Grain-Size Controls on Acoustic Evolution and P-Wave Velocity–Saturation Relationships during CO2 Hydrate Sequestration in Unconsolidated Sandy Sediments

Abstract CO2 hydrate-based geological sequestration technology offers a viable strategy for mitigating atmospheric CO2 emission, yet acoustic monitoring interpretation is highly restricted by sediment grain size. This study systematically investigated the influence of pressure, temperature, and grain size on acoustic properties of hydrate-bearing sediments, and revealed grain-size-dependent hydrate formation kinetics and acoustic characteristics, and further calibrate the Lee weighted rock-physics model. The results show that hydrate formation universally experiences three stages. Temperature exerts competing thermodynamic and mass-transfer effects, while 4.0 MPa liquid CO2 accelerates reactions but induces early pore blockage. Fine sands with dense grain contacts and high capillary pressure form contact-cement hydrates and achieve higher ultimate saturation, whereas coarse sands support rapid early CO2 migration with limited storage capacity. P-wave velocity rises monotonically with saturation: fine sediments yield higher absolute velocities, while coarse samples show larger relative increments. Severe acoustic scattering attenuation dominates fine media, yet coarse sands display mild amplitude recovery in later stages. Measured data lie between Wood suspension and time-average cement end-members. Negative fitted parameters of fine sands verify full cementation, and positive values of coarse sands correspond to grain-coating transition. Universal velocity-saturation formulas trigger large inversion errors. This study extends the Lee model to CO2 hydrate systems and proves grain-size constraints are indispensable for reliable field acoustic monitoring.