F. Tabasinejad, SPE, Y. Barzin, SPE, G. R. Moore, SPE, S. A. Mehta, SPE, K.
C. Van Fraassen, SPE, University of Calgary, J. A. Rushing, SPE, Anadarko
Petroleum Corp., K. E. Newsham, SPE, Apache Canada LTD
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
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.