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Geological sequestration of CO2 has been proposed as a potential option to reduce the volume of CO2 emissions into the atmosphere and thus to stabilise the earth’s climate. However, monitoring, verification and environmental safety of CO2 storage are important issues that must be addressed before the option can be accepted by the public for wide-scale implementation. So far, seismic monitoring provides the most attractive approach for obtaining the spatial coverage required for mapping the location and movements of CO2 in the subsurface. Therefore, the development of monitoring techniques has a large impact on the application of geological sequestration of CO2. In this article, a laboratory study to improve the seismic monitoring techniques has been outlined in view of capturing the change of petrologic properties of typical porosity rocks due to CO2 replacing water. The obtained high-quality seismic data enabled detailed determination of the relative velocity and attenuation coefficient of the P-wave using difference seismic tomography. CO2 migration and water displacement were clearly mapped using tomographic images of relative velocity and the attenuation coefficient. As a function of gas saturation, both the velocity and attenuation data are in good agreement with results obtained using the White and Dutta-Odé model for partial saturation, indicating that viscous losses due to fluid diffusion are of significant importance for P-waves travelling at ultrasonic frequencies in porous rocks saturated with water and CO2. By assuming some scaling rules, models obtained in a laboratory at ultrasonic frequencies are helpful in the interpretation of field data at sonic and seismic frequencies.
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