In the CCS project, the monitoring of distributed underground CO2 is required during the CO2 injection in order to alarm the CO2 leakage. Many quantitative monitoring techniques have been investigated and applied at various CCS projects, however, the seismic methods, e.g., seismic reflection, seismic tomography, and sonic logging, are believed as the most promising technique. By using seismic methods, it is desired that the CO2 salutation values at geological strata are estimated. For this purpose, precise understanding of the relationship between seismic velocity and CO2 saturation in the rock pore is required. In this review manuscript, three models of rock physics such as the uniform saturation model, the patchy saturation model, and the critical patchy saturation model will be studied by using the time-lapse well logging data obtained at the site of CCS pilot project of Nagaoka, Japan. From our study, the best fitted model to the sonic logging data is the critical patchy saturation model. Additionally, we discuss the size of CO2 patch in the pore space in relation to the applied seismic wavelength to validate this model.

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