# Acoustic Propagation Characteristics of Unsaturated Porous Media Containing CO\({}_{2}\) and Oil
## 6 Conclusions  
In this paper, we proposed [PERSON]'s model to study the effect of CO\({}_{2}\) injection on the acoustic responses of the porous media containing two immiscible fluids. We calculated the variation of phase velocities and attenuations of waves, with frequency and CO\({}_{2}\) saturation. In addition, we analyzed the sensitivities of phase velocities and attenuations to various parameters, especially CO\({}_{2}\) saturation. Moreover, we proposed a method to modify the velocity and density models of the CO\({}_{2}\)-oil mixture based on experimental data. Finally, the wave propagation was compared between [PERSON]'s model and the modified model under [PERSON]'s model. The conclusions are as follows:  
1. There are four kinds of mode waves (three compressional waves and one shear wave) in the unsaturated porous media with pore fluid composed by CO\({}_{2}\) and oil. However, only two mode waves, P1 and S, can be observed at seismic frequencies. The velocity and attenuation characteristics are sensitive to CO\({}_{2}\) saturation, and the combined P1 and S waves can monitor the CO\({}_{2}\) migration more accurately.
2. In the analysis of acoustic wave modes, the accuracy of the features of pore fluid is crucial. The pore fluid density has an effect on both the velocities and attenuations of P1 and S, while the pore fluid velocity only has an effect on the velocity and attenuation of P1.
3. The fluid parameter of [PERSON]'s model is based on hydrocarbon gas, which cannot accurately describe the physical parameters of the CO\({}_{2}\)-oil mixture. Compared with experimental data, the modified model can provide more exact data.  
The numerical simulations show that [PERSON]'s model, combined with the modified parameters models of live oil, can realistically describe the wave propagation characteristics of unsaturated porous media. The velocities and attenuations of waves in unsaturated porous media are closely related to CO\({}_{2}\) saturation, considering the effect of dissolution, temperature, and pressure. The model of pore fluid parameters and the theory of unsaturated porous media in this paper can be used to explore the changing pattern of the acoustic field after CO\({}_{2}\) injection. Meanwhile, this study provides a more accurate basis to monitor the security of CO\({}_{2}\) injection in CGUS.  
Conceptualization, Y.Q. and X.Z.; methodology; Y.Q., X.Z. and L.L.; validation, Y.Q., X.Z. and L.L.; formal analysis, Y.Q.; investigation, Y.Q. and L.L.; resources, X.Z.; data curation, Y.Q. and L.L.; writing--original draft preparation, Y.Q.; visualization, Y.Q., X.Z. and L.L.; supervision, X.Z.; project administration, X.Z.; funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (Grant Nos. 11974018, 11734017) and the Strategic Pilot and Technology Special of Chinese Academy of Sciences, China (Grant No. XDA14020303), and the Research Instrument and Equipment Development Project of Chinese Academy of Sciences-Key Technology Team Project, Grant No. GJJSTD20210008.  
Not applicable.  
The authors declare no conflict of interest.