Fuente: Molecules - Revista científica (MDPI) Molecules, Vol. 31, Pages 620: CO2–Binder Reaction Mechanisms in Geopolymer Wellbore Cements: Alternatives to API Class G Cement in CO2-Rich Environments (CCS)
Molecules doi: 10.3390/molecules31040620
Authors:
Omer Mohamed Bakri
Ahmed Abdulhamid Mahmoud

API Classes of cement are susceptible to three major problems: carbonation, decalcification, and increased porosity of cement sheaths in CO2-rich environments. These degradation pathways in American petroleum institute (API) Class/ordinary Portland cement (OPC) systems are well documented in laboratory and field observations for CO2-rich wellbore service. In contrast, while geopolymer/alkali-activated binders have been increasingly studied as alternatives, the evidence remains distributed across different precursor chemistries, exposure conditions, and test protocols, and a consolidated, mechanism-based synthesis specific to CO2 sequestration wells is still limited. Accordingly, this article presents a critical, narrative (non-systematic) review that synthesizes published laboratory and field studies on geopolymer/alkali-activated binders for CO2 sequestration wells, with emphasis on permeability, strength retention, and microstructural stability under CO2-rich exposure. The main outcome of this review is a mechanism-based synthesis that links CO2–binder reaction pathways (gel chemistry/phase evolution) to pore-network and transport changes, and consolidates quantitative performance benchmarks (permeability and strength retention) relative to API Class G/OPC, while defining the key validation gaps for qualification (HPHT, cyclic/tensile integrity, mixed fluids, and long-term monitoring). Laboratory tests have already demonstrated that geopolymer samples have ultralow permeability and preserve 90% of their strength after being treated with supercritical CO2 concentrations, while OPC loses its strength and produces macropores causing substantial growth of cement sheath porosity. Microstructural studies have shown that geopolymers do not contain portlandite but only N–A–S–H/C–A–S–H gels with low Ca content in concentrations high enough to create N–A–S–H/C–A–S–H gels, but do not suffer from multi-zone carbonation, as occurs for OPC concrete. Key challenges being tackled include slurry rheology, setting control and variability of precursors by designed admixture use and new performance specifications for higher-quality geopolymers. On the whole, geopolymers emerge as a sustainable and reliable alternative to traditional well cementing techniques for their sustainability well integrity.