Defining the brittle failure envelopes of individual reaction zones observed in CO₂-exposed wellbore cement

In order to predict the behaviour of the cement sheath after CO₂ injection and the potential for leakage pathways, it is key to understand how the mechanical properties of the cement evolves with CO₂-exposure time. We performed scratch hardness tests on hardened samples of class G cement before and after CO₂-exposure. The cement was exposed to CO₂-rich fluid for one to six months, at 65°C and 8 MPa P_total. Detailed SEM-EDX analyses showed reaction zones similar to those previously reported in the literature: 1) an outer-reacted, porous silica-rich zone; 2) a dense, carbonated zone; and 3) a more porous, Ca-depleted inner zone. The quantitative mechanical data (brittle compressive strength and friction coefficient) obtained for each of the zones suggest that the heterogeneity of reacted cement leads to a wide range of brittle strength-values in any of the reaction zones, with only a rough dependence on exposure time. However, the data can be used to guide numerical modelling efforts needed to assess the impact of reaction-induced mechanical failure of wellbore cement, by coupling sensitivity analysis and mechanical predictions.