Modeling CO₂ vertical migration based on seismic frequency-dependent AVO responses

Injection of CO₂ into a geological subsurface will cause seismic dispersion and attenuation, which will change with the migration of the CO₂ plume. Therefore, temporal attenuation changes from the seismic section with the CO₂ plume can provide additional information about CO₂ storage, migration, and possible leakage. The frequency-dependent amplitude variation with offset (FDAVO) attribute, as a seismic characteristic, which quantifies frequency-dependent anomalies from various kinds of sources, is widely applied in fields of hydrocarbon indication and fluid identification. In this work we aim to investigate the potential of the FDAVO attribute as a monitoring tool during the process of CO₂ migration. We combine the Buckley–Leverett equation and the patchy-saturation model to simulate the substitution process. We then apply a novel tool, the FDAVO inversion method, to estimate the dispersion level at the target interface. What is more, the characteristic of the frequency-dependent attribute versus time is studied for the first time. The result of the numerical simulation reveals that: (1) caused by CO₂ injection, there is significant dispersion and attenuation within the seismic band; (2) the level of dispersion reflects the migration of CO₂ plume; (3) dispersion anomalies are well quantified by the inverted FDAVO attributes, which can be utilized to characterize the vertical distribution of the CO₂ plume. (paper)