Defining the Brittle Failure Envelopes of Individual Reaction Zones Observed in CO_2-Exposed Wellbore Cement

摘要

To predict the behavior of the cement sheath after CO_2 injection and the potential for leakage pathways, it is key to understand how the mechanical properties of the cement evolves with CO_2 exposure time. We performed scratch-hardness tests on hardened samples of class G cement before and after CO_2 exposure. The cement was exposed to CO_2-rich fluid for one to six months at 65 ℃ 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 compressrve 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 modeling efforts needed to assess the impact of reaction-induced mechanical failure of wellbore cement by coupling sensitivity analysis and mechanical predictions.