Cement degradation in CO2-saturated water and the impact of liquid to solid ratio: an experimental and thermodynamic modelling study

Cement is an important sealing material for geological CO2 storage (GCS) reservoirs.The durability of cement exposed to CO2-saturated water environments is critical for the long-term integrity of GCS structures, as groundwater and CO2 interact with cement, which may significantly affect its microstructure and mechanical properties.In this study, Class G oil well cement was exposed to CO2-saturated water under various liquid-to-solid ratios (L/S), temperatures, and pressures for up to four months.After the exposure, the carbonation depths and the mineral compositions of the specimens were analysed using XRD and EDS techniques.The results revealed that exposure to CO2-saturated water caused the formation of distinct carbonation zones in the specimens.Especially, the dissolution of cement might occur under specific conditions, and it could result in a significant decrease in mechanical properties.This could pose a serious threat to the integrity of the cement, where the influence of the L/S ratio was found critical.Higher L/S ratios led to marked dissolution of the cement, accelerating the loss of mechanical properties.While in the cases of lower L/S ratios, abundant calcium carbonate precipitation was observed in the cement specimens, leading to densification of microstructure and strength enhancement, which might instead help to maintain structural integrity.Furthermore, thermodynamic modelling based on GEMS was performed to provide an evaluation of cement mineral composition changes.The importance of L/S ratio, temperature, and pressure on cement degradation in CO2-rich environments was emphasised, and the critical L/S corresponding to cement dissolution at different environments was revealed.The findings of this research implied that the local exposure conditions are very important for the structural integrity and sustainability of GCS structures in the long term.