Effect of very Low pH on Degradation of Class G Cement: Implications for CO₂ Geological Storage

Depleted hydrocarbon reservoirs from the North Sea are considered suitable and safe for injection and permanent CO2 storage
but the potential cement deterioration around the wellbore constitutes one of the main risks for potential CO2 leakage pathways.
A series of laboratory experiments using Class G cement were carried out to improve our understanding of the reactivity of a
CO2-brine-well cement system at high pressure and temperature conditions (PCO2=150-250-265 bar and T=50-80-90 C) and low
pH. The analysis showed variations in the content of Na+, Ca2+, K+, Mg2+, S2-, Sr2+, Li+, and Si4- in reacted brines depending on
the pressure and temperature conditions at which cement samples were exposed. At PCO2=150 bar and T=50 C conditions, K+,
Si4-, Mg2+ and Li+ were leached from the cement sample while the concentration of Ca2+, S2- and Sr2+ in the brine dropped. In
contrast, cement samples which were exposed at higher pressure and temperature conditions (PCO2=250-265 bar and T=80-90C)
showed an opposed tendency where the concentrations of Ca2+ increased in the brine sample. Based on the results of μm-CT
analyses, a series of degraded layers in the cement samples could be observed which were confirmed to correspond to an external
calcium carbonate-rich layer (carbonated rim) and an inner layer depleted of calcium hydroxide (CH-depleted