Summary In this study, we investigate the long-term integrity of well cements exposed to supercritical carbon dioxide (SC-CO2) and SC-CO2-saturated brine (sodium chloride, NaCl) solution for carbon capture and storage (CCS) applications. By analyzing the correlation among cement composition, carbonation patterns, and mechanical properties, the aim of our research is to demonstrate how exposure environments affect cement chemically and physically. Our findings provide information toward the design of durable cement formulations to enhance long-term zonal isolation in CCS application. Two cement systems—Class G (G), Class G with fly ash (GF)—are exposed to SC-CO2 and CO2-saturated brine at 70°C and 28.0 MPa (4,060 psi) for up to 6 months. Post-exposure characterization includes phenolphthalein tests, X-ray diffraction (XRD), thermogravimetric analysis (TGA), X-ray microscopy (XRM), permeability measurements, Vickers hardness testing, and uniaxial compressive strength (UCS) measurements. These methods link mineralogical changes, microstructural alteration, flowability assessment, and mechanical performance, providing insights into carbonation mechanisms and long-term cement integrity assessment under simulated downhole CCS conditions. High-portlandite cement (G) under SC-CO2 exposure demonstrates a competing mechanism between carbonation and CO2 diffusion. The rapid carbonation near the exposure surface forms calcium carbonate (CaCO3), which blocks pores and restricts further CO2 diffusion. Meanwhile, continuing CO2 diffusion through remaining pathways drives more carbonation. This competing process results in a distinct multilayer structure. While carbonation increases density and hardness, it raises integrity concerns due to the formation of brittle layers. Low-portlandite systems (GF) show uniform carbonation throughout, maintaining structural integrity. The transport mechanism in CO2-brine exposure is different. CO2 dissolves in brine to form carbonic acid, which then moves into the pores. Carbonation rates are slower than SC-CO2 exposure because CO2 concentration is lower in brine. This slower carbonation results in lower levels of CaCO3 formation and deep penetration of CO2-brine solutions into the cement matrix. Consequently, both G and GF cement in CO2-brine shows uniform carbonation with lower hardness increment when compared with SC-CO2 exposure samples. Permeability remains low in both cement systems despite structure and chemical alteration, validating cement’s sealing capacity within the test duration. In conclusion, cement composition affects carbonation mechanisms—high portlandite content drives stratified carbonation, while a well-engineered blend system with fly ash enables uniform carbonation. With this study, we provide a systematic comparison of cement carbonation mechanisms under both SC-CO2 and CO2-brine exposure over extended durations (up to 6 months). By correlating the results of chemical, physical, direct, and indirect techniques, our research reveals environment-specific carbonation mechanisms. These findings contribute to the understanding of the long-term performance of well cement systems in CCS applications, offering the pathway for optimizing cement designs and derisking well construction for CO2 storage.
Liu et al. (2026) studied this question.