PHREEQC at high T and P

[Peng]

Dear Colleagues,

Please find an online version of the USGS program PHREEQC that now allows speciation – solubility,  reaction path, and 1D reactive transport modeling at T >300 oC. To facilitate modeling of reaction kinetics, a library of rate equations and parameters for about 100 minerals in the form of PHREEQC RATES Basic language scripts are included in the data files phreeqc-kinetics.dat, llnl-kinetics.dat, geothermal.dat, and diagenesis.dat.

https://models.earth.indiana.edu/applications_index.php

Please note that we are not doing high T & P modeling ourselves; So we have not tested the code and databases extensively. Kindly report any errors to supcrt@indiana.edu with your full name and academic affiliation. We make the online version available because we think it might be helpful for your teaching and research.

Dr. David Parkhurst made the change of the PHREEQC code to allow T>300 oC calculations. The future release of PHREEQC distributed by USGS may include this functionality. Meanwhile, users can use the online version for now.

[Mouadh]

Great news! Thank you.

[Mouadh]

Dear all,

I was hoping this update will improve the CO2 solubility estimation at high pressure using llnl.dat. I tested the following example on the new online version using the Geothermal.dat and it seems it is still over estimating CO2(aq).

Code: [Select]

SOLUTION 1
-pressure 400
units            mmol/kgw
C 0.001
temp             100
GAS_PHASE 1
   -fixed_pressure
   -pressure 400
   -temperature 100
   -volume 100
   CO2,g 400

#output
C(4)          4.325e+00
   CO2             4.323e+00   4.325e+00     0.636     0.636     0.000     (0) 
   HCO3-           1.520e-03   1.445e-03    -2.818    -2.840    -0.022     (0) 
   CO3-2           1.012e-10   8.258e-11    -9.995   -10.083    -0.088     (0) 

   

From example 22 the CO2 sol. at these conditions should be around 1.5M.
what am I missing?

Best,
Mouadh

[Peng]

Please use diagenesis.dat, instead of geothermal.dat. The former has the pressure correction up to 1000 bar by using molar volumes for aqueous species and solids and P_c, T_c, Omega (Peng-Robinson EOS) for gases, while the later follows the pressure curve of Psat.

These are the results from diagenesis.dat. Please check.

C(4)          1.324e+00
   CO2             1.323e+00   1.323e+00     0.121     0.121     0.000     34.47
   HCO3-           9.061e-04   8.721e-04    -3.043    -3.059    -0.017     25.65
   CO3-2           1.507e-10   1.291e-10    -9.822    -9.889    -0.067     -8.16


Note that using phreeqc.dat, we can get the result of CO2 solubility at ~1.5 M. However, this is achieved by adding an extra (CO2)2  as activity correction for CO2 solubility at high P, T, which contributes 2*0.14 = 0.28 M extra! The prediction from actual thermodynamic data of phreeqc.dat is only ~ 1.23 M. 

C(4)          1.511e+00
   CO2             1.230e+00   1.230e+00     0.090     0.090     0.000     37.85
   (CO2)2          1.404e-01   1.404e-01    -0.853    -0.853     0.000     75.70
   HCO3-           8.532e-04   8.220e-04    -3.069    -3.085    -0.016     25.70
   CO3-2           1.283e-10   1.106e-10    -9.892    -9.956    -0.065     -8.82

***********************************************************
2CO2 = (CO2)2 # activity correction for CO2 solubility at high P, T
   -log_k -1.8
   -analytical_expression  8.68  -0.0103  -2190
   -Vm   14.58  1.84  4.14  -2.46  -3.20
***********************************************************

[dlparkhurst]

I disagree somewhat with the statement "However, this is achieved by adding an extra (CO2)2  as activity correction for CO2 solubility at high P, T, which contributes 2*0.14 = 0.28 M extra! The prediction from actual thermodynamic data of phreeqc.dat is only ~ 1.23 M."

The solubility is the amount of CO2 that dissolves in the solution, so you should consider the sum of CO2 and 2*(CO2)2 to be the solubility assuming other species are negligible. The fugacity (SR) and partial pressure (SR/phi) can be determined from the species CO2(aq). It is true that the (CO2)2 was added simply to fit the empirical data more closely, but there are many species that are included in ion-association models (including llnl-type databases) purely to enhance the fits to data.