A COMPOSITIONAL MODELING STUDY OF CO₂ LEAKAGE THROUGH WELLBORE ANNULAR CEMENT

Abstract

Legacy wellbores represent a primary risk for containment loss in geological carbon storage projects, necessitating tools to quantify long-term leakage. This study aimed to develop and apply a numerical simulation workflow to assess the CO₂ leakage risk through a legacy well, focusing on the impact of annular cement quality. A detailed 2D Cartesian numerical model was constructed, integrating a robust fluid characterization with a realistic geological and wellbore geometry. The model simulated a 30- year CO₂ injection period followed by a 3000-year post-injection monitoring phase, with sensitivity analyses performed on cement permeability over a wide range. The main finding is that the permeability of the annular cement is the single most dominant factor controlling containment integrity. For a base case of 1 mD cement, breakthrough into a shallow aquifer occurred after several centuries, while permeabilities of 10 mD or higher reduced this time to decades, causing significant leakage. This study provides a tangible, physics-based framework for a quantitative, risk-based approach to well abandonment. The workflow allows operators to move beyond prescriptive guidelines, enabling more informed decisions by identifying and prioritizing high-risk wells, thereby enhancing the safety and economic viability of large-scale CCS deployment.