14. Reactions between CO2 and borehole infrastructures
The engineered seals around boreholes may provide routes for CO2 migration regardless of how effective the caprock seal is. It is therefore very important to understand the behaviour of borehole materials in the longer term in the presence of CO2, and in particular the rock/cement and cement/steel interfaces, as these could provide a focus for CO2 migration.
The aim of this study was to provide an improved quantitative understanding of how CO2 interacts with borehole cement and steel, over both short and long timescales. Laboratory experiments were used to investigate shorter-term reactions of CO2 with borehole materials under realistic in-situ conditions found deep underground.
Longer-term reactions of cement were investigated through the study of relatively rare naturally-occurring cement minerals (natural analogues) – in this case in Northern Ireland. Information from these two areas was used to improve our ideas of how borehole materials are likely to behave in a CO2 storage scheme. This helped to calibrate predictive computer modelling simulations of the evolution of borehole systems, and will ultimately improve our confidence that borehole seals will be effective for the timescales necessary to contain stored CO2.
The laboratory experiments resulted in carbonation of the outer few mm of the cement samples, and the formation of carbonate minerals such as calcite. Other tests assessed changes in the physical properties of the cement compared to samples that have not been exposed to CO2, some of which suggest that limited reaction with CO2 may actually improve the sealing properties of borehole cement, but more extensive reaction led to leaching of the cement and a reduction of its sealing properties.
Naturally-occurring cement minerals reacted with atmospheric CO2 for up to approximately 10000 years showed very similar types of reaction. 1 cm thick ‘rinds’ of carbonate minerals that formed around lumps of cement phases have apparently protected them from further reaction, lending further support to the idea that limited carbonation might be able to improve the sealing properties of borehole cement.
Results of a lab test using a poorly-cemented cement/steel block showing the evolution of fluid flow (permeability) over time. Initially the permeability dropped (i.e. sealing was improved), but prolonged flushing with CO2-rich water appeared to have led to increased dissolution and an increase in permeability (i.e. sealing was reduced).