Acoustic Propagation Characteristics of Unsaturated Porous Media Containing CO2 and Oil

Carbon dioxide geological utilization and storage (CGUS) is an effective way to mitigate climate warming. In this paper, we resorted to Lo's model to analyze the dispersion and attenuation characteristics of unsaturated porous media. Based on this, we analyzed the sensitivity of the first compressional wave (P1) and the shear wave (S) to various physical parameters. In addition, the modified models of live oil's velocity and density were proposed, which were verified by experimental data under the consideration of CO2 dissolution. It is shown that the velocities and attenuations of P1 and S waves are influenced by various parameters, especially CO2 saturation and pore fluid parameters, such as density and velocity. In particular, with increasing CO2 saturation, the sensitivity of P1 velocity decreases, while that of the S velocity increases. Better monitoring results can be achieved by combining P1 and S waves. Finally, the acoustic response was analyzed under the modified model. With the increase in CO2 saturation, the P1 velocity decreases, while the S velocity becomes almost constant and then linearly increases, with the trend changing at the critical saturation. The study provides a more precise basis for monitoring the security of CO2 injection in CGUS.