Chemical and mechanical deterioration of wellbore cement by injected supercritical (sc) CO2 raises a serious concern for the long-term operation of geologic carbon dioxide sequestration (GCS). Sulfate is known to adversely affect cement integrity. Recently, however, we found that introducing 0.15 M sulfate to scCO2-acidified brine under conditions relevant to GCS operation can unexpectedly retard scCO2 attack on Portland cement and result in a much stronger cement than found in a sulfate-free system. To systematically study the underlying mechanism, we combined experimental results with reactive transport modeling (CrunchTope). In particular, to analyze the thicknesses of CO2 attacked zones, we imaged the reacted samples using a scanning electron microscope–back scatter electron (SEM-BSE). We found that the presence of sulfate promoted the growth of CaCO3 in the cement, filling more nanopore spaces and decreasing its porosity. This pore-filling should reduce the intrusion of acidic brine into the cement, alleviating scCO2 attack. Together, these two mechanisms driven by the presence of sulfate resulted in a thick, compact, and protective CaCO3 layer on the cement. Consequently, the co-existence of scCO2 and sulfate ions can lessen the impact of CO2 attack on cement’s mechanical strength. Our findings provide new insights into the interactions between CaCO3 and sulfate and CaCO3 formation under conditions relevant to CO2-related subsurface operation sites. The principles that learned here can also be applied to green engineering of stronger building materials.

Sulfate ions yield compact CaCO3 formations in Portland cement pores under supercritical CO2 attack

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.