multiphase multicomponent reactive transport example

[Christinali91]

Hi David,

I am working on a multiphase multicomponent reactive transport test case using phreeqc. I start with Example 11 -- Transport and cation exchange given by phreeqc website. Instead of pure water, I want to have a gas phase(CO2) in the domain and see how the chemical reactions(e.g. pH change) affect CO2 solubility or fugacity.

I tried to find some examples to follow but haven't found any. Below is the code I wrote based on my understanding but unfortunately it doesn't work well. First, I am not very clear how to introduce an initial gas phase in this problem. Second, I want to do a CO2 fugacity plot vs. injection volume but didn't get it to work. Could you give me some suggestions on these?

Thanks so much for your help!

Best.
Christina Li

Code: [Select]


SOLUTION 0  CaCl2
        units            mmol/kgw
        temp             25.0
        pH               7.0     charge
        pe               12.5    O2(g)   -0.68
        Ca               0.6
        Cl               1.2
SOLUTION 1-40  Initial solution for column
        units            mmol/kgw
        temp             25.0
        pH               7.0     charge
        pe               12.5    O2(g)   -0.68
        Na               1.0
        K                0.2
        N(5)             1.2
END
EXCHANGE 1-40
        -equilibrate 1
        X                0.0011
COPY cell 1 101
END

ADVECTION
        -cells           40
        -shifts          100
        -punch_cells     40
        -punch_frequency 1
        -print_cells     40
        -print_frequency 20
PRINT; -reset false; -status false

USE SOLUTION 20
GAS_PHASE 1
   -fixed_volume
   -pressure 20
   -temperature 25
   -volume 0.00016
   CO2(g) 1.0

SELECTED_OUTPUT
   -reset false
   -totals C
   -saturation_indices CO2(g)
   -file co2.dat
   USER_PUNCH
   -headings PRESSURE PHICO2 lPCO2 Pore_vol
   10 PUNCH  PRESSURE
   20 PUNCH  PR_PHI("CO2(g)")
   30 PUNCH  log10(PR_P("CO2(g)"))
   40 PUNCH (STEP_NO + .5) / 40.
END

USER_GRAPH 1
    -headings               Pore_vol_added Fugacity(CO2)
    -axis_titles            "Pore_vol_added, moles" "Fugacity" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 x = (STEP_NO + 0.5) / cell_no
20 graph_x x
30 graph_y log10(pressure)
  -end
END




[dlparkhurst]

I have changed and simplified your problem considerably, but perhaps it will do as a starting point.

The calculation below assumes there is a fixed pressure for the system of 10 atm. So, the gas phase always has a pressure of 10 atm, or a gas phase does not exist.

I have used the same solution for initial condition and infilling, and removed the exchanger. I have added a little carbon to the solution so that the fugacity of CO2(g) is always defined.

The simulation starts with 10 L of CO2(g) at a pressure of 10 atm in cell 1. As infilling solution is advected into cell 1, the gas dissolves, decreasing the gas volume and moles in the gas phase. At some point the partial pressure of CO2 is less than the ambient 10 atm, and the gas phase disappears. Ultimately, the fugacity of CO2(g) is determined by the fugacity of the infilling solution.

This simulation probably does not match your conceptual model. However, a fixed volume gas has the problem that the pressure in the system varies with the amount of gas and would go to zero if the infilling solution had no CO2. Fixed pressures seem more compatible with most groundwater systems. You can expand the calculation to multiple cells, but the one cell model will help clarify your conceptual model.

Code: [Select]

SOLUTION 0-1  CaCl2
        -pressure 10
        units            mmol/kgw
        temp             25.0
        pH               7.0     charge
        pe               12.5    O2(g)   -0.68
        Ca               0.6
        Cl               1.2
        C                0.001
GAS_PHASE 1
   -fixed_pressure
   -pressure 10
   -temperature 25
   -volume 10
   CO2(g) 10
END
ADVECTION
        -cells           1
        -shifts          25
        -punch_frequency 1
        -time_step 1
USER_GRAPH 1
    -headings               Pore_vol CO2(g) log10(Fugacity(CO2))
    -axis_titles            "Pore volume" "CO2 in gas phase, moles", "Log10(fugacity(CO2(g))"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  t
  -start
20 graph_x TOTAL_TIME
30 graph_y GAS("CO2(g)"),
40 graph_sy SI("CO2(g)")
  -end
END