how to calculate the max CO2 to be dissolved at high temperature and pressure

[maria]

Dear all,
I know that my question could be similar to others but I am a bit confused in terms of how to use the high pressure. I have a dataset of pH, Ca, Mg, Na, K, Cl, SO4, SiO2, HCO3-, CO32-, NH4+ from geothermal waters at very high temperature T=245oC and high pressure P=210 atm. I want to calculate the maximum CO2 that can be dissolved in these water at these high T (245oC) and P (210 atm).

I have made a solution with these parameters; I used C(4) to include both CO32- and HCO3- concentrations and the phreeqc.dat database. I simply run it and I got the C species distribution with CO2 at 0.029 M.

However, on the results I get a P value of 23 atm at the 'Description of solution", which is not right. So, the CO2 concentration of 0.029 M does not correspond to my pressure (210 atm) that I have.

I tried to use the gas phase for CO2(g) but it asks for a partial pressure of CO2. I tried to use the water pressure that I have (210 atm) as the partial pressure of CO2 but I am not sure if this is right. Under the gas phase section, I got the following

Moles in gas
Initial      Final      Delta
5.32e+00   3.65e+00   -1.67e+00

And under the gas phase species distribution I got a CO2 of 1.25M and a saturation index of 2.26; which means a partial pressure CO2 of 182 atm. That cannot be right again, as I know from literature that the partial pressure of CO2 is around 3-4 atm.

I tried to use equilibrium phase. But again, it asks for a saturation index of CO2 and the amount of moles. I am not sure what to else to do.

Thank you very much in advance. Your help could be tremendous. I really appreciate it.

Kind regards,
Maria



 

[dlparkhurst]

First, a couple of issues with your SOLUTION definition.

When you define C(4), you are including both HCO3- and CO3-2, but you are also including CO2(aq), which looks like the most dominant species. Thus, when you distribute the species, you get far less HCO3- (and CO3-2) than you originally measured. I think you should enter the carbon as Alkalinity (CO3-2 is negligible, so don't bother trying to correct), but with the proper conversion. I tried "Alkalinity 2343 as HCO3", which gives a very good charge balance. Distribution of carbon species is CO2(aq) 0.08, HCO3- 0.04, CO3-2 1e-5 mol/kgw.

This is a fairly concentrated solution, so you should probably consider density, which is needed to convert from mg/L to mol/kgw. You can have the program calculate a density with "density 1 calc" in the SOLUTION definition.

My guess is that the pH is the pH measured at 1 atm and 25 C, rather than at the formation temperature and pressure. So the trick would be to use the analysis from these conditions and increase temperature and pressure to formation conditions. You can decide whether this is correct or not.

To calculate pressure, you need to use GAS_PHASE to be able to calculate the fugacity coefficients, and I would use pitzer.dat for the high ionic strength. The strategy is to increase the temperature and equilibrate the solution with a very small gas volume that does not substantially change the solution composition. However, you state the total pressure is 210, but CO2 is only 3-4 atm, so there must be another component or components in the gas phase. I have assumed there are 200 atm of methane to produce the total pressure of the gas phase. The methane partial pressure is fixed by EQUILIBRIUM_PHASES. The gas phase of CO2 and methane then adjusts to be in equilibrium with the solution (only slightly changing the solution composition). The calculated fugacity coefficient for CO2(g) is then 0.888, the fugacity is 10^0.27, and the pressure is 2.1 atm.

I don't know how your data are collected, but degassing during sampling would tend to produce an estimated PCO2(g) too low.

Code: [Select]

SOLUTION 1
#temp 220
#pressure 210
pH 6.8
units mg/L
density 1 calc
Ca 1.19
Cl 101
#F 21
Fe 0.0025
K 165
Mg 0.5
#N(-3) 8.2
Na 1147
S(6) 572
Si 469 as SiO2
Alkalinity 2343 as HCO3
END
REACTION_TEMPERATURE
220
USE solution 1
EQUILIBRIUM_PHASES
Mtg(g) 2.3 10
GAS_PHASE 1
-fixed_volume
-pressure 210
-volume 1e-10
CO2(g) 0
Mtg(g) 0
END


[maria]

Dear David,

Thank you so much for your reply and taking the time to explicitly explain this. It is very clear.

Actually you are right; pH was taken at 22 C and 10 atm. The high temperature (245 C) and (gauge) pressure (210) atm are bottom hole T and P. I tried to use the pitzer.dat database but I was getting a warning 'Could not find element in database, F. Concentration is set to zero.' and 'Could not find element in database, N(-3). Concentration is set to zero.' Shall I ignore this?

Once again thank you so much.

Kind regards,
Maria

[dlparkhurst]

pitzer.dat does not have data for fluorine or nitrogen. These elements will be ignored when using this database, or you can explicitly remove or comment (#) in the SOLUTION definition. It should not make too much difference in your calculation.

Pitzer.dat is the best database for high ionic strength, but it lacks any redox reaction capability and has a limited set of elements.

[maria]

Dear David,

Many thanks. Everything is clear now.

I have one more question, if that is ok. How can add some salt contents (e.g. a mix of 150mg/lt KCl, 6 mg/lt CaCO3 and 400 mg/lt Na2SO4) in my initial solution and based on that to calculate the amount of CO2 that needs to be added to that so to get a partial pressure of 3-4 atm?

I tried equilibrium and/ or solid phases but I could only find calcite. I was not able to add KCl and Na2SO4.

Thank you very much in advance.

Best,
Maria

[dlparkhurst]

Use REACTION.

[maria]

Code: [Select]

Dear David,

Thank you very much for your suggestion.

I tried to use your suggested code to Phreeqc (for Windows) but which ever database I would use, I was getting an error. Actually I was getting

ERROR: Gas not found in PHASES data base, Mtg(g).
ERROR: Phase not found in data base, Mtg(g).
ERROR: Calculations terminating due to input errors.

I am a bit confused why Mtg is not recognised. I tried to use Phreeqc Interactive, although I could add Mtg in the equilibrium and gas phase; however, in the gas phase I had to add an initial temperature (I used 220oC) and in the initial solution I could not define density 1 calc in the initial solution and also I had to add a pe. I used pitzer.dat.

When I tried to run this, I got a Warning: Numerical method failed.

[SOLUTION 1
    temp      220
    pH        6.8
    pe        1.8
    redox     pe
    units     mg/l
    density   1
    Ca        1.19
    Cl        101
    F         21
    Fe        0.0025
    K         165
    Mg        0.5
    N(-3)     8.2
    Na        1147
    S(6)      572
    Si        469 as SiO2
    Alkalinity 2343 as HCO3
    -water    1 # kg


REACTION_TEMPERATURE 1
    220
GAS_PHASE 1
    -fixed_volume
    -pressure 210
    -volume 1e-010
    -temperature 220
    CO2(g)    0
    Mtg(g)    0
EQUILIBRIUM_PHASES 1
    Mtg(g)    2.3 10
]
Your help would be tremendous.

Kind regards,
Maria

[dlparkhurst]

The database you are using should be printed as the third line of the output. Mtg(g) is defined in phreeqc.dat, Amm.dat, and pitzer.dat. Either you are not using one of these databases, or you have an old version of PHREEQC. The current version of PHREEQC is 3.6.2 at https://www.usgs.gov/software/phreeqc-version-3.

[maria]

Dear David,

I have been using pitzer.dat as a database. I had an old version; I uninstalled the old one and I tried to install the newer version through this

Windows 64-bit: phreeqc-3.6.2-15100-x64.msi [15.6M] - Executable, database files, examples, PDF documentation

But it only has the user guide and description of input and examples. I couldn't find the software.

I tried to go to Phreeqc for Windows link but I am getting a warning that the connection is not safe and it is blocking me.

Thank you very much.

Kind regards,
Maria

[dlparkhurst]

I think you installed the batch version of PHREEQC that can be run from a command line.

You probably want the interactive version, PhreeqcI, which you can download and install from this link:

https://water.usgs.gov/water-resources/software/PHREEQC/phreeqci-3.6.2-15100.msi

[maria]

Dear David,

Thank you very much. I was able to install the Interactive Phreeqc.

I just want to ask if there is any chance to be able to insert the code above as a command line as it is rather than just clicking.

The only problem is that I need to specify a pe in the initial solution and then I need also to specify an initial temperature in the gas_phase through phreeqc interactive since reaction_temperature is using

Also, there is no possibility to calculate the density as 'density 1 calc' since the solution is quite condensed.

When I used Mtg(g) 2.3 10 in equilibrium_phases, I was getting a warning; when i used Mtg(g) 0 0 it somehow run. But I am not sure is using 0 makes sense.

Thank you.

Kind regards,
Maria

[dlparkhurst]

> I just want to ask if there is any chance to be able to insert the code above as a command line as it is rather than just clicking.

You can cut and paste in the main edit window, the big window (usually) on the right side of the app that shows the input.

 >The only problem is that I need to specify a pe in the initial solution and then I need also to specify an initial temperature in the gas_phase through phreeqc interactive since reaction_temperature is using

Not sure what is the problem you are having. You can define the pe through the SOLUTION screen. You can open a new SOLUTION by double clicking on the SOLUTION icon (small blue flask). You can edit an existing SOLUTION definition through the input tree on the left side of the screen; open the + items in the tree until you see SOLUTION and double click. You can also simply type in the edit screen, but if you make a mistake, you will not be able to access the input tree until you remove the error.

You can edit a GAS_PHASE definition in the same ways. Note if there are zero moles of all gases, the temperature definition in GAS_PHASE has no effect.

>Also, there is no possibility to calculate the density as 'density 1 calc' since the solution is quite condensed.

The option to calculate the density has not been added to the SOLUTION screens. You will have to type "calc" in the edit screen. For example:

Code: [Select]

SOLUTION
    density   1 calc


>When I used Mtg(g) 2.3 10 in equilibrium_phases, I was getting a warning; when i used Mtg(g) 0 0 it somehow run. But I am not sure is using 0 makes sense

You will have to attach an input file with that produces the warning. Using Mtg(g) 0 0 means that you want a log10 partial pressure of 0 (the first 0), and there are no moles of Mtg(g) initially available to react (second 0). In this case, Mtg(g) would partition into the equilibrium phase only if Mtg were defined in the SOLUTION definition and the resulting partial pressure were greater than 1.0 atm.

[maria]

Dear David,

Thank you so much for your help. It worked perfectly; I used pitzer.dat as a database and I didn't have any warnings or errors. The simulations run very smoothly.

Once again, many thanks.

Kind regards,
Maria