Effects of confined thermal cycling on sealants with different thermomechanical properties for CCS

In geological CO₂ storage, the integrity of seals between well and caprock is crucial for ensuring permanent CO₂ storage. One key mechanism that might affect this integrity is thermal cycling when cold CO₂ is injected periodically into a hot reservoir. Our study aims to identify which properties of a sealant are key to its ability to withstand thermal cycling under in-situ conditions. We investigate sealants of four different compositions, namely two ordinary Portland cement-based blends (S1 and S2), and two blends designed for carbon capture and storage applications, namely a novel ordinary Portland cement-based blend with CO₂-sequestering additives (S3), and a calcium aluminate cement-based blend (S4). These four sealants possess different thermomechanical properties, such as Young's modulus, tensile strength, unconfined compressive strength, thermal diffusivity, and thermal expansion coefficient, also characterized in this study. Samples of these sealants were placed in a triaxial deformation apparatus, and subjected to either 1.5 or 10 MPa confinement at 120 °C. Then we applied eight thermal cycles by injecting 20 °C water through a central bore in these samples. To assess the effects of the thermal treatment, we used X-ray micro-computed tomography (micro-CT), helium pycnometry, and compressive strength testing on thermal-treated as well as intact samples. Micro-CT results indicate that all sealant samples maintained integrity without cracking (above 32 µm) after thermal cycling under confinement. For all four sealants, post-treatment porosity (determined by either micro-CT or pycnometer) was reduced, which is ascribed to compression during confinement. This reduction in porosity was associated with an increase in compressive strength. Compared to experiments conducted under 1.5 MPa confinement, those at 10 MPa exhibit a greater reduction in porosity, and more enhanced compressive strength. The application of confinement suppressed the potential of crack formation by increasing the effective strength including tensile strength of the sealant during thermal cycling by a reduction in porosity. Based on these findings, we conclude that to limit potential damage to seal integrity induced by thermal cycling, sealants for carbon capture and storage should ideally have high tensile strength and thermal diffusivity, but low Young's modulus and thermal expansion coefficient.