Interface-Controlled Wellbore Leakage in CCUS Wells: Microannulus Formation, Pathway Evolution, Detection, and Risks

Wellbore integrity is central to geological CO2 storage because wells can intersect formations intended to contain injected CO2 and displaced brine. This critical narrative review examines how defects at casing–cement and cement–formation interfaces form, evolve, and contribute to hydraulic communication and potential leakage. Laboratory experiments, field observations, numerical models, and diagnostic studies are compared with attention to what each type of evidence can establish. Cement alteration, structural debonding, measurable fluid movement, and consequential leakage are distinct observations. Interface conductance depends on hydraulic aperture, connectivity, effective stress, thermal and pressure cycling, fluid chemistry, and flow history; mechanical closure or precipitation may restrict pathways, whereas renewed loading or dissolution may preserve or enlarge them. Diagnostic interpretations require method- and well-specific calibration: an undetected anomaly does not establish zero flow. Brine migration, dissolved-CO2 transport, and free-phase CO2 entry have different pressure, capillary, and saturation controls. The proposed interface-to-risk framework links defect evidence, exposure, phase-specific migration, and possible receptor consequences without treating the framework as a calibrated risk model. Priorities include cyclic testing, coupled multiphase characterization, field validation, and laboratory-to-field uncertainty quantification.