Well Cement Technology for CO₂ Geological Sequestration Injection Wells: Corrosion Mechanisms, Modification and Application Advances

Background Wellbore integrity in CO₂ geological sequestration injection wells is crucial for ensuring the long-term safe operation of storage projects. As the primary barrier for maintaining wellbore integrity, the durability of well cement under CO₂ storage conditions has become a research hotspot worldwide. This review systematically summarizes the chemical corrosion mechanisms of well cement under CO₂ storage conditions, anti-corrosion modification technologies, and research advances in non-Portland cement systems, while also providing an outlook on future development directions.

Advances in Research  The corrosion of Portland cement essentially stems from the reaction of its hydration products, Ca(OH)₂ and C−S−H, in an acidic CO₂ environment. Ca(OH)₂ is preferentially consumed, whereas C−S−H exhibits relatively stronger stability. In the modified design of Portland cement, appropriate retention of Ca(OH)₂ can play a buffering role during carbonation, contributing to long-term performance maintenance. Although most laboratory accelerated tests indicate significant degradation of Portland cement in CO₂ environments, field evidence from sites such as the SACROC block in the United States demonstrates that Portland cement can still provide effective sealing for decades under good bonding conditions. This discrepancy highlights the inadequacy of current experimental evaluation systems in simulating real downhole environments (e.g., confining pressure, formation water chemistry, dynamic temperature-pressure conditions, etc.). The main approaches to enhancing the CO₂ corrosion resistance of Portland cement include reducing matrix permeability, incorporating inert or active fillers to regulate reaction products, and applying surface protective coatings. Among these, nano-SiO₂, with its synergistic effects of microstructural optimization and pozzolanic reaction, serves as an effective modifying material for improving the carbonation resistance of Portland cement. Modified Portland cement has demonstrated sufficient sealing capability in some CCS-EOR projects. However, its thermodynamic metastability raises concerns about long-term durability under extreme conditions. For CCS-GS wells aimed at permanent storage, non-Portland cement systems represent a superior alternative.

Prospects  Currently, there is still a lack of unified standards for the experimental evaluation of the long-term integrity of cement under CO₂ storage conditions, and methodological differences lead to insufficient comparability of data. In the future, it is necessary to establish a standardized testing system that covers low-temperature and dynamic corrosion conditions and emphasizes settlement stability, so as to enhance the engineering guidance value of evaluation results. Furthermore, although non-Portland cement systems can fundamentally avoid carbonation risks, they still face limitations in practical applicability and on-site operability. Further systematic research is required in areas such as material performance regulation, construction compatibility, and cost-effectiveness.