ECO2 

ECO2 is a "slight" variant and offshoot of EWASG, a TOUGH2 module for flow of variably saline water and non-condensible gas, which includes salt dissolution and precipitation, and associated porosity and permeability effects. EWASG was developed primarily for geothermal applications (Battistelli et al., 1997), with elevated temperatures and modest CO2 partial pressures (of order of a few bar). The essential enhancement in ECO2 is that for a broad range of temperature and pressure conditions of interest to CO2 sequestration, accurate values for CO2 density, viscosity and enthalpy are obtained by interpolation from tables. The "CO2TAB" fluid property table supplied with ECO2 covers the pressure range from 73.82 bar (= Pcrit) to 469.82 bar, and the temperature range from 5.04 deg-C to 103.04 deg-C. Outside of the range of tabular data, CO2 properties "fall back" to the correlations used in EWASG. (Users may generate their own CO2TAB property table for any pressure and temperature range desired; however, ECO2 has no provisions to recognize phase change between liquid and gaseous CO2 at subcritical conditions.)

The other enhancement in ECO2 relative to EWASG is an improved solubility model with fugacity corrections. However, solubilities are still somewhat overpredicted for P > 100 bar for lack of a "Poynting correction." Also, no allowance is presently made for dependence of aqueous phase density on dissolved CO2 (density should increase by 1-2 % at most). Further information on thermophysical property descriptions in ECO2 is available in (Pruess and Garcia, 2002; Pruess et al. 2003; Garcia, 2003).

Except for changes in thermophysical property descriptions, the EWASG documentation in the TOUGH2 user's guide applies to ECO2 (Pruess et al., 1999).

The ECO2 module distributed with TOUGHREACT should be considered a preliminary "work in progress." Although the range and accuracy of its thermophysical property descriptions are limited, it was included in the TOUGHREACT package in order to provide a capability for analyzing chemical interactions between rocks and fluids that would be induced by CO2 injection. More accurate models for dependence of aqueous phase density on CO2 concentration (Garcia, 2001) and for phase partitioning in the H2O-CO2 system have been developed (Spycher et al., 2003; Spycher and Pruess, 2004) and are currently being implemented in a fluid property module that also treats transitions between super and sub-critical fluid and phase change between liquid and gaseous CO2 (Pruess, 2004).

References

Battistelli, A., C. Calore and K. Pruess.  The Simulator TOUGH2/EWASG for Modeling Geothermal Reservoirs with Brines and Non-Condensible Gas, Geothermics, Vol. 26, No. 4, pp. 437 - 464, 1997.

Garcia, J.  Density of Aqueous Solutions of CO2, Lawrence Berkeley National Laboratory Report LBNL-49023, Berkeley, CA, 2001.

Garcia, J.E.  Fluid Dynamics of Carbon Dioxide Disposal into Saline Aquifers, PhD Thesis, University of California at Berkeley, December 2003.

Spycher, N., K. Pruess and J. Ennis-King.  CO2-H2O Mixtures in the Geological Sequestration of CO2. I. Assessment and Calculation of Mutual Solubilities from 12 to 100 ¡C and up to 600 bar, Geochim. Cosmochim. Acta, Vol. 67, No. 16, pp. 3015 - 3031, doi:10.1016/S0016-7037(03)00273-4, 2003. (LBNL-50991)

Spycher, N. and K. Pruess.  CO2-H2O Mixtures in the Geologic Sequestration of CO2. II. Partitioning in Chloride Brines at 12–100 ûC and up to 600 bar, submitted toGeochim. Cosmochim. Acta, September 2004. (LBNL-56334)

Pruess, K.  Numerical Simulation of CO₂ Leakage from a Geologic Disposal Reservoir, Including Transitions from Super- to Sub-Critical Conditions, and Boiling of Liquid CO₂, Soc. Pet. Eng. J., pp. 237 - 248, June 2004. (LBNL-52423)

Pruess, K., C. Oldenburg and G. Moridis.  TOUGH2 User’s Guide, Version 2.0, Lawrence Berkeley National Laboratory Report LBNL-43134, Berkeley, CA, November 1999.

Pruess, K. and J. Garcia.  Multiphase Flow Dynamics During CO2 Injection into Saline Aquifers, Environmental Geology, Vol. 42, pp. 282 - 295, 2002.

Pruess, K., T. Xu, J. Apps and J. Garcia.  Numerical Modeling of Aquifer Disposal of CO2, Paper SPE-83695, SPE Journal, pp. 49 - 60, 2003.