Determination of CO2Solubility in Water and NaCl Solutions under Geological Sequestration Conditions Using a Fused Silica Capillary Cell with in Situ Raman Spectroscopy

To investigate the solubility of carbon dioxide (CO2) in geological fluids at different temperatures and pressures, we developed a method for measuring solubility using an optically transparent fused silica capillary cell as a high-pressure optical cell combined with Raman spectroscopy under geological sequestration conditions (temperatures of up to 353.15 K and pressure of up to 30.0 MPa). On the basis of the fact that the band intensity of an active Raman species is proportional to its concentration, we determined the linear correlation between the known CO2concentration and the ratio of the Raman peak heights of CO2to H2O (νCO2/νH2O) in homogeneous CO2–H2O and CO2–H2O–NaCl systems. Our results indicate that the Raman peak height ratio is a function of the CO2concentration and that the pressure and temperature do not significantly affect the relationship between the CO2concentration and νCO2/νH2Owithin the experimental P–T–xconditions. The νCO2/νH2Ovalues of the CO2-saturated solutions were determined by simulating the CO2geological sequestration T–P–xconditions. Then, the CO2solubilities were calculated using the linear relationship. Our results indicate that the method is feasible and that the solubility of CO2in H2O or NaCl solutions decreases with increasing temperature, increases with increasing pressure, and decreases with increasing salinity. On the basis of a comparison between our experimental data and the results of the previous model, our method provides satisfactory results.