While ensuring the long-term integrity of wellbore sealants is critical for the success of geological carbon storage (GCS), the chemical degradation of conventional materials under CO₂-rich conditions remains a major challenge. This study investigates the carbonation behavior of a one-part granite-based geopolymer, integrating a novel pore-scale simulation framework with experimental validation. A new model, ReacSan, is developed to simulate CO₂ transport and carbonation reactions within the evolving microstructure of the geopolymer under GCS-relevant conditions. The framework incorporates CO₂ dissolution using the Redlich–Kwong equation of state, gel dissolution via transition state theory, ion transport using the Lattice Boltzmann Method, and chemical reactions through thermodynamic modeling. The model was validated through experiments exposing equivalent geopolymer samples to CO₂ under in-situ conditions. The experimentally observed rapid carbonation, leading to a decrease in pore fluid pH and the precipitation of CaCO₃ matched the numerical simulations well, demonstrating the ability of the novel ReacSan framework to capture both temporal and spatial variations in the microstructure and carbonation mechanisms of alkali-activated materials (AAMs) exposed to supercritical CO₂. Based on the demonstrated validity of the model, the model is capable of providing detailed predictions of carbonation progression of AAMs or any other sealants over longer time- and length-scales required to ensure long-term GCS integrity.

This paper reports exposure of five different sealants to simulated well conditions with supercritical CO2 containing H2S or H2SO4 as impurities. Three of these sealants are based on Portland Cement, while the other two are based on Calcium Aluminate Cement, and a rock-based geopolymer specifically developed for Geological CO2 Storage (GCS). The impact of the impurities on these sealants was assessed through scanning electron microscopy with energy-dispersive X-ray spectroscopy, computed tomography scanning, and fluid chemical analysis, and compared to previous research where the same five sealants were exposed to clean CO2 under otherwise identical conditions.

The results show that during exposure to CO2-saturated water, the presence of H2S mostly resulted in enhanced sealant alteration depths, and reduced carbonate precipitation. During exposure to wet supercritical CO2, the presence of H2S or H2SO4 resulted in reduced carbonate precipitation, and enhanced alteration depths in some (H2S) or all (H2SO4) sealants. Additionally, relatively minor degradation was observed in the outer 100–200 μm of samples exposed in the presence of H2SO4. Overall, the impacts of impurities were more pronounced for sealants that were more affected by exposure to clean CO2.

Maintaining well integrity is a key challenge to the secure geological storage of CO2. Here, sealants based on Portland Cement form a key component, providing seals between the steel wellbore and surrounding caprock, as well as plugs for sealing wells that will no longer be used. However, exposure of sealants based on Portland Cement to CO2-bearing fluids may lead to carbonation, potentially followed by degradation of these materials during prolonged exposure or flow, which may thus negatively impact well integrity. Therefore, new sealant materials need to be developed to help ensure long-term well integrity.

This paper reports exposure of five different sealants to CO2-saturated water and wet supercritical CO2 at in-situ conditions (80 °C and 10 MPa). Three of the sealants investigated are based on Portland Cement, while the other two are based on Calcium Aluminate Cement, and a rock-based geopolymer specifically developed for Geological CO2 Storage (GCS). The five sealants were selected to represent different methods for improving wellbore seal integrity, such as restricting permeability (and porosity), or modifying how the material interacts with CO2-bearing fluids. Exposures were carried out in a purpose-built batch apparatus, enabling simultaneous exposure of up to 10 samples in total to CO2-saturated water and wet supercritical CO2.

After exposure, changes in the sealants’ microstructures and chemical and mineralogical compositions were assessed using scanning electron microscopy with energy-dispersive X-ray spectroscopy, computed tomography scanning, and fluid chemical analysis. The impact of exposure to CO2-bearing fluids was interpreted in terms of alteration and degradation of the materials, to compare how different sealant design modifications can be employed to enhance wellbore integrity.

The development of new geological storage applications and other uses of subsurface reservoirs requires tailored wellbore sealants, able to withstand application-specific exposure conditions. For example, wellbore sealants used in reservoirs targeted for CO2-storage will be exposed to CO2-rich fluids, that may chemically attack OPC-based sealants, leading to carbonation and potential degradation. During CO2-injection, local temperature changes around the injection well may also affect the integrity of the sealant-wellbore system, for example causing the formation of annuli between sealant and steel casing due to differences in thermal expansion. The research project CEMENTEGRITY aims to identify the key sealant properties that may help to ensure the long-term integrity of the wellbore-seal system during CO2-injection and storage, as well as the best testing methods for these properties. This is done by testing five different sealant compositions, exposing them to potentially deleterious impacts under different conditions. Here, we report some of the key findings of our project. While CEMENTEGRITY is researching sealants specifically for CO2-storage, other applications, such as hydrogen storage, or geothermal energy exploitation, will require purpose-built sealants that are similarly tailored to the expected chemical and physical conditions.