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Author Topic: Hydrothermal dolomitization  (Read 1071 times)

Eivind_Vagle

  • Contributor
  • Posts: 2
Hydrothermal dolomitization
« on: 18/01/24 17:47 »
Dear phreeqc community,

I'm working on a problem where I try to simulate hydrothermal dolomitization (calcite dissolution and dolomite precipitation) as a function of pressure and temperature. I use seawater brine evaporated to dolomite, anhydrite, and halite equilibrium as the input solution. The brine is warmed up to 250 C and pressure is varied (1, 250, 500, 750, 1000 atm). I expected mineral solubility to increase when pressure increased. However, the output shows that the saturation indices increase (i.e. solubility decreases) when pressure increases. The opposite happens if I lower the temperature to 100 C. Does anyone know why this happens? Could it be a valid result or is there something wrong with my code?

Code: [Select]
TITLE Hydrothermal dolomite

SOLUTION 1
#seawater brine
-units mol/kgw
-temp 40
pH 6.77
#anions
Alkalinity 0.000260977
Cl 6.15E+00
S(6) 2.20E-01
#cations
Ca 1.10E-02
Mg 5.92E-01
K 1.15E-01
Na 5.28E+00

SELECTED_OUTPUT
-file Pressure.txt
-reset false
-alk
-solution
-temp
-pH
-alkalinity
-totals S(6) Ca K Na Mg Cl #concentrations (moles/kgw)
-saturation_indices Calcite Dolomite Anhydrite Halite CO2(g)
END

USE solution 1
EQUILIBRIUM_PHASES
#Calcite
#Dolomite
REACTION_TEMPERATURE
250
REACTION_PRESSURE
1 250 500 750 1000
END

Kind regards,

Eivind
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dlparkhurst

  • Global Moderator
  • *****
  • Posts: 4030
Re: Hydrothermal dolomitization
« Reply #1 on: 18/01/24 20:34 »
I don't think there is anything wrong with your code. I'm not very familiar with higher temperature and pressure. I can tell you that the main cause of the increase in SI is the increase of the ion-activity product, which in turn is due mainly to the activity coefficients.

The following script shows the ratio of quantities at pressure to the value of the quantity at 1 atm (all at 250 C).

Code: [Select]
TITLE Hydrothermal dolomite

SOLUTION 1
#seawater brine
-units mol/kgw
-temp 40
pH 6.77
#anions
Alkalinity 0.000260977
Cl 6.15E+00
S(6) 2.20E-01
#cations
Ca 1.10E-02
Mg 5.92E-01
K 1.15E-01
Na 5.28E+00
END
USE solution 1
REACTION_TEMPERATURE
250
SAVE solution 1
USER_PRINT
10 PUT(MOL("Ca+2"), 1)
20 PUT(ACT("Ca+2"), 2)
30 PUT(GAMMA("Ca+2"), 3)
110 PUT(MOL("CO3-2"), 11)
120 PUT(ACT("CO3-2"), 12)
130 PUT(GAMMA("CO3-2"), 13)
210 PUT(SR("Calcite"), 21)
220 PUT(ACT("Ca+2")*ACT("CO3-2"), 22)
230 PUT(10^LK_PHASE("Calcite"), 23)
END
PRINT
-user_print false
END
USE solution 1
REACTION_PRESSURE
1 250 500 750 1000
USER_GRAPH 1
    -headings               pressure SR(Calcite) IAP(Calcite) K(Calcite)
    -axis_titles            "Pressure" "Relative change"
  -start
10 GRAPH_X PRESSURE
20 GRAPH_Y SR("Calcite")/GET(21)
30 GRAPH_Y ACT("Ca+2")*ACT("CO3-2") / GET(22)
40 GRAPH_Y 10^LK_PHASE("Calcite") / GET(23)
  -end
USER_GRAPH 2
    -headings               pressure MOL(Ca+2) ACT(Ca+2) GAMMA(Ca+2)
    -axis_titles            "Pressure" "Relative change" ""
  -start
10 GRAPH_X PRESSURE
30 GRAPH_Y MOL("Ca+2")/GET(1), ACT("Ca+2")/GET(2), GAMMA("Ca+2")/GET(3)
USER_GRAPH 3
    -headings               pressure MOL(CO3-2) ACT(CO3-2) GAMMA(CO3-2)
    -axis_titles            "Pressure" "Relative change" ""
  -start
10 GRAPH_X PRESSURE
40 GRAPH_Y MOL("CO3-2")/GET(11), ACT("CO3-2")/GET(12), GAMMA("CO3-2")/GET(13)
  -end
END
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Eivind_Vagle

  • Contributor
  • Posts: 2
Re: Hydrothermal dolomitization
« Reply #2 on: 19/01/24 09:45 »
Hi dlparkhurst,

Thanks for your quick reply. I have looked at my output files a bit more and see that increasing the pressure leads to more dissolution of calcite which drives more precipitation of dolomite. This makes sense to me.

Thanks again,

Eivind
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