Comparative analysis of FLAG and NIST CO₂ modelling methods and their impact on CCS seismic monitoring

This study compares two methods for modeling CO₂ properties: the FLAG 2023 calculator and the NIST reference database. While FLAG uses regressions and is limited to specific temperature and pressure ranges, NIST employs an Equation of State approach with broader applicability. The research reveals significant differences between the methods, particularly near CO_2's critical point, with variations up to 460% in bulk modulus calculations. The study also examines the importance of accurately modeling CO₂ interactions with reservoir fluids, rather than treating CO₂ as a separate phase. When modeling CO₂ injection into a depleted gas field, considering CO₂-hydrocarbon gas mixtures produced a 50% higher normalized root mean square (NRMS) response compared to treating them as separate phases. These findings have important implications for CCUS project planning and monitoring, particularly for initiatives like the UK's East Coast Cluster and HyNet hub. The research emphasizes that accurate modeling of CO₂ properties and its interactions with reservoir fluids is crucial for developing effective monitoring strategies.