Two-Phase Flow Dynamics and Transient Modeling for CO₂ Injection in Complex Well Geometries

Carbon Capture, Utilization, and Storage (CCUS) is an increasingly important strategy in the global response to climate change. A key challenge in implementing CCUS, particularly in offshore environments, is understanding the complex behavior of CO₂ and reservoir fluids during multiphase flow under subsurface conditions. The interactions between CO₂, water, and potentially hydrocarbons can significantly affect the safety and efficiency of injection operations. However, obtaining reliable field data in such conditions is often unfeasible, making numerical simulation an indispensable tool for both design and operational decision-making. Most models in the existing literature emphasize steady-state conditions, which are computationally simpler and applicable to much of an injection well’s operational lifespan. Nonetheless, these models fail to capture transient behaviors that occur during early-stage operations, sudden perturbations, or system failures. Such events may be triggered by pressure surges, formation damage, leakage, wellbore integrity issues, rapid changes in fluid properties, or geomechanical instabilities. Moreover, evolving regulatory frameworks and environmental conditions further necessitate more dynamic simulation approaches. This study introduces a transient multiphase flow model capable of simulating complex offshore well geometries, including flowlines, risers, and injection tubing. The model incorporates phase-change phenomena and flow-pattern-dependent parameters to more accurately represent two-phase CO₂ interactions. It was validated using field data available in the literature and was used to investigate the effects of injection rate, pressure, and temperature on both thermal and structural well responses. Particular attention is given to Annular Pressure Change (APC), a phenomenon that can cause significant stress on well casings and may lead to catastrophic failure. By capturing such transient effects, the model offers a more robust framework for predicting operational risks and improving the design and management of CO₂ injection systems.