Wellbore integrity is crucial for the safe and effective long-term geological storage of CO2. Over time, downhole loading and corrosion by acidic fluids can cause the mechanical strength of the cement sheath to decrease and permeability to increase, which can lead to COO2 leakage. This paper presents a corrosion test of cement under supercritical carbon dioxide with varying times. The corrosion depth and mechanical properties of cement specimens before and after corrosion are determined through SEM tests and uniaxial and triaxial tests. A 3D numerical FEM model for cement sheath integrity is developed, which considers the coupled effects of mechanical deformation and chemical corrosion. The model is innovative and represents a new approach to analyzing cement sheath integrity. The leakage mechanism of cement sheaths under the combined effects of complex loads and chemical corrosion is analyzed by the proposed model. The results show that the corrosion depth of the cement specimens gradually increased, and the mechanical properties progressively weakened after corrosion. The radial tensile stress, micro-annulus size and equivalent plastic strain of the cement sheath increase with increasing corrosion time, indicating that the cement sheath is more susceptible to damage and debonding after corrosion. This study provides a theoretical foundation for designing new wells and assessing the suitability of existing wells for CO2 storage.
Keywords Cement sheath integrity, Mechanical-chemical coupling, CO2 corrosion, CCUS, Cement failure