Background and Merhod  The wellbore integrity of CO₂ injection wells plays a critical role in maintaining the long-term safe operation of geologic CO₂ sequestration (GCS) projects. Well cement serves as the foremost wellbore integrity barrier, and its durability under GCS has emerged as a hot research topic both at home and abroad. This study systematically reviews the advances in research on the chemical corrosion mechanism of well cement under the operating conditions of GCS, technologies for cement modification for enhanced resistance to CO₂ corrosion, and non-Portland cement systems. Furthermore, the directions for the future development of well cement are proposed.

Advances The corrosion of Portland cement essentially stems from the reactions of its hydration products, Ca(OH)₂ and C-S-H, in an acidic CO₂ environment. Among these, Ca(OH)₂ is preferentially consumed, while C-S-H exhibits a higher stability. Therefore, in the modification design of Portland cement, Ca(OH)₂ can be retained appropriately as a buffer during carbonation reactions. This measure helps maintain the cement performance for the long term. Although most laboratory acceleration experiments have revealed significant degradation of Portland cement when exposed to a CO₂ atmosphere, field evidence from sites exemplified by the SACROC block in the United States indicates that Portland cement can provide effective sealing for decades under sound cementation conditions. This discrepancy highlights the inadequacy of current experimental evaluation systems in simulating actual downhole environments (e.g., confining pressure, formation water chemistry, dynamic temperature and pressure conditions). Primary approaches to enhancing the resistance to CO₂ corrosion of Portland cement include reducing the cement matrix permeability, incorporating inert or active fillers to regulate the products of chemical reactions, and applying surface protective coatings. Among these, nano-SiO₂ can optimize the microstructures of cement and produce a synergistic effect by participating in pozzolanic reactions, emerging as an effective material for the modification of Portland cement to enhance its carbonation resistance. Despite having exhibited sufficient sealing performance in partial carbon capture and storage-enhanced oil recovery (CCS-EOR) projects, modified Portland cement faces the risk related to long-term durability under extreme conditions due to its thermodynamic metastability. Therefore, non-Portland cement systems prove a better choice for CCS-geologic sequestration wells aimed at permanent CO₂ sequestration.

Prospects  Presently, there remains a lack of unified standards for the experimental evaluation of long-term cement integrity under GCS conditions, leading to insufficient comparative data arising from methodological differences. In the future, it is necessary to establish standardized testing systems that cover low-temperature, dynamic corrosion conditions while also highlighting the settlement stability of cement. The purpose is to enhance the value of evaluation results for guidance on engineering practices. Additionally, although non-Portland cement systems can fundamentally circumvent the carbonation risk, they suffer from limitations of practical applicability and field operability. Therefore, it is recommended to further conduct systematic research on material performance regulation, construction suitability, and cost effectiveness.