New Insight of CO₂ Corrosion Performance on Cement for Enhanced Oil Recovery and Carbon Geological Storage Based on Nuclear Magnetic Resonance Technology

As a major means of reducing carbon emissions and achieving the carbon-zero target, CO2 geological storage has been widely applied in depleted oil or gas reservoirs for enhanced oil recovery. However, during CO2 injection and long-term geological storage, the carbonation-induced corrosion of the cement sheath is the main threat as CO2 may leakage from the well to atmosphere. In this study, G-grade oil well cement was placed in different CO2 corrosion environments to investigate the effects of the CO2 pressure and water saturation on long-term cement corrosion. Mechanical and pore permeability properties, as well as changes in the microstructure and composition, were analyzed using uniaxial compression, X-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy testing methods, respectively. Specially, nuclear magnetic resonance (NMR) technology was used to evaluate the pore changes in cement during the CO2 corrosion. Results showed that wet-phase corrosion facilitated the occurrence of carbonation reactions and the migration of corrosive medium and products. Moreover, the microstructure and composition of the CO2-corroded cement exhibited different characteristics at different stages of corrosion. The T2 spectrum curve indicated that the degree of CO2 corrosion of cement was related to the diffusion rate of the CO2. When the pressure was low, CO2 was difficult to penetrate deep into the cement, resulting in the smallest change in the NMR curve area under dry-phase CO2 corrosion conditions at 5 MPa. This study employed nuclear magnetic technology to further analyze the mechanism of CO2 corrosion on oil well cement at the microscopic level. It will contribute to a deeper understanding of the mechanism of CO2 corrosion of cement and lay a theoretical analysis foundation for practical engineering applications of cement in CO2 geological storage and enhanced oil recovery.