A review on cementing system for CO2 storage wells: Mechanisms, testing protocols and screening criteria

Long-term containment of CO2 in geological formations demands cementing systems that can withstand highly aggressive downhole environments, where carbonic acid, brine, and other reactive species threaten the integrity of cement sheath. This review critically examines the deterioration mechanisms of Portland-based cements under CO2-rich conditions, emphasizing acid-induced decalcification, carbonation, and leaching processes. It further categorizes and compares a broad range of acid-resistant alternatives including modified Portland systems, non-Portland systems (CAC: Calcium Aluminate Cement, CAPC: Calcium Aluminate Phosphate Cement, MPC: Magnesium Phosphate Cement, geopolymers), resin-based sealants, and nano-engineered formulations, focusing on their chemistry, resistance mechanisms, performance metrics, and field applicability. The review then proposes a detailed laboratory testing system aligned with API standards and introduces a phase-wise testing protocol. A novel conceptual screening criterion (HSR framework) is developed based on operational, design, and sustainability parameters to guide material selection and validation for CO2-brine exposed cementing systems. Among the systems evaluated, CAC, CAPC, and MPC have shown superior results in terms of post-exposure compressive strength retention and reduced carbonation depth. Key knowledge gaps and challenges are identified, particularly regarding long-term behavior under thermobaric and acidic exposure. Finally, a future roadmap is outlined, calling for systematic field validation, deeper mechanistic insights into CAC and CAPC systems, and innovations in multifunctional, low-carbon binders suited for emerging CCS frontiers.<br/>