Evaluating gas phase composition from solution and pressure data

[larsaste]

Hello, I'm working on a groundwater source heat pump project, where we suspect that degassing in the piping system causes precipitation. In this regard, I'm trying to determine the gas phase composition from my solution composition (determined from field measurements and water samples). I have also measured the ph of the pipe fluid at various pipe pressures. Would this be suitable to backmodel an estimate of the gas phase composition, or do I have to do gas sampling of the pipe fluid?

I have tried the backmodeling approach with the script below, to see if I got the same pco2 pressures by changing the reaction_pressure as I got from direct measurement. As you can see from the attached picture, this approach was unsuccessful.

I understand that I may be "far off and out" here, but intuitively I thought the detected pH responses to pressure changes in the pipes could be used to estimate gas composition.

Anyways hope all of this makes at least a bit sense, and that you could help me if possible!

SOLUTION 1
-units mg/L
-pressure 1.122
Ca 132.30
Cl 280.51
Fe(2) 3.42
Alkalinity 354 as HCO3                                          
K 10.48                              
Mg 24.94
Mn 0.64
Na 179.44                                             
S(6) 60.36
TEMP 5.2
O(0) 0.05
ph 7.24
pe 2.44

GAS_PHASE 1 Fixed-pressure gas phase
-fixed_pressure
-pressure 1.122
-temperature 5.2
CO2(g) 1.99E-02
O2(g) 9.61E-04
N2(g) 1.101139

USE SOLUTION 1
USE GAS_PHASE 1

REACTION_PRESSURE 1
0.93 1.122 in 10 steps

USER_GRAPH 1 Gasslek
-plot_tsv_file pco2pressure.tsv
-headings pressure logpco2_modellert
-chart_title "Pressure effect on CO2"
-axis_scale x_axis 0.9 1.2 0.05 0
-axis_scale y_axis auto
-axis_titles "Pressure (atm)" "log(pCO2)"

initial_solutions false
-start
10 graph_x pressure
20 graph_y SI("CO2(g)")
-end

END

[dlparkhurst]

In your GAS_PHASE definition, you assume that the gas volume is 1 L. This is part of the calculation of the number of moles of gas in the gas phase. If you increase the gas volume (in the extreme, making it infinitely large) then the transfer between gas and solution is negligible and the gas phase remains constant. A volume of 10 produces a larger variation of the PCO2 in solution.

However, the change in PCO2 will be directly proportional to the change in overall pressure. So, the maximum variation in log10(PCO2) will be log10(1.12/.93) or 0.08 units.

[larsaste]

Thank you for your quick reply and help!

But will this proportional relation between total pressure and partial pressure also hold for a closed, limited gas reservoir?

[dlparkhurst]

The pressure relation will hold when the gas phase is large relative to the solution solubility. In the other direction, when the gas volume is smaller, increasing pressure may be enough to dissolve the gas phase entirely. The large-gas-phase case will produce the greatest range in the calculated solution partial pressure, while smaller gas phase volumes will mute the range of calculated aqueous partial pressures (as you saw with your 1-L gas volume calculation.)

[larsaste]

Very clarifying, thanks again!

[larsaste]

I have now taken a new approach to estimate the gas phase volume in my piping system.

This includes measuring the pressure drop from the top part of the piping system (right after the heat exchanger) to the injection well screen. From this measured pressure drop and knowledge of the gas phase (mostly nitrogen), I have used the process simulator Aspen HYSYS to calculate how much gas should be present in the pipes to produce the measured pressure drop. I found this to be a very small amount (0,0006 ratio of gas to liquid). When I input this into my PHREEQC model (see below) I get a very small pH change.

Could you please help me with some input on whether this approach seems applicable, and whether the results seems reasonable from your experience?

Kind regards
Lars

"SOLUTION 1
-units mg/L
-pressure 1.1
Ca 132.30
Cl 280.51
Fe 3.42 pe
Alkalinity 329.5 as HCO3                  
K 10.48                        
Mg 24.94
Mn 0.64 pe
Na 179.44                     
S 180 pe
TEMP 4.6
O(0) 0.05
ph 7.35
pe 2.44

GAS_PHASE 1 Fixed-pressure gas phase
-fixed_pressure
-pressure 1.1
-volume 0.0006
-temperature 5
CO2(g) 1.16E-2
O2(g) 0.0006
N2(g) 1.08

USE SOLUTION 1
USE GAS_PHASE 1

REACTION_PRESSURE 1
0.1 1.0 in 10 steps

USER_GRAPH 1 Gasslek
-plot_tsv_file pco2pressure.tsv
-headings pressure logpco2_modellert
-chart_title "Pressure effect on CO2"
-axis_scale x_axis 0.0 1.0 0.1 0
-axis_scale y_axis auto
-axis_titles "Pressure (atm)" "log(pCO2)"

initial_solutions false
-start
10 graph_x pressure
20 graph_y SI("CO2(g)")
-end

USER_GRAPH 2 GasslekPH
-chart_title " CO2 degassing effect on pH"
-axis_scale x_axis 0.0 1.0 0.1 0
-axis_scale y_axis auto
-axis_titles "Pressure (atm)" "pH"

initial_solutions false
-start
10 graph_x pressure
20 graph_y -LA("H+")
-end

END"

[dlparkhurst]

It seems correct that there will be little change in pH.

The only gas component that will affect pH is CO2, so another way to run the problem is simply to remove CO2 by REACTION to see the effect on pH. (SOLUTION 1...; REACTION; CO2 1; ...). You can estimate the amount of CO2 in the gas phase from volume, temperature, and pressure. I'm not sure how you deal with the flow-through system to establish how much gas evolves/ingases per liter of water.

I'm not sure about your input though. You have defined a gas phase at 25 C, and your input solution has no N2 and little O2. With a fixed pressure GAS_PHASE, the gas completely dissolves into the solution for all pressures in your simulation, especially considering the reaction temperature is 4.6 C where gases are more soluble. Doesn't seem like what you want. Also, make sure you are using absolute pressure, rather than gauge pressure. A fixed pressure gas phase will exist only when the sum of the partial pressures of the gas components in the gas phase is greater than the specified pressure.

Ntg(g) will be a better choice for nitrogen gas (Ntg(g)) and dissolved nitrogen (Ntg). It is defined in phreeqc.dat and is disconnected from the nitrogen (N) system, so it is unreactive. For the same reason Oxg and Oxg(g) would be better for O2 to avoid unwanted redox reactions. Ntg in equilibrium with the atmosphere would be defined

Code: [Select]

SOLUTION
Ntg 1 Ntg(g) -0.155 10


[larsaste]

Thanks for a very rapid reply and useful comments, David, really appreciate it!

The problem is I'm not sure about the volume of gas, so I have tried to use a two-phase flow model for pressure loss to estimate approximately how much gas would be present. But again this won't consider the change in gas volume with distance along the pipeline.

[dlparkhurst]

Sorry, looking back, the GAS_PHASE is defined for 5 C, not 25, and log10(P(N2)) should be -0.1 for the atmosphere.

Pressure effects on equilibrium constants are minimal up to 10s of atm. Given constant solution composition and small gas volume, the gas composition will vary mostly because of temperature. Here the solution is defined in equilibrium with atmospheric O2 and N2 at 4.6 C and then heated to 25 C allowing a gas phase to form. The gas composition changes with temperature slightly; CO2 and O2 increase in the gas phase relative to N2. The pH drops from ~7.35 to ~7.15. If temperature remains constant, you can pick out the gas composition from the graph. Your conditions may vary from the atmospheric equilibrium that I assumed.

Code: [Select]

SOLUTION 1
-units mg/L
-pressure 1.1
Ca 132.30
Cl 280.51
Fe 3.42 pe
Alkalinity 329.5 as HCO3                 
K 10.48                       
Mg 24.94
Mn 0.64 pe
Na 179.44                     
S 180 pe
TEMP 4.6
O(0) 0.05
ph 7.35
pe 2.44
Ntg 1 Ntg(g) -0.1 10
Oxg 1 Oxg(g) -0.7 10
END
GAS_PHASE 1 Fixed-pressure gas phase
-fixed_pressure
-temperature 4.6
CO2(g) 0
Oxg(g) 0
Ntg(g) 0
END
USE solution 1
USE gas_phase 1
REACTION_TEMPERATURE
5 10 15 20 25
USER_GRAPH 1
    -headings               TC pH CO2(g) Ntg(g) Oxg(g)
    -axis_titles            "Temperature" "Pressure, atm" "pH"
    -axis_scale y_axis      auto auto auto auto log
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TC
20 GRAPH_SY -LA("H+")
30 GRAPH_Y PR_P("CO2(g)"), PR_P("Ntg(g)"), PR_P("Oxg(g)")
  -end
    -active                 true
END



[larsaste]

Thanks a lot again, David, and interesting results!

I will continue and experiment with your script and different reaction temperatures and pressures if that is okay:)