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

Abstract

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 Ptotal. 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.