EXPERIMENTAL DETERMINATION OF THE SOLUBILITY OF CO2 IN ELECTROLYTES: APPLICATION TO CO2 SEQUESTRATION IN DEEP-SALINE AQUIFERS

The burning of fossil fuels and other anthropogenic activity have caused a dramatic increase in atmospheric CO₂. The disposal of this excess CO₂ into deep aquifers offers advantages of high capacity and permanence. A combination of fixed-volume titanium and flexible-gold reaction-cell technology was developed to investigate the geochemical interactions between supercritical CO₂ and potential fluids and host rocks characteristic of deep-saline aquifers. This equipment was used to make new determinations of the solubility of CO₂ in electrolytes at elevated temperature and pressure. Initial experiments focused on the binary CO₂ - pure water system where data are well known to validate the experimental design. Other experiments include the reaction of supercritical CO₂ with mixtures of synthetic and natural brines to obtain essential data lacking in the literature.

Isotherms of the solubility of CO₂ were determined in NaCl solutions of 5%, 10% and 22% at 21°, 50°, 100°, 125° and 150°C over a pressure range of 100 to 600 bars in 100 bar intervals. The solubility of CO₂ was also measured in CaCl₂ at an ionic strength equivalent to 10% NaCl (I = 1.7) and in a mixed Na-Ca-Cl solution over the same temperature and pressure range. An isotherm of the solubility of CO₂ was also determined for a natural brine from Paradox Valley, Colorado (PVB), equivalent in ionic strength to 22% NaCl (I = 3.8), at 21°C from 100 to 600 bar.

Results indicate a systematic decrease in the solubility of CO₂ with increasing ionic strength and is reduced ~threefold in 22% NaCl relative to pure water at 21°C and 100 bars. In general the solubility of CO₂ increases with increasing pressure and decreases with increasing temperature. At 21° and 50°C, the solubility of CO₂ increases as linear function of pressure. At 100°C and above, the solubility of CO₂ increases as second order polynomial functions of pressure. Isobars from 21 to 150°C display distinct minima that vary with pressure. The solubility of CO₂ in CaCl₂ shows a similar pattern but is elevated relative to the solubility in NaCl of equivalent ionic strength. An important finding is that the solubility of CO₂ in PVB is offset from and higher than the solubility in NaCl alone. This is apparently due to the presence of divalent salts in the natural brine and is consistent with the solubility data for CaCl₂.