Optimizing Class G Cement for CO ₂ Storage (CCS): A Comprehensive Study on the Effects of Fly Ash and Eggshell Blends on Wellbore Cement Integrity

Carbon capture and storage (CCS) is a critical pathway for achieving net-zero emissions; however, maintaining long-term wellbore cement integrity under CO2-rich conditions remains a major technical challenge. Conventional Class G cement is highly vulnerable to carbonation–dissolution reactions, which increase porosity and permeability, weaken mechanical strength, and ultimately compromise zonal isolation. These degradation processes create potential leakage pathways that threaten storage security and reduce CO2 storage efficiency. This study presents a systematic optimization of Class G cement using fly ash (FA) and waste-derived eggshell powder (ESP) as supplementary cementitious materials. The objectives are to develop CO2-resistant formulations, quantitatively evaluate geomechanical, petrophysical, and geochemical performance under simulated downhole conditions, and identify the microstructural mechanisms responsible for enhanced durability. A novel formulation strategy is introduced, leveraging synergistic calcium–silica interactions between FA and ESP to fundamentally redesign the pore structure. Beyond conventional pore size reduction, this approach promotes pore network disconnection as the dominant sealing mechanism while simultaneously enabling a self-healing (autonomous sealing) response under CO2 exposure. The presence of reactive calcium phases from ESP enhances carbonate precipitation, allowing microcracks and pore throats to be progressively sealed through in situ mineralization. Cement systems [base cement (BS), FA/ESP (75%/25%), and FA/ESP (50%/50%)] were subjected to high-pressure, high-temperature aging (2000 psi, 170 °C) in CO2-saturated brine for up to 60 days. Comprehensive characterization was performed using Nuclear Magnetic Resonance, ultrasonic wave propagation (Auto-Lab 1500), X-ray diffraction, and Scanning Electron Microscopy. The FA/ESP (75%/25%) formulation exhibited superior performance, achieving a 75% reduction in permeability (0.01125 mD vs 0.045 mD for base cement) and an 82.22% decrease in porosity, while base cement showed a 56.7% increase. Mechanical properties improved significantly, with increases of 19.5% in Young’s modulus and 19.3% in Poisson’s ratio, indicating enhanced structural resilience. Mineralogical analysis revealed progressive calcite formation (46.4% to 56.5%), confirming active CO2 mineralization that transforms chemical degradation into a strengthening mechanism. These findings directly enhance wellbore integrity and CO2 storage efficiency by reducing permeability, minimizing leakage risk, and improving long-term containment. Pore network disconnection and improved mechanical strength ensure sustained zonal isolation, while CO2-driven mineralization enables self-sealing through calcite precipitation within pores and microfractures. The novel integration of industrial byproducts and agricultural waste offers a sustainable, cost-effective solution, establishing a new paradigm for durable, self-healing cement systems in geological carbon storage.