Carbonated water

[hagar.haithm@gmail.com]

Hello everyone,
I am Haitham, I am a new phreeqc user. I am coming from petroleum background. basically I am working on carbonated water injection into oil reservoir.

I searched in the manual about carbonated water. However, It was a simple method by using "EQUILIBRIUM_PHASES" my question: Is it correct if I do EQUILIBRIUM_PHASES to other water solution composition (not pure water) like seawater. Will it be carbonated water? I hope my question is understandable.

Why I doing this?
Because I want to injected carbonated water to be mixed with formation Water (aquifer) to check the change in pH.

Thank you for your support and contribution
Many thanks,
Haitham

[dlparkhurst]

Yes, PHREEQC will calculate the solubility of CO2(g) at the partial pressure specified in EQUILIBRIUM_PHASES for whatever water composition you define, either by SOLUTION, or SOLUTION plus additional reactions (equilibrium with calcite, for example).

Note that phreeqc.dat, Amm.dat, and pitzer.dat will use the Peng-Robinson fugacities. Further, with these databases, the use of GAS_PHASE will also use Peng-Robinson for the gas-phase components. 

All other databases will use the ideal gas law for the calculations of gas components in EQUILIBRIUM_PHASES and GAS_PHASE.

[hagar.haithm@gmail.com]

Thank you very much dlparkhurst :)

[hagar.haithm@gmail.com]

Thank dlparkhurst for your help,

I have one question regarding dissolved CO2 concentration.

is it possible to calculate the concentration of the dissolved CO2 from the above method. I ran the simulation and get different pH values, the  pCO2 (-2.0,-1.5, -0.5) was the key parameter now I want to find the concentration of dissolved CO2 in each solution or is it possible to extracted it from the output sheet, if yes please indicate which section.

Many thanks,
Haithm

[dlparkhurst]

The attached file generates seawater with 20 atm CO2. The concentration of CO2 is about 0.52 mol/kgw. With phreeqc.dat (used here) you should probably add add CO2 + 2*(CO2)2 if you want to know the solubility of CO2. (CO2)2 is something of a fitting parameter that was added to fit solubilities at very high pressures.

Code: [Select]

                                              Log       Log       Log    mole V
   Species          Molality    Activity  Molality  Activity     Gamma   cm³/mol
...
C(4)          5.329e-01
   CO2             5.187e-01   6.059e-01    -0.285    -0.218     0.068     34.43
   (CO2)2          5.768e-03   6.739e-03    -2.239    -2.171     0.068     68.87
   HCO3-           2.070e-03   1.398e-03    -2.684    -2.855    -0.171     26.98
...


[hagar.haithm@gmail.com]

Hello dlparkhurst,

Is there an equation to calculate pH value during CO2 dissolution which involves CO2 partial pressure (PCO2)?

Many thanks,

[dlparkhurst]

The Henry's law constant defines

K = a(CO2(aq))/f(CO2(g)), a = activity, f = fugacity ~ partial pressure.

Given f(CO2(g)) you can calculate a(CO2(aq)). For a pure system, you can work through to get a(HCO3-) from

CO2 + H2O = HCO3- + H+; K = [a(HCO3-)a(H+)]/[a(CO2)a(H2O)]

assuming H+ = HCO3-, but your are making assumptions about a pure system, activity of water, ionic strength, fugacity coefficients, and activity coefficients. It is simpler just to run a PHREEQC calculation.