Durability of Well Cement Systems in Carbon Capture and Storage: Insights into Long-Term Carbonation Under Supercritical CO2 and CO2-Saturated-Brine Exposure

Abstract This study investigates the long-term integrity of well cements exposed to supercritical CO2 (SC-CO2) and SC-CO2-saturated brine (NaCl) solution for Carbon Capture and Storage (CCS) applications. By analyzing the correlation between cement composition, carbonation patterns, and mechanical properties, the research aims to demonstrate how exposure environments affect cement chemically and physically. Findings will provide information towards 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 (158°F) and 28 MPa (4,060 psi) up to 6 months. Post-exposure characterization includes phenolphthalein tests, XRD, TGA, X-ray microscopy (XRM), permeability measurements, Vickers hardness testing, and uniaxial compressive strength 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 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 multi-layer structure. While carbonation increases density and hardness, it raises integrity concerns due to the formation of brittle layers. Low-portlandite systems (GF) shows 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 is 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 to SC-CO2 exposure samples. Permeability remains low in both cement systems despite structure and chemical alteration, validating cement’s sealing capacity within test duration. In conclusion, cement composition affects carbonation mechanisms - high portlandite content drives stratified carbonation, while well engineered blend system with fly ash enables uniform carbonation. This study provides 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, the 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.