Effect of Gas Volume on pH in Water?Gas?Rock System

[ahmadreza_shojaee]

Dear PHREEQC community,

I am encountering a technical issue in a closed-system simulation and would appreciate your insights.

I am comparing two PHREEQC input files where the aqueous solution is identical. The only difference is the pressure:

Code: [Select]

SOLUTION 1
-pressure 147.0132
-temp 60
pH 5.96 charge
units mol/kgw
K 0
Na 2.865
Mg 0.1086
Ca 0.2878
Cl 3.655
C(4) 0.004167
S(6) 0.004664
S(-2) 0
Fe(3) 0
Fe(2) 0
Si 9.723e-05
-water 1
END
Gas_Phase 1
-pressure 147.0132
-temp 60
-fixed_volume
-volume 5.2367
Hdg(g) 0
CO2(g) 0
CH4(g) 146.4546
H2S(g) 0
N2(g) 0
Gas_Phase_Modify 1
-type 1
-total_p 147.0132
-volume 5.2367
-component Hdg(g)
-moles 0
-component CH4(g)
-moles 32.2706
-component CO2(g)
-moles 0
-component H2S(g)
-moles 0
-component N2(g)
-moles 0
END
EQUILIBRIUM_PHASES 1
Calcite 0 13.2199
Dolomite 0 3.5877
Anhydrite 0 0.048595
Quartz 0 1068.0591
Goethite 0 0
Pyrite   0 0
Gypsum 0  0
END
USE solution 1
USE EQUILIBRIUM_PHASES 1
USE gas_phase 1


Code: [Select]

SOLUTION 1
-pressure 151.8681
-temp 60
pH 5.96 charge
units mol/kgw
K 0
Na 2.865
Mg 0.1086
Ca 0.2878
Cl 3.655
C(4) 0.004167
S(6) 0.004664
S(-2) 0
Fe(3) 0
Fe(2) 0
Si 9.723e-05
-water 1
END
Gas_Phase 1
-pressure 151.8681
-temp 60
-fixed_volume
-volume 0.051602
Hdg(g) 0
CO2(g) 0
CH4(g) 158.4763
H2S(g) 0
N2(g) 0
Gas_Phase_Modify 1
-type 1
-total_p 151.8681
-volume 0.051602
-component Hdg(g)
-moles 0
-component CH4(g)
-moles 0.34434
-component CO2(g)
-moles 0
-component H2S(g)
-moles 0
-component N2(g)
-moles 0
END
EQUILIBRIUM_PHASES 1
Calcite 0 2.2245
Dolomite 0 0.60369
Anhydrite 0 0.0081769
Quartz 0 179.7185
Goethite 0 0
Pyrite   0 0
Gypsum 0  0
END
USE solution 1
USE EQUILIBRIUM_PHASES 1
USE gas_phase 1

Case 1: Pressure = 151 atm
Case 2: Pressure = 147 atm

Each case includes a gas phase, defined using both GAS_PHASE and GAS_PHASE_MODIFY. The gas mixture includes:

1. CH4 (non-zero amount)
2. CO2, H2S, N2, and Hdg (each with initial amount = 0)

The gas phases differ only in:

1. Total pressure
2. Gas volume
3. Amount of CH4

The rock phase, defined using EQUILIBRIUM_PHASES, contains calcite, dolomite, anhydrite, and quartz, with different quantities between the two cases.

Despite the same solution composition, the two cases yield different pH values:

One gives pH ≈ 6

The other gives pH ≈ 5.3

However, when I remove CO2 (and the other zero-amount gases) from the GAS_PHASE, both cases return nearly identical pH.

My questions are:

1. Is it reasonable for the gas volume or pressure to impact pH in this case, even when CO2 is initially set to zero?

2. Is the pH difference caused by more CO2 being released from the rock phase, due to the larger gas volume?  When I increase the gas volume in the second model, final pH increases!

Any explanation about the mechanism driving this pH sensitivity would be greatly appreciated.

Best regards,
Ahmad

[dlparkhurst]

You need to look at your output file closely.

First, the pH of your initial solution is 2.3. Is this really what you had in mind?

Second, by adding CH4 to solution from the gas, it will reduce S(6) to S(-2), which will affect pH. If you include N2 in your system, CH4 will also reduce it. So, consider which redox reactions you want to occur. If you use Ntg, it will not participate in redox reactions.

However, in regard to your questions,

1. Is it reasonable for the gas volume or pressure to impact pH in this case, even when CO2 is initially set to zero?

Of course.

2. Is the pH difference caused by more CO2 being released from the rock phase, due to the larger gas volume?  When I increase the gas volume in the second model, final pH increases!

Not really, although the dissolution of calcite and other minerals does affect the result. You would get an effect even if you had no EQUILIBRIUM_PHASES reactions.

Assuming you do not make pH 2 in your initial solution, and the redox reactions can be ignored, and there are no mineral reactions, the change in pH will be due to the amount of CO2 (and H2S) that are lost from solution to the gas phase. The pH is determined by the ratio of HCO3- to CO2(aq) (and HS- to H2S(aq)). As these ratios increase (denominator decreases), pH increases. More CO2 will be lost to a large gas volume; the amount will be determined by Henry's law K = act(CO2(aq))/P(CO2(g)).

[ahmadreza_shojaee]

Thank you very much for your comprehensive response, David.

I think something is wrong with my initial solution and final pH should around 6. I will double check it.

I am wondering how CH4 contributes in reduction of S(6) to S(-2). I could not find any reaction that CH4 contributes to this reaction. I know that adding H2 will cause reduction of S(6) to S(-2) and because of that I have decoupled database to see the kinetic of that reduction by sulfate reducing bacteria. [I think you may have meant H2 instead of CH4.]

I think the explanation for this pH behaviour could be that I am entering brine-rock data and ignoring gaseous CO2 which drives CO2(aq) in the equations. When a gas phase is introduced, CO2(aq) and CO2(g) will appear and the amount of gaseous CO2 depends on the partial pressure of CO2 which is a function of volume. This is why different gas volumes leads to different pH.

Again thanks for your feedback.

Kind regards,
Ahmad

[dlparkhurst]

Apparently, you have a large negative imbalance in your analysis. By charge balancing with pH, the pH is lowered to 2.3 to generate cations.

I don't know what database you are using, but in databases that contain C(4), C(-4), S(6), and S(-2),  sulfate will be reduced by methane. Look at your output file and see if S(-2) is generated.

In simplest terms, pH is determined by the ratio of HCO3- to CO2(aq). Reactions that increase that ratio increase the pH; reactions that decrease that ratio decrease the pH. If CO2 exsolves from the solution--like it would if you introduce a gas phase with no CO2 component--pH will increase.

[ahmadreza_shojaee]

Dear David,

Thanks again for your informative response.

I am using PHREEQC.DAT database. Could you please tell me what is the exact reaction for reduction of S(6) to S(-2). I cannot find in the database while when I run the model with CH4 and Anhydrite, HS- will appear in the solution. Shall be something like:

Code: [Select]

SO4-2 + CH4 = HS- + HCO3- + H2O

To avoid that, I think I should decouple C(-4) or use Mtg as non reacting methane. Am I right?

Kind regards,
Ahmad

[dlparkhurst]

All redox reactions are connected through e-. So the same activity of e- applies to the following reactions:

Code: [Select]

CO3-2 + 10 H+ + 8 e- = CH4 + 3 H2O
SO4-2 + 9 H+ + 8 e- = HS- + 4 H2O

I'm not sure you are right to decouple the reactions, but if you do, you can use Mtg and Mtg(g) for CH4 and CH4(g), which are defined in phreeqc.dat. Mtg will not react with any redox species (because it has no e- in any reactions).

[ahmadreza_shojaee]

So interesting. Thanks for clarification. That reaction I sent is somehow the combination of these two redox reaction. I did not know that!

Thank you!

Kind regards,
Ahmad