Processes > Dissolution and precipitation

Correct way to model porosity change

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dlparkhurst:
Runs for me with PhreeqcI version 3.8.5.

Are you running PhreeqcI, batch, or Tony's version from Hydrochemistry.edu?

Latest versions of codes are now being released at https://github.com/usgs-coupled/. For each program, the Release is located on the right-hand-side of the page.

Nourah:
Dear Dr Parkhurst,

I have downloaded the latest version of Phreeqc which is 3.8.5
The code runs with the new version. But I seem to have the same issue with get por.

Am I doing something wrong with my code or calculation ?
The original calculation calculated based on number of moles is correct, the original porosity is almost 27% with the 10 moles of calcite and 95 quartz, the porosity increases to 31 , as calcite dissolved and part if quartz. But the porosity in get pore goes up to almoat 38 which is unreal.

dlparkhurst:
I did not check your logic, but it is probably right.

I think these are approximately the initial conditions.

95*23.7/1000 = 2.25 L quartz
10*36.9/1000 = 0.37 L calcite
                         1.    L water

Initially, calculated porosity is 1 / 3.62 = 0.28.

With enough CO2, all the calcite dissolves, and the porosity is 1.37/3.62 = 0.38.

Here is a PHREEQC calculation that shows the same result.


--- Code: ---# diffusion equilibrium only

PHASES
CO2(g)
CO2 = CO2
-log_k -1.468
-delta_h -4.776 kcal
-analytic   10.5624  -2.3547e-2  -3972.8  0  5.8746e5  1.9194e-5
-T_c  304.2 # critical T, K
-P_c   72.86 # critical P, atm
-Omega 0.225 # acentric factor

SiO2(a)
SiO2 + 2 H2O = H4SiO4
-log_k -2.71
-delta_h  3.340 kcal
-analytic -0.26 0.0 -731.0


SOLUTION 0
    temp      65
    pH        7
    pe        4
    redox     pe
    units     mg/l
    density   1
     Al        0.5
     C         230 as HCO3
     Ca        1300
     Cl        32330 charge
     K         110
     Mg        310
     Na        19350
     Si        13
     Fe        5
     -water    1


GAS_PHASE 0
    -fixed_pressure
    -pressure 300
    -volume 1000
    -temperature 65
    CO2(g)    300

SAVE solution 0
END

SOLUTION 1-50      # Seal
temp  65         
pH     7
pressure 300
units   mg/l
 Al        0.5
 C         230 as HCO3
 Ca        1300
 Cl        32330 charge
 K         110
 Mg        310
 Na        19350
 Si        13
 Fe 5
 -water    1

SAVE SOLUTION 1-50

EQUILIBRIUM_PHASES 1-50  #seal
    Calcite   0 10       
    Quartz    0 95   
    CO2(g)    2.25  10

USE solution 1
USE equilibrium_phases 1
END

TRANSPORT
-cells 3                                 
-shifts 300                             
-time_step  10.00 yr                     
-flow_direction diffusion_only           
-boundary_conditions constant closed     
-diffusion_coefficient  5e-11
-lengths  5*0.5
-punch_frequency   1                     
-multi_D true  1e-9 0.27 0 1               
-porosities    5*0.27
-punch_cells 1-3
-punch_frequency 300
-print_cells 1


USER_GRAPH 4                # works correctly
   -headings                Time Porosity Calcite_volume Water_volume
    -axis_titles            "Time years" "Porosity" "Calcite volume, L"
    -chart_title            "CO2 rich water diffuses through mudstone"
    #-axis_scale x_axis      auto auto auto auto log
    -initial_solutions      false
    -connect_simulations    false
    -plot_concentration_vs  x
  -start

110 rock_v_original = (10 * 36.93 + 95 * 23.69)/1000   
120 rock_v_current = (EQUI("Calcite") * 36.93 + EQUI("Quartz") * 23.69)/1000   
130 cell_v = 1.0 + rock_v_original
140 water_v_current = 1 + (rock_v_original - rock_v_current)
150 porosity_current = water_v_current / cell_v
155 calcite_v_current = EQUI("Calcite") * 36.93 / 1000
160 graph_x DIST
170 graph_y porosity_current
180 GRAPH_SY calcite_v_current, water_v_current
1000 END
    -active                 true
END

--- End code ---

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