Dynamic Fluid Flow Effects on Acoustic Propagation Characteristics of Unsaturated Porous Media in CO2 Geological Sequestration

CO₂ geological sequestration (CGS) is a crucial strategy to mitigate the greenhouse effect. The quantitative correspondence between CO₂ saturation and acoustic response serves as the essential basis for monitoring CO₂ migration. However, due to dynamic fluid interactions between supercritical CO₂ and brine/oil in porous media, acoustic propagation behaviour is extremely complicated, even at the same saturation during drainage and imbibition processes. This study is motivated to evaluate the acoustic characteristics of the above porous stratum containing CO₂. To do so, pore fluid parameter models specific to CGS are consolidated and refined, with the consideration of CO₂ solubility. Meanwhile, Lo's theory is modified to describe both partial flow and global flow in CO₂-saturated porous media, capturing key mechanisms of patchy distribution and alterations in capillary pressure and relative permeability during drainage and imbibition. By combining these procedures, the wave propagation characteristics within CGS scenarios are systematically analysed. It is shown that CO₂ exhibits higher solubility than gases, leading to a distinct two-stage acoustic response, corresponding to its dissolved and free states. Relative permeability affects both compressional and shear waves, whereas capillary pressure and patchy distribution mainly affect compressional wave propagation. Notably, compressional waves exhibit heightened sensitivity to free CO₂ content and fluid flow dynamics, especially at ultrasound frequencies. The modified acoustic propagation theory demonstrates superior performance in characterizing compressional velocities during both drainage and imbibition. These findings highlight the dynamic fluid flow effects in CGS, providing a theoretical framework for analysing acoustic propagation characteristics.