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Abstract
Accurate density description of saturated liquid and vapor (L-V) phases for the water-CO2 system is important in many fields of engineering and science such as CO2 sequestration, supercritical fluids-based extraction and purification processes, and CO2-related enhanced oil recovery methods. There are only a few studies, mostly dedicated to low pressure and temperature conditions, on densities of equilibrium liquid and vapor phases for this system.

Due to paucity of experimental data at high pressure and high temperature conditions, a series of experiments have been performed to measure the density of both liquid and vapor phases of water/CO2 system from 382 K to 478 K and pressures from 3.48 MPa to 129 MPa. In order to measure the mass of water and volume of gas in the vapor phase, in each experiment the vapor phase of an L-V equilibrium system is transferred to an equilibrium flash separator equipped with a desiccant and a gasometer. The gas volume is converted to density based on the ideal behavior of gases at standard conditions. The density of the liquid phase is directly measured by a densitometer.

In addition, a “Two-fluid Model” consisting of the Cubic-Plus-Association equation of state (CPA EOS) and the Henry’s law is implemented in phase equilibrium modeling of this system to predict the density of both phases. A comparison between our experimental data, literature data and the results of the model shows the reliability of this model for density prediction of L-V phases of water-CO2 system over a wide range of pressure and temperature conditions.

Introduction
The phase behavior of the water-CO2 system is a subject of great importance in chemical and petroleum industries. CO2 related enhanced oil recovery; CO2 sequestration in aquifers and oceans; extraction and purification processes based on supercritical fluids such as CO2 and also waste treatment of industrial liquids are chemical and petroleum engineering subjects dealing with the phase behavior of water-CO2 mixture.

For example, dissolution of CO2 in brine causes an increase in the density of brine and as a result, natural convection takes place in the aquifer. This process significantly accelerates the rate of dissolution of CO2 in the water and decreases the time scale of the CO2 storage in the aquifers. Therefore, accurate density data for the CO2-water system are required to better model the storage of CO2 in the aquifers or oceans. In CO2-related enhanced oil recovery, the injected CO2 could be dissolved in the connate water and aquifer and therefore the amount of CO2 in the reservoir decreases which could affect the recovery of in-situ oil (Ji et al., 2005).

There are some experimental density data for both saturated vapor and liquid phases of the water-CO2 system in the literature. King et al. (King et al., 1992) have reported the density of the water-rich phase at 288. 15 K, 293.15 K, and 298.15 K from 6.08 MPa to 24.32 MPa. Density of the water-saturated CO2 phase is reported by Fenghour et al. (Fenghour et al., 1996) from 405 K to 613 K. Yaginuma et al. (Yaginuma et al., 2000) measured the saturated density of both vapor and liquid phases at 304.1 K in vapor-liquid and vapor-liquid-liquid equilibria. More density data of CO2-saturated water have been published by Tegetmeier et al.