Conceptual Models > Equilibrium assumptions

Gas phase volume

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Hamed:
I am currently simulating mineral scaling at the bottomhole of a geothermal reservoir during a pressure drop. The reservoir is fully saturated with geothermal brine, and I first establish gas-brine-rock equilibrium under reservoir conditions. This is followed by a pressure drop in the next step to calculate mineral precipitation.

However, I?m unsure about the appropriate gas phase volume to use in the simulation. The results of both the initial reservoir equilibration and the subsequent bottomhole step are sensitive to the input gas volume, and I observe changes in the output as I vary this volume. Here is my code:


--- Code: ---SOLUTION 1   
-pressure    400
-temperature 120
-pH   7
-units   ppm

Na   55000
Cl   190000  charge
K   4000
Ca   32000
Mg   700
Ba   50
Fe      160
Mn      500
Pb   70
S   60
C   50   as HCO3

GAS_PHASE 1   
      -fixed_pressure
      -pressure     400
      -temperature   120         
     Mtg(g)    96     
     CO2(g)    4     
     Ntg(g)    300

EQUILIBRIUM_PHASES 1 #Reservoir equilibration

Quartz 0  10
Calcite  0  10
Anhydrite 0 10
Dolomite 0 10
Halite 0 10
     
Barite 0 0
Talc 0 0   

SAVE solution 1
END

USE solution 1

REACTION_TEMPERATURE 1
117

GAS_PHASE 2   
      -fixed_pressure
      -pressure      200
     Mtg(g)    0   
     CO2(g)    0       
     Ntg(g)    200

EQUILIBRIUM_PHASES 2 #scaling because of pressure drop

Talc 0  0
Anhydrite 0 0
Halite 0 0     
Barite 0 0 
dolomite 0 0 
calcite 0 0

END

--- End code ---

dlparkhurst:
I don't think you should specify a gas volume, rather I would set the partial pressures of the gasses for the downhole conditions with EQUILIBRIUM_PHASES. The downhole reaction calculation (user number 1 definitions) tries to account for all of the gases and minerals that are lost from solution to bring the solution back to downhole conditions.

The next reactions (user number 2, solution, GAS_PHASE, and EQUILIBRIUM_PHASES, REACTION_TEMPERATURE, REACTION_PRESSURE  should recreate the conditions at 200 atm, 127 C.

It's not my specialty, but consider if this script represents your situation.


--- Code: ---SOLUTION 1
    temp      120
    pH        7
    pe        4
    redox     pe
    units     ppm
    density   1
    Ba        50
    C         50 as HCO3
    Ca        32000
    Cl        190000 charge
    Fe        160
    K         4000
    Mg        700
    Mn        500
    Na        55000
    Pb        70
    S         60
    -water    1 # kg
END
EQUILIBRIUM_PHASES 1
    Anhydrite 0 10
    Barite    0 0
    CO2(g)    0.6 10
    Calcite   0 10
    Dolomite  0 10
    Halite    0 10
    Mtg(g)    1.98 10
    Ntg(g)    2.48 10
    Quartz    0 10
    Talc      0 0
END
USE solution 1
USE equilibrium_phases 1
SAVE solution 2
END
USE solution 2
GAS_PHASE 2
    -fixed_pressure
    CO2(g)    0
    Mtg(g)    0
    Ntg(g)    0
REACTION_TEMPERATURE 2
    117
REACTION_PRESSURE
    200
EQUILIBRIUM_PHASES 2
    Anhydrite 0 0
    Barite    0 0
    Halite    0 0
    Talc      0 0
    calcite   0 0
    dolomite  0 0
END
--- End code ---

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