Effect of supercritical CO2 exposure on the high-pressure CO2 adsorption performance of shales

The interactions between shales and supercritical carbon dioxide (ScCO2) as well as the influence of these interactions on the pore structure and the surface functional groups play a key role in long-term CO2 geological sequestration. To investigate the effect of ScCO2 treatment on the performance of high-pressure CO2 adsorption, three shale samples selected from different formations were treated with ScCO2 in a geochemical reactor to simulate the in situ geological sequestration process. The high-pressure adsorption behaviors of CO2 on untreated and ScCO2-exposed shale samples were measured by gravimetric method at 45 degrees C and pressures up to 18 MPa. Characterization methods included scanning electron microscope (SEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), low-pressure gases adsorption (LP-GA), element analysis (EA) and X-ray photoelectron spectroscopy (XPS). The experimental results indicated that ScCO2 fluid could significantly alter the structural and physicochemical properties of shales, leading to the variations in high-pressure CO2 adsorption capacity of shales. The moisture and oxygen contents in the ScCO2-exposed samples were lower than those in the untreated samples. XPS analysis showed that the relative content of the organic oxygen-containing functional groups (C-O, C=O, O-C-O and COO-) in the raw shale samples decreased after the ScCO2 treatment. After long-term ScCO2 immersion, the maximum CO2 adsorption capacity of the shale samples exhibited a decreasing trend when compared to the raw samples, which was linked to the changes in the specific surface area of the micro- and mesopores as well as the oxygen-containing functional groups. The results obtained in this paper provide a basis for further evaluation of CO2 storage mechanisms.