EQUATION OF STATE FOR THE NACL#H2O#CO2 SYSTEM: PREDICTION OF PHASE EQUILIBRIA AND VOLUMETRIC PROPERTIES

Abstract

An equation of state (EOS) has been developed for the NaCl#H2O#CO2 system which consistently predicts various properties including PVTX, immiscibility or phase equilibria, solubilities, and activities with an accuracy close to that of experimental data from 300 to about 1000°C and 0-6000 bar with NaCl concentrations to about 30 wt% of NaCl (relative to NaCl + H2O) or to about 50 wt% with less accuracy. The EOS predicts that excess volumes can be over 30% of the total volume under some T-P conditions. Adding NaCl to the H2O#CO2 system dramatically increases the T-P range of immiscibility. The immiscibility field is minimal around 400-500°C. Above or below this temperature, it expands for a constant pressure. A moderately saline brine can evolve into a very saline brine by phase separation at high temperatures. The presence of NaCl can substantially decrease the activity of H2O and increase that of CO2, thus affecting decarbonation and dehydration reactions. Compared to the EOS of Bowers and Helgeson (1983a), the EOS of this study is more reliable in the calculation of volumetric properties particularly in the low pressure range. In addition, BH EOS cannot predict phase equilibria.