The purpose of this study is to analyze the CO2 state in the porous media by means of &$181;-focus X-ray CT. In order to extract the information of the captured CO2 in pores, a new method, the histogram subtraction method, was introduced. Here, we have conducted µ-focus X-ray tomographying under the pre- and post- conditions of CO2 injection into the water saturated porous media. Between the pre- and post- conditions of CO2 injection tests, the density distribution will change. This method utilizes the difference of the CT value distribution. Moreover, the other extra information, such as noise components and heterogeneity, can be eliminated. In this study, the replacement ratio, which represents the ratio of pore spaces where water-CO2 replacement occurred against the total pore space, was evaluated from µ-focus X-ray CT image data. It was found that the approximately 40% of pore spaces are replaced by CO2. However, it is also confirmed that almost all spaces are replaced by CO2 in some analytical region. It means the replacement ratio is not uniform and it is influenced by the local pore conditions. It was found that water and CO2 exist simultaneously in pores in many cases, and that CO2 is located at the center of pores.

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