Processes > Dissolution and precipitation

H2O is zero in solubility of CO2

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Mohamadreza:
I used a multiphase reactive transport package at the pore scale, coupled with PHREEQC (phreeqcRM, IPhreeqc, IPhreeqcPhast, etc.), to simulate the solubility of supercritical CO₂ in a 2 molal CaCl₂ salt solution at a pressure of 10 MPa and a temperature of 60 ?C. The geometry of the model is at the microscale, with very small cells and a high concentration of salt ions. However, when running the simulation, I encountered the following error:
"A(H₂O): Activity of water has not converged."
Notably, when I ran the simulation in pure water or simulate with cacl2 solution with (Dilute calcium chloride solution)0.01 molal, no such error occurred.
Could you please advise on how to resolve this issue?

--- Code: ---SOLUTION 0 formation water
pressure 100
temp 60
water 1 # kg
pe 4
redox pe
units   Mol/L
C 1e-13
Ca 2
Cl 4
KNOBS
    diagonal_scale true
    iterations         2000
    convergence_tolerance   1e-12

SOLUTION_MASTER_SPECIES
#
#element species alk gfw_formula element_gfw
#
H H+ -1.0 H 1.008
H(0) H2 0 H
H(1) H+ -1.0 0
E e- 0 0.0 0
O H2O 0 O 16.0
O(0) O2 0 O
O(-2) H2O 0 0
Ca Ca+2 0 Ca 40.08
Cl Cl- 0 Cl 35.453
C CO3-2 2.0 HCO3 12.0111
C(+4) CO3-2 2.0 HCO3
Alkalinity CO3-2 2.0 61.0173 61.0173

SOLUTION_SPECIES
H+ = H+
-gamma 9.0 0

-dw 9.31e-9
e- = e-
H2O = H2O
-dw 2.299e-9  -254  1e-10  1e-10
H2O + 0.01e- = H2O-0.01
        -log_k -9 # aids convergence
Ca+2 = Ca+2
-gamma 5.0 0.1650
-dw 0.793e-9
-Vm  -0.3456  -7.252  6.149  -2.479  1.239  5  1.60  -57.1  -6.12e-3  1 # ref. 1
Cl- = Cl-
-gamma 3.5   0.015
-gamma 3.63  0.017 # cf. pitzer.dat
-dw 2.03e-9
-Vm  4.465  4.801  4.325  -2.847  1.748  0  -0.331  20.16  0  1 # ref. 1
CO3-2 = CO3-2
-gamma 5.4 0
-dw 0.955e-9
-Vm  5.95  0  0  -5.67  6.85  0  1.37  106  -0.0343  1 # ref. 1
# aqueous species
H2O = OH- + H+
-analytic  293.29227  0.1360833  -10576.913  -123.73158  0  -6.996455e-5
-gamma 3.5 0
-dw 5.27e-9
-Vm  -9.66  28.5  80.0 -22.9 1.89 0 1.09 0 0 1 # ref. 1
2 H2O = O2 + 4 H+ + 4 e-
-log_k -86.08
-delta_h 134.79 kcal
-dw 2.35e-9
-Vm  5.7889  6.3536  3.2528  -3.0417  -0.3943 # supcrt
2 H+ + 2 e- = H2
-log_k -3.15
-delta_h -1.759 kcal
-dw 5.13e-9
-Vm 6.52  0.78  0.12 # supcrt
CO3-2 + H+ = HCO3-
-log_k 10.329
-delta_h -3.561 kcal
-analytic 107.8871 0.03252849 -5151.79 -38.92561 563713.9
-gamma 5.4 0
-dw 1.18e-9
-Vm  8.472  0  -11.5  0  1.56  0  0  146  3.16e-3  1 # ref. 1
CO3-2 + 2 H+ = CO2 + H2O
-log_k 16.681
-delta_h -5.738 kcal
-analytic 464.1965 0.09344813 -26986.16 -165.75951 2248628.9
-dw 1.92e-9
-Vm   7.29  0.92  2.07  -1.23  -1.60 # ref. 1 + McBride et al. 2015, JCED 60, 171
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

-delta_h -0.396 kcal
-Vm  6.16  0  29.4  0  0.9 # ref. 2
Ca+2 + H2O = CaOH+ + H+
-log_k -12.78
Ca+2 + CO3-2 = CaCO3
-log_k 3.224
-delta_h 3.545 kcal
-analytic -1228.732 -0.299440 35512.75 485.818
-dw 4.46e-10 # complexes: calc'd with the Pikal formula
-Vm  -.2430  -8.3748  9.0417  -2.4328  -.0300 # supcrt
Ca+2 + CO3-2 + H+ = CaHCO3+
-log_k 11.435
-delta_h -0.871 kcal
-analytic 1317.0071 0.34546894 -39916.84 -517.70761 563713.9
-gamma 6.0 0
-dw 5.06e-10
-Vm  3.1911  .0104  5.7459  -2.7794  .3084 5.4 # supcrt
2Cl- + Ca+2  =  CaCl2   
        log_k           -0.6436
-delta_h -5.8325 kJ/mol # Calculated enthalpy of reaction CaCl2
# Enthalpy of formation: -211.06 kcal/mol
        -analytic 1.8178e+002 7.6910e-002 -3.1088e+003 -7.8760e+001 -4.8563e+001
#       -Range:  0-300

H+ + Cl- = HCl
-log_k  -0.5
-analytical_expression  0.334  -2.684e-3  1.015 # from Pitzer.dat, up to 15 M HCl, 0 - 50°C
-gamma  0  0.4256

END



--- End code ---


Error information:

--- Code: ---SOLUTION_RAW                 2304 Solution after simulation 1.
  -temp                      60
  -pressure                  100
  -total_h                   0.095811131926754
  -total_o                   0.047908152856899
  -cb                        -2.823231909479e-05
  -density                   5.3998185596486
  -totals
    C                        4.0364888717517e-06
    Ca                       0.043624548285507
    Cl                       0.087288301058543
    H2O                      0
  -pH                        2.4144543407868
  -pe                        1.6050376215329
  -mu                        14.291834424749
  -ah2o                      0.02504013734053
  -mass_water                0.00086320996138556
  -soln_vol                  0.0010861515257731
  -total_alkalinity          -4.9209075364978e-06
  -activities
    C(4)                     -14.557124780346
    Ca                       1.9142597622842
    Cl                       0.85511383656121
    E                        -1.6050376215329
    H(0)                     -11.369300642517
    O(0)                     -62.818347710313
  -gammas
  -species_map
    0 2.2368336837094e-06
    1 1.0276889800347e-12
    2 0.0037581979967257
    3 2.6578823195791e-14
    4 4.0759235929202
    5 3.4643761868298e-11
    6 36.855862360579
    7 0.00029962982747638
    8 9.9048503640988e-13
    9 8.1661248579579
    10 0.0048304248887352
    11 1.3617743923983e-13
    12 47.527553493865
    13 2.2064441141925e-11
    14 2.1056574872548e-07
    15 5.8398132073799e-09
    16 0
    17 1.2943077267615e-12
  -log_gamma_map
    0 1.4291834424749
    1 1.4291834424749
    2 0.94326107203345
    3 -1.0490566960506
    4 1.2366375606398
    5 1.4291834424749
    6 1.4291834424749
    7 -0.23909330015695
    8 1.8981769210125
    9 -0.12429834682639
    10 -0.16583584166974
    11 1.4291834424749
    12 0
    13 0.00018981769210125
    14 -0.26226417401264
    15 6.0826047311733
    16 1.4291834424749
    17 -0.37838544448794
SURFACE_RAW                  2304
  # SURFACE_MODIFY candidate identifiers #
  -type                      2
  -dl_type                   0
  -only_counter_ions         0
  -thickness                 1e-08
  -debye_lengths             0
  -DDL_viscosity             1
  -DDL_limit                 0.8
  # SURFACE_MODIFY candidates with new_def=true #
  -new_def                   0
  -sites_units               0
  -solution_equilibria       0
  -n_solution                -999
  # Surface workspace variables #
  -transport                 0
  -totals                   

--- End code ---

dlparkhurst:
PHREEQC works best with approximately 1 kg water, so it is best to scale your problem to larger water volumes.

However, I do not think the mass of water alone is the problem. The concentration in your solution is approximately 100 molal Cl. That is way past the  concentration range for any of the databases. So, somehow you have gone from 2 m CaCl2 to 50 m CaCl2. PHREEQC will fail with concentrations that large.

Mohamadreza:
I did not set the concentrations of Cl and Ca to 100 and 50 m; could the non-convergence be due to this? I set them to 4 and 2 instead.
When the simulation does not converge, it reaches unrealistic values and fails. What could be the underlying issue causing this behavior? What's your opinion to solve this error? Thanks.

dlparkhurst:
The values of SOLUTION_RAW should be those after transport, but before reaction. That is where I am getting the values to calculate concentration. Could your transport be incorrect?

Try conservative transport with no reactions to see if the CaCl2 concentrations are transported conservatively.

Mohamadreza:
How do I change it to conservative transport with no reaction?

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