Huge oil deposits (> 8 billion barrels) were recently discovered off the coast of Brazil below 2000 m of seawater and 1000–2000 m of reservoir, below 2000 m of salt layers. The high pressures (~ 700 bars) make these reservoirs excellent candidates for high pressure miscible gas injection, e.g., CO2 injection. For the geothermal gradients encountered in this area the temperatures are around 473 K. At high pressures it is possible that the mass density of carbon dioxide is higher than of oil, which will allow gravity stabilized injection. We consider the CO2-brine system, firstly for its storage as a greenhouse gas. Secondly, it is an initial step to describe displacement in the CO2-oil-brine system.

In this paper we study the phase behavior of carbon dioxide in a brine bearing layer at a temperature of 473 K and pressures between 600 and 800 bars. The quantification of the displacement process requires the thermodynamic equilibrium relations in the oil-water-salt-CO2 system at the prevailing pressures. Here we limit our interest to the thermodynamic behavior of the carbon dioxide-brine system. We show that it is possible to use the PRSV equation of state to estimate the composition of the vapor and aqueous phases. The PRSV is a modification of the Peng-Robinson equation of state. We use an activity-coefficient based mixing rule for the thermodynamic calculations. A volume shift procedure is applied to improve liquid density.

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