Sulfate Ions Chemically and Mechanically Protect the Portland Cement during Supercritical CO2 Attack: Implications for Geologic Carbon Sequestration

Abstract:

To mitigate the increased atmospheric CO₂ level, geologic carbon sequestration (GCS) has been considered as a promising strategy. Through wellbores, GCS injects pressurized CO₂ into geologic formations such as depleted oil reservoirs, un-mineable coal seams, and deep saline aquifers, and eventually aims to form metal carbonate solids for long-term storage. Under high temperature and high pressure subsurface environments, supercritical (sc) CO₂ and brine interactions can chemically and mechanically deteriorate the CO₂ injection wells made of Portland cement, affecting the safety of the CO₂ storage by creating potential CO₂ leakage. Traditionally, sulfate is an unwelcome species for cement materials because sulfate induces the formation of expansive minerals that adversely affect the mechanical integrity of cement. To evaluate the effects of sulfate during scCO₂ attack on cement, we reacted Portland cement specimens with 0.5 M NaCl or with 0.15 M Na₂SO₄ and 0.05 M NaCl under 95℃, 150 bar scCO₂. The ionic strength of brine was controlled as 0.5 M. After 10 days reaction time, we measured their mechanical properties using a three-point bending test. Interestingly, high concentrations of sulfate significantly alleviated scCO₂’s attack on Portland cement compared to those without sulfate, resulting in cement with much higher mechanical properties (i.e., modulus of rupture and elastic modulus). Microscopic observation, spectroscopy measurement, and reactive transport modeling analysis in this study revealed three underlying mechanisms to explain such findings: First, sulfate promotes the formation of a bassanite (CaSO₄·0.5H₂O) layer protecting the cement from scCO₂ saturated acidic brines. Second, sulfate enables the calcium carbonate (CaCO₃) to fill more nanopore spaces in the cement and decreases the permeability of acidic brine in the cement matrix. Third, sulfate promotes the growth of CaCO₃ in the highly complex cement matrix. In this study, through concerted efforts from detailed mechanical and chemical analyses with reactive transport modeling, we elucidated interfacial interactions among sulfate, cement, and newly formed CaCO_3, which supports safer geologic carbon sequestration and other CO₂-enabled subsurface engineering processes.