Acoustic Monitoring of CO2 Hydrate Sequestration in Marine Sediments Based on a Multiphase Poroelastic Model

CO2 hydrate sequestration in marine sediments has attracted increasing attention as a potential offshore carbon-storage strategy. However, the acoustic response associated with CO2 injection, fluid migration, hydrate formation, and subsequent storage stabilization remains insufficiently understood, which limits the development of reliable in situ monitoring methods. In this study, we develop a staged acoustic modeling framework based on multiphase poroelastic formulations. According to the evolution of pore components, the sequestration process is divided into three representative stages: a CO2–water–sediment skeleton system during CO2 injection, a CO2–water–CO2 hydrate–sediment skeleton system during hydrate formation, and a CO2 hydrate–CO2–sediment skeleton system during stable sequestration. The acoustic responses at these stages are analyzed in terms of wave velocity, attenuation, and velocity ratio under different CO2 and hydrate saturation conditions. The results show that the injection stage is mainly controlled by pore-fluid substitution and changes in fluid compressibility. In this stage, P-wave velocity, P-wave attenuation, and VP/VS are sensitive to CO2 saturation, whereas S-wave velocity varies only weakly. During hydrate formation, the acoustic response is jointly affected by fluid substitution, hydrate-induced solid stiffening, interphase coupling, and viscous dissipation. As hydrate saturation increases, both P1- and S1-wave velocities increase, while the velocity ratio decreases, indicating that hydrate formation significantly enhances sediment shear stiffness. In the stable sequestration stage, hydrate saturation becomes the dominant control on the elastic and dissipative properties of the medium. A comparison with published laboratory P-wave velocity data further supports the ability of the formation-stage model to reproduce the velocity increase associated with CO2 hydrate generation. Therefore, the combined use of P-wave velocity, S-wave velocity, attenuation, and VP/VS provides a theoretical basis for identifying CO2 migration, hydrate formation, and stable storage states in marine sediments.