Supercritical CO₂ (ScCO₂) invades oilwell cement under geological CO₂ sequestration conditions. With the penetration of ScCO₂, cement structure prone to damage when the coupled effects of capillarity and carbonation were found. Microstructural evolution of oilwell cement samples was investigated by the CT scanning and the quantitative image-based analysis and show that ScCO₂ with the high humid condition would penetrate much deeper than the dry ScCO₂ because of the capillarity effects. Due to the deep saline condition in the sequestration formation, the penetration of ScCO₂ was retarded by the salt deposition, comparing with the ultrapure water (UP water) conditions. For further assessment of this coupled mechanism, the permeability property and contact angle changes were proposed to analyse the interface region between ScCO₂, saline/UP water and oilwell cement.