Gas phase weathering

[ylg]

Hi,
I am trying to model if H2S (gas) is geneerated  during the contact of a gas phase with a pyrite rich carbonate reservoir at some time during the gas storage operational lifetime.
This aims to mimic the case of a constant gas composition wheathering a solution and rock overtime. I assume quartz, calcite, pyrite, albite as kinetic minerals as a starting point.
This modeling may represent 1 grid block of a reaction transport <ith immobile solution and moving gas ... But I do not know how to set this up  within PHREEQC.

When I specify  H2S  in the EQUILIBRIUM_PHASES, I do not see any change in H2S(g) nor in H2S molalitywhen defining the GAS PHASE at fixed-pressure.
When I specify  H2S  in GAS PHASE ( partial pressure = 0), PHREEQC predicts some H2S(g) generation.
However, the volume of gas is limited (0.6% of a liter) due to low rock porosity and  the other components of the gas are dissolved in water but not renewed .
In the case I want to model, axcept for H2S, the  gas components should have a constant partial pressure.

DATABASE C:\Program Files (x86)\USGS\Phreeqc Interactive 3.4.0-12927\database\wateq4f.dat
TITLE  brine +rock initial
SAVE SOLUTION 0
SAVE EQUILIBRIUM_PHASES 0
SOLUTION 0 :
   units mg/l
   temp 64
   pressure 130
   pH 6.9
   Na 29575
   Ca 5265
   Mg 1425
   K  960
   Cl 59108
   S(6) 566
   Al 16
   Si 29.
   Fe 31
   C(4) 662
   N(-3)    80

EQUILIBRIUM_PHASES 0
   Calcite 0.0 0.0233
   Quartz 0.0 .00166
   Albite 0.0 1.95e-5
   Siderite 0.0 3.37e-5
   Pyrite 0.0 0.001
   Kaolinite 0.0 4.98e-5
   Chlorite14A 0.0 6.54e-5
   Illite 0.0 3.04e-4
   Anorthite 0.  9.82e-6
   Chalcopyrite 0. 2.31e-06
   Magnesite 0. 1.17e-06
   Sphalerite 0. 4.15e-06
   Galena 0. 3.14e-06
   Kmica 0  7.46e-06
   FeS(ppt) 0 0
   Dolomite 0 0
   Anhydrite 0 0
   Halite 0 0
   Brucite 0 0
   
SELECTED_OUTPUT
   -file initial-equilibrium.xls
   -selected_out True
   -simulation True
   -solution True
   -state True
   -pH True
   -ionic_strength true
   -water true
   -saturation_indices    Calcite Quartz Albite Anorthite Siderite Pyrite Kaolinite
   Chlorite14A Kmica Illite Chalcopyrite Sphalerite Galena Magnesite
   -equilibrium_phases Calcite Quartz Albite Anorthite Siderite Pyrite Kaolinite
   Chlorite14A Kmica Illite Chalcopyrite Sphalerite Galena Magnesite FeS(ppt) Dolomite
   Anhydrite Halite Brucite
   -totals Al C(-4) C(4) Ca Cl Fe(2) Fe(3) K Mg N(-3) N(0) Na S(-2) S(6) Si
   -activities H+ Ca+2 CO2 HCO3- CO3-2 NH4+ HS- SO4-2
END
TITLE   storage gas Injection
USE SOLUTION 0
USE EQUILIBRIUM_PHASES 0
SAVE EQUILIBRIUM_PHASES 1
SAVE SOLUTION 1
SAVE GAS_PHASE 1
GAS_PHASE 1 storage gas at 132 bars
   -fixed_volume
   -pressure 130
   -volume 0.006
   -temperature 64
   CH4(g) 125.7
   N2(g)  2.6
   CO2(g) 1.95
   H2S(g) 0.
INCREMENTAL_REACTIONS true
KINETICS 1
   Pyrite
         -m      103
         -parms  0.3     0.67     .5      -0.11
   Calcite
      -m     2328.5
      -parms 1.67e5   0.6
   Albite
      -m  2           
      -parms 6.04  0.1
   Quartz
      -m 161.3
      -parms 0.146 1.5
-steps       25 year in 60 steps
-step_divide 1
-runge_kutta 3
-cvode true
-bad_step_max 1000
-cvode_order 5

KNOBS
-iterations 300

SELECTED_OUTPUT
   -file storage.xls
       -time                 true
   -selected_out True
   -solution True
   -simulation True
   -state True
   -pH True
   -ionic_strength true
   -water true
   -gases CH4(g) N2(g) CO2(g) O2(g) H2(g) H2S(g) H2O(g)
   -saturation_indices    Calcite Quartz Albite Anorthite Siderite Pyrite Kaolinite
   Chlorite14A Kmica Illite Chalcopyrite Sphalerite Galena Magnesite
   -equilibrium_phases Calcite Quartz Albite Anorthite Siderite Pyrite Kaolinite
   Chlorite14A Kmica Illite Chalcopyrite Sphalerite Galena Magnesite FeS(ppt) Dolomite
   Anhydrite Halite Brucite
   -kinetic_reactants Albite Calcite Pyrite Quartz
   -totals Al C(-4) C(4) Ca Cl Fe(2) Fe(3) K Mg N(-3) N(0) Na S(-2) S(6) Si
   -activities H+ Ca+2 CO2 HCO3- CO3-2 NH4+ HS- SO4-2
END

I tried to split the computation on year basis with USE and SAVE for the  EQUILIBRIUM_PHASES, SOLUTION  but does not allow to preserve the number of mole of kinetic mineral in between simulations.
The only solution I could figure out is to run PHREEQC  several times with each simulation an edition of the number of moles of each kinetic minerals as shown below

TITLE  Injection storage gas  year 1
USE SOLUTION 0
USE EQUILIBRIUM_PHASES 0
SAVE EQUILIBRIUM_PHASES 1
SAVE SOLUTION 1
SAVE GAS_PHASE 1
GAS_PHASE 1 storage gas at 132 bars
   -fixed_volume
   -pressure 130
   -volume 0.006
   -temperature 64
   CH4(g) 125.7
   N2(g)  2.6
   CO2(g) 1.95
   H2S(g) 0.
INCREMENTAL_REACTIONS true
KINETICS 1
   Pyrite
         -m      103
         -parms  0.3     0.67     .5      -0.11
   Calcite
      -m     2328.5
      -parms 1.67e5   0.6
   Albite
      -m  2           
      -parms 6.04  0.1
   Quartz
      -m 161.3
      -parms 0.146 1.5
-steps       1 year in 12 steps

SELECTED_OUTPUT
   -file storage-1.xls
       -time                 true
   -selected_out True
   -solution True
   -simulation True
   -state True
   -pH True
   -ionic_strength true
   -water true
   -gases CH4(g) N2(g) CO2(g) O2(g) H2(g) H2S(g) H2O(g)
   -saturation_indices    Calcite Quartz Albite Anorthite Siderite Pyrite Kaolinite
   Chlorite14A Kmica Illite Chalcopyrite Sphalerite Galena Magnesite
   -equilibrium_phases Calcite Quartz Albite Anorthite Siderite Pyrite Kaolinite
   Chlorite14A Kmica Illite Chalcopyrite Sphalerite Galena Magnesite FeS(ppt) Dolomite
   Anhydrite Halite Brucite
   -kinetic_reactants Albite Calcite Pyrite Quartz
   -totals Al C(-4) C(4) Ca Cl Fe(2) Fe(3) K Mg N(-3) N(0) Na S(-2) S(6) Si
   -activities H+ Ca+2 CO2 HCO3- CO3-2 NH4+ HS- SO4-2
END
TITLE  Injection storage gas year 2
USE SOLUTION 1
USE EQUILIBRIUM_PHASES 1
SAVE EQUILIBRIUM_PHASES 2
SAVE SOLUTION 2
USE GAS_PHASE 1
INCREMENTAL_REACTIONS true
KINETICS 1
   Pyrite
         -m      103
         -parms  0.3     0.67     .5      -0.11
   Calcite
      -m     2328.5
      -parms 1.67e5   0.6
   Albite
      -m  1.999995629           
      -parms 6.04  0.1
   Quartz
      -m 161.3
      -parms 0.146 1.5
-steps       1 year in 12 steps

SELECTED_OUTPUT
   -file storage-1.xls
       -time                 true
   -selected_out True
   -solution True
   -simulation True
   -state True
   -pH True
   -ionic_strength true
   -water true
   -gases CH4(g) N2(g) CO2(g) O2(g) H2(g) H2S(g) H2O(g)
   -saturation_indices    Calcite Quartz Albite Anorthite Siderite Pyrite Kaolinite
   Chlorite14A Kmica Illite Chalcopyrite Sphalerite Galena Magnesite
   -equilibrium_phases Calcite Quartz Albite Anorthite Siderite Pyrite Kaolinite
   Chlorite14A Kmica Illite Chalcopyrite Sphalerite Galena Magnesite FeS(ppt) Dolomite
   Anhydrite Halite Brucite
   -kinetic_reactants Albite Calcite Pyrite Quartz
   -totals Al C(-4) C(4) Ca Cl Fe(2) Fe(3) K Mg N(-3) N(0) Na S(-2) S(6) Si
   -activities H+ Ca+2 CO2 HCO3- CO3-2 NH4+ HS- SO4-2
END


This solution is quite cumbersome and, of course, depends  on the time split (daily, monthly, yearly)
used as the adjustment of number of moles (here albite) is edited out by the user in between simulations.
Is there an alternative or a smarter way to do this?
Thanks for your advices

[dlparkhurst]

You have SO4-2 in your water, so my guess is that H2S is derived from sulfate reduction by organic material. I think you are more likely to be precipitating pyrite by reaction of H2S with iron than to be dissolving it.

[ylg]

Thanks for the comment.
H2S is generated from SO4-2 but pyrite stays  at equilibrium and seems unaffected by the solution. This is the reason why we wanted to check if adding fresh gas overtime  could alter pyrite stability. Thus, I tried to split the computation steps because the kinetic reactions are quite fast.
As far as I could understand Phreeqc modeling concept, adding fresh gas to a solution and rock (static which have already been in contact with the same gas phase) is not possible i.e. gas phase is renewed (we need to keep track of H2S which is generated) and put in contact in altered rock and solution.
Can you suggest a different approach?

[dlparkhurst]

Do you think pyrite should dissolve by adding more H2S? I guess I don't understand what reaction you are trying to model.

It may be awkward, but you can always use SAVE, USE, and new definitions to bring reacted solutions and equilibrium phases with a fresh gas phase. For example:

Code: [Select]

SOLUTION 1
...
END
EQUILIBRIUM_PHASES 1
...
END
GAS_PHASE 1
...
END
USE solution 1
USE equiuilibrium_phases 1
USE gas_phase 1
SAVE solution 1
SAVE equilibrium_phases 1
END
USE solution 1
USE equiuilibrium_phases 1
USE gas_phase 1
...

Two reaction calculations are defined. The solution and equilibrium_phases are saved after the first, but the GAS_PHASE is not. Therefore, in the second reaction calculation, the reacted solution and equilibrium_phases are used, but the initial GAS_PHASE condition is the same as in the first calculation.

[ylg]

In this case, the site operator assumed  that methane could react with pyrite

𝐹𝑒𝑆2+(1−𝑥) 𝐶𝐻4 ⇔2(1−𝑥)𝐻2𝑆+𝐹𝑒𝑆_(1+𝑥)+ (1−𝑥)𝐶

as indicated in "Proposed Explanation of Hydrogen-Sulfide Formation in Underground Natural-Gas Storage Structures by Reduction of Mineral Sulfides in the Reservoir Rock"
J. P. Bourgeois, N. Aupaix, R. Bloise, J. L. Millet (DOI https://doi.org/10.2516/ogst:1979013)

I just wanted to make sure that I did a proper PHREEQC modeling.
Thank for your quick replies and comments

[dlparkhurst]

Reduction of S in pyrite is possible, but you still have SO4-2 in your system, which thermodynamically should be preferred. Using equilibrium modeling of S with PHREEQC will reduce sulfate first, unless you somehow separate the S redox states (see example 8 for Fe).