pH variations with reaction pressure and temperature

[Franco Galarce]

Hi,

I simulated the rise of a geothermal fluid sample through a well: from 255 ° C, 130 bar pressure -->to 100 ° C with 7 bar pressure.

First I depressurized and then I cooled down. However, curiously the pH increases during cooling, instead of decreasing.

Why could this happen?

Input:

    temp        255
    pressure  128.3
    pH           6.89
    pe        -3.8
    redox     pe
    units     mg/l
    density   1.017
    Al        0.5
    As        71
    B         273
    Br        10.4
    Ca        543
    Cl        10066
    Cs        32
    F         10.9
    Fe        0.33
    K         871
    Li        59
    Mg        0.17
    Na        5182
    Rb        9.5
    S(6)      22
    Sb        9.5
    Si        531 as SiO2
    Sr        13.4
    Ti        0.01
    C(4)      34
    -water    1 # kg
EQUILIBRIUM_PHASES 1
    Ar(g)     -4.5686 10
    CH4(g)    -5.0528 10
    CO(g)     -5.5864 10
    CO2(g)    -0.2749 10
    H2(g)     -3.5433 10
    H2O(g)    -0.3373 10
    H2S(g)    -2.4823 10
    He(g)     -5.1215 10
    N2(g)     -2.2720 10
REACTION_PRESSURE 1
    128.3 6.8 in 100 steps
REACTION_TEMPERATURE 1
    255 100 in 100 steps

Regards

[dlparkhurst]

The gases H2, CO, CO2, and CH4 are reacting.

2H2 + 2CO = CO2 + CH4

You have defined 10 moles of each are available to react, so all of the H2 and CO have reacted to make 5 moles of CO2 and 5 moles of CH4. The addition of the CO2 decreases the pH.

Consider whether 10 moles of each of these gases is available to react.

You may also want partial equilibrium, where some of these gases are present, but you do not want them to react with each other. phreeqc.dat has definitions for unreactive versions of H2 (Hdg), O2 (Oxg), CH4 (Mtg), H2S (Sg), and N2 (Ntg). It would be possible to define a similar CO.

You may also want to consider using a fixed pressure or fixed volume GAS_PHASE. Currently, you are allowing 10 moles of each gas to react with the solution to attain specified partial pressures. With GAS_PHASE, you can allow the solution to equilibrate with an initially absent gas phase. For fixed pressure, a gas bubble (of a calculated volume) will form if the sum of the partial pressures of the gases exceeds the specified total pressure. For fixed volume, a gas phase will always be present in equilibrium with the solution, but the pressure of the gas phase will vary.

Note also that phreeqc.dat and pitzer.dat use the Peng-Robinson equation of state for calculations, rather than the ideal gas law. You may want to consider the effects of the PR approach for your temperatures and pressures by using a system simplified to elements and gases available in these databases.