawsonite. A full 3D dynamic modelling study still needs to be undertaken, where flow, transport and chemical processes are coupled in order to quantify the temporal and spatial changes in CO2 solubility and mineral trapping, and consequent porosity changes.

Although CO2 has accumulated over millions of years in the Montmiral sandstone reservoir, observed chemical reactivity is low. Detailed mineralogical and fluid characterisation combined with numerical modelling show that the dissolution of feldspars is the main reaction and that the porosity has increased by less than 3%. Mineral trapping is very limited and is restricted to minor precipitation of dolomite. Dawsonite was neither observed nor modelled at Montmiral, although often referred to in the literature as an important CO2-trapping mineral.

Overall, therefore, CO2 reactivity is highly site-specific and time dependent. Investigations need to be based on a very precise characterisation of minerals and fluids from the host formation. They also benefit from the combined outputs of laboratory investigations, numerical modelling, field monitoring and comparisons with natural analogues.

Reactivity can be minor for certain types of reservoirs or cap rocks, but significant for others. It is worth noting that chemical reactions are accelerated by high reservoir temperatures, and can be amplified in the presence of a flow regime. The impact of reactivity, either minor or significant, on the geomechanical properties of the reservoir and cap rock needs to be assessed to investigate long term storage stability and security.

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