A methodology for reactive transport modelling and geomechanical investigation of wellbores in CO2 storage sites

The movement of injected CO2 from injection wells in CO2 storage sites creates an acidic environment that comes into contact with the cement sheath with a possible consequential effect on its sealing performance. Thus the durability of the cement sheath can be threatened due to the degradation process and the stress state resulting from the new conditions found underground. The integrity of the cement sheath is a function of the cement composition, surrounding rock type, fluid pressure, stress state, temperature, and the type of well. This paper provides a methodology to investigate the stability of the rock-cement-casing assemblage. The assemblage’s mechanical behaviour is coupled with the chemical alterations resulting from it being in contact with CO2-bearing fluids. The cement matrix is a quasi-brittle material which shows a plastic behaviour. The character of the geomechanical part and the geochemical aspect mutually affects each other which is studied in this paper. A plastic-damage approach benefiting from the concept of embedded bands is also introduced to characterise the performance of the cement sheath within CO2 storage sites. This approach considers both the failure and the deformation phenomena occurring within altered zones. In this paper, injection and abandoned wells are hypothesised at depths between 800 and 2500 m and their performance investigated. It is shown that abandoned wells are considered more likely to remain safe while the interfacial transition zones in injection wells are predicted to fail, which converts them to potential leakage pathways. Nevertheless, caution should be taken when the real wells are considered to match the required assumptions in the Introduction Section. In injection wells at depths of 1225–2500 m the cement-casing interface encounters pure dilation. However, the compaction process deforms the rock-cement interface at depths of 800–2500 m, which increases its durability again.