The supercritical (sc) CO2 used in geologic carbon sequestration chemically and mechanically deteriorates wellbore cement, raising concerns for long-term operation. In contrast to the conventional view of “sulfate attack”, we found that adding 0.15 M sulfate to the acidic brine can significantly alleviate scCO2 attack on Portland cement, resulting in a much stronger cement compared to sulfate free system. To systematically study the mechanism in the scCO2–brine–cement system, we combined reactive transport modeling and scanning electron microscope–back scatter electron (SEM-BSE) analysis. By defining a minimum porosity term in the reactive transport modeling, we found that sulfate could promote CaCO3 ability to fill more nanopore spaces in the cement, and the resulting decreased porosity hindered the intrusion of acidic brine. Moreover, using the transition-state theory in reactive transport modeling, we found that sulfate could promote the growth rate of CaCO3 in cement. These two mechanisms enabled cement to form a thick and compact CaCO3 protective layer on the cement. By harnessing the discovered mechanisms, we incorporated the Na2SO4 and CaSO4 particles into the Portland cement to synthesize a new engineering material that exhibited strong resistance to scCO2 attack. These newly discovered interfacial interactions between CaCO3 and sulfate can provide useful principles for engineering mechanically strong, green materials for safer geologic carbon sequestration.

Sulfate induces a compact and thick CaCO3 formation that helps protect Portland cement from supercritical CO2 attack

ACS Fall 2020 Virtual Meeting

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