CO2 stability of Portland cement based well cementing systems for use on carbon capture & storage (CCS) wells

Three Portland cement based systems formulated with specific inorganic particles and organic admixtures were tested against conventional API Class G oil well cement with respect to CO2 tolerance. Hardened specimens (30 × 50 mm) were prepared and stored under supercritical CO2 (90 °C/400 bar) for one and six months, respectively. CO2 ingress was probed via phenolphthalein test and thermogravimetry. In all samples, formation of different CaCO3 modifications was observed, proving carbonation. Carbonation rates were relatively low and similar, except for one sample. Most detrimental was cracking of specimens as a result of massive CaCO3 formation which comes along with expansion. Best CO2 resistance was obtained from a slag cement (CEM III) blended with a reactive filler which can bind large quantities of portlandite, and by providing pore space in the cementitious matrix for growing of CaCO3. The addition of latex polymers or of other organic admixtures did not provide much improvement over conventional API oil well cement.