A Comprehensive Review of Cement Degradation Analysis under Downhole Conditions: CCS/CCUS/CO‑EOR Applications.

The long-term performance of wellbore cement in CO-rich environments remains a critical challenge for carbon capture and storage (CCS), carbon utilization and storage (CCUS), and CO-enhanced oil recovery (CO-EOR) operations. Under downhole conditions, cement degradationprimarily driven by carbonation and bicarbonationcan lead to microcracking, increased permeability, and loss of well integrity. This review presents a comprehensive synthesis of degradation mechanisms, evaluating the interplay between CO exposure, pressure, temperature, and curing conditions on cement properties. We critically examine both traditional and advanced cement systems, including Portland-based formulations, pozzolanic blends, calcium aluminate, geopolymers, and polymer-enhanced cements. Particular focus is given to experimental findings on mechanical property evolution (e.g., compressive, tensile, and bond strength) and transport behavior under simulated downhole conditions. Modern characterization techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), computed tomography (CT), and Fourier-transform infrared spectroscopy (FTIR) are reviewed, alongside emerging machine learning approaches for predicting degradation and optimizing cement design. Despite decades of research, significant gaps remain in testing standards, long-term performance prediction, and integration of mechanical and chemical deterioration models. This paper identifies those gaps and proposes strategies to enhance cement durability, including the use of nanomaterials, particle-engineered systems, and tailored additives. Ultimately, this review serves as both a critical reference and a practical guide for developing robust, CO-resistant cement systems capable of maintaining zonal isolation in high-stress, corrosive subsurface environments.