Thermo-mechanical behaviour of the sealant-steel interface under thermal cycling for CCS

Abstract

In CCS wells, cyclic injection of cold CO₂ into the hot subsurface may lead to debonding between sealant and steel casing. We test how thermal cycling affects the sealing ability of five different types of sealant (S1 to S5) surrounding a simulated steel wellbore. We use cylindrical sealant samples with a stainless steel (AISI 316 L) pipe in the centre, cured at 150°C and 30 MPa for 28 days. Using 3 bar N₂ leak tests at room temperature, we test how much the sealant-steel interface leaks before and after thermal cycling under unconfined and confined (1.5 MPa) conditions. We also conduct push-off experiments using a 500 kN loading frame before and after. For the unconfined test, we place the sample on a custom-built jig, whereas for confined tests we have a similar assembly inside a conventional triaxial vessel. The samples are brought to 60°C. Subsequently, we inject 5°C water through the central pipe at 80 mL/min for 2 mins, and let the sample reheat for 12 mins. We repeat this 16 times. Afterwards, we allow the sample to cool to room temperature, and repeat the N₂ leak test in-situ. The results show that under unconfined conditions, the interface leaks more for all sealant types except S3. The key parameter controlling performance is the linear thermal expansion coefficient, where an expansion coefficient closer to that of steel indicates better performance. Under confinement, all sealant types perform better post-thermal cycling, due to the prolonged exposure to confining pressure.