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 (P_CO2=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⁺, S^2-, Sr²⁺, Li⁺, and Si^4- in reacted brines depending on the pressure and temperature conditions at which cement samples were exposed. At P_CO2=150 bar and T=50 ◦C conditions, K⁺, Si^4-, Mg²⁺ and Li⁺ were leached from the cement sample while the concentration of Ca²⁺, S^2- and Sr²⁺ in the brine dropped. In contrast, cement samples which were exposed at higher pressure and temperature conditions (P_CO2=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).