Threshold for methane gas emission

[Zixuan Chen]

Dear all,

I'm working on a reactive transport modeling in a wetland. There is a problem that our partial pressure of gas is always larger than 1 atm, and we have way more methane in the liquid phase. 

To solve this problem, our current model is using a constant outgassing velocity by assigning a constant gas flow velocity. However, it would outgas in a lower partial pressure. 

Thus, I'm wondering if there is a way to set up a limitation. The model would outgas when only its partial pressure reached this limitation. 

Thank you so much for any help!

Best,

Zixuan

[Hammond, Glenn E]

Zixuan,

 

Please send a drawing of what you are attempting to accomplish.

 

Please note that the gas phase from the flow process model is independent of the gas species in the reactive transport code. The flow model gas phase state variables solely determine the gas phase saturation and Darcy velocity for reactive transport. But these state variable may have nothing to do with what you are asking…; it may be an issue with boundary conditions.

 

Glenn

 

From: pflotra...@googlegroups.com <pflotra...@googlegroups.com> On Behalf Of Zixuan Chen
Sent: Thursday, September 7, 2023 11:20 AM
To: pflotran-users <pflotra...@googlegroups.com>
Subject: [pflotran-users: 7267] Threshold for methane gas emission

 

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[Zixuan Chen]

Hi Glenn,

Thank you so much!

Here is a diagram for illustration.

Have a good week。

Best,

Zixuan

[Hammond, Glenn E]

Zixuan,

 

With the current approach to coupling reactive transport to the gas phase in multiphase flow, there is no way to simulate off gassing of gas generated through chemical reaction using a pressure constraint in the flow portion of the code; it has to be from the reactive transport side.

 

If I were to attempt this using the reactive transport process model, I would create a reaction sandbox with a sink term for the dissolved aqueous species that is off gassing. Yes, the gas is off gassing, but the primary dependent variable is an aqueous species that is equilibrated with a gas partial pressure. The sink term would involve a kinetic mass transfer process that drives the gas partial pressure (and associated aqueous concentration) to equilibrium with 1 atm. A large rate constant will approximate equilibrium conditions.

 

Glenn

 

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[Peter Alt-Epping]

Hi all,

I am running a simulation in MPHASE mode using an explicit unstructured grid on our cluster and on my PC. On the cluster I get the following error on startup:

I don't get this error on my PC.

I use an Intel compiler on the cluster and gfortran (Ubuntu 20.04) on the PC. The PFLOTRAN version on the cluster is a few months older than the one on the PC.

Can you let me know what the error means and how it can be fixed?

Thanks.

Regards,

Peter

-- 
Peter Alt-Epping, PD PhD
RWI Group, Institute of Geological Sciences
University of Bern
Baltzerstrasse 3
3012 Bern
Switzerland

Tel.: 0041 31 684 8765

[Hammond, Glenn E]

Peter,

 

Please pull the latest source code and try the same input file. The following commit from August fixed that error message:

 

From: pflotra...@googlegroups.com <pflotra...@googlegroups.com> On Behalf Of Peter Alt-Epping
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To: pflotra...@googlegroups.com
Subject: Re: [pflotran-users: 7270] Face ID error on cluster

 

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Hi all,

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[Hammond, Glenn E]

Peter,

 

Please pull the latest source code and try the same input file. The following commit from August fixed that error message:

 

https://bitbucket.org/pflotran/pflotran/commits/4e96db47c3a2b6e9e563ac54ceff6252a6c9218b

 

However, the error message is only invoked when there are faces defined, and for explicit unstructured grids, regions do not have face IDs. If I recall correctly, the array of face ids was not being properly nullified during initialization.  The commit should fix the issue.

Glenn

 

From: pflotra...@googlegroups.com <pflotra...@googlegroups.com> On Behalf Of Peter Alt-Epping

Sent: Monday, September 11, 2023 10:50 AM
To: pflotra...@googlegroups.com
Subject: Re: [pflotran-users: 7270] Face ID error on cluster

 

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Hi all,

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[Peter Alt-Epping]

This fixed it. Thanks Glenn!

Peter

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[Zixuan Chen]

Thank you so much!

I will try the sandbox.

Have a great week.

Best,

Zixuan

[Zixuan Chen]

Hi Glenn,

Thank you so much for your response! I misunderstood the method that we were going to use after talking with my advisor. Let me start over with my question.

The general idea is that I am trying to simulate observed aqueous CH4 concentrations in a 1D vertical wetland sediment profile (from the ground surface to about 50 cm depth). We know that some CH4 must be outgassing, but our main focus is on aqueous conditions, so I'm not concerned about accurately representing gas transport. I just need a way to allow for some CH4 to outgas in the model so that I can match observed aqueous CH4 concentrations. I do not have any measurements of the CH4 outgassing flux to constrain any simulations of gas transport.

We previously set up a similar problem with CH4 using PHREEQC, and I'd like to know if there is a similar option in PFLOTRAN. Specifically, in PHREEQC, it is possible to have the model simulate gas phase ONLY after the sum of calculated partial pressures (using Henry's Law and the simulated aqueous concentrations) exceeds a user-specified threshold, like 1 atm. This would be the equivalent of setting a lower pressure threshold for when a bubble can form in the solution. Only when that threshold is met will some of the aqueous phase be converted into moles of gas phase. We then emulated fast gas transport in PHREEQC to remove the gas phase from solution (so it can't dissolve back in) by adding a fast kinetic reaction to convert any gas phase to an immobile and inert component.  

When I played around with PFLOTRAN gas options, I found that I can invoke ACTIVE_GAS_SPECIES and specify a gas velocity. This allows outgassing of CH4, but the problem is that the gas phase will form (and be transported away) whenever there is any partial pressure > 0. I think it would work well if I could set a threshold for when gas phase is allowed to form (e.g., sum of calculated partial pressures > 1 atm), so that gas phase is not transported away before that threshold. Is there a way to do that? This is depicted in the diagram here. Do you think the reaction sandbox can still do this?

Or if you have other ideas on how I can easily represent CH4 outgassing in PFLOTRAN, I would also be interested to know. I see that with GENERAL, I can explicitly simulate gas transport, but I worry that I don't have enough data to constrain the various gas-transport parameters. 

Best,

Zixuan

[Peter Alt-Epping]

Hi all,

I am running a simulation in MPHASE mode on an explicit unstructured
grid. The injection zone comprises 8 cells and I have a given CO2
injection rate in kg/s. I noticed that there is no volume scaling of the
injection rate in MPHASE. Also, there is a MASS_RATE and
TOTAL_MASS_RATE. What would be the correct rate input in a flow
condition assigned to a SOURCE_SINK. Is it

TYPE
 RATE MASS_RATE

or

TYPE
RATE TOTAL_MASS_RATE

Thanks,

[Hammond, Glenn E]

Peter,

Sorry for the last response. I was teaching a reactive transport short course all week.

You are correct that mphase currently does not handle distribution of mass when multiple cells are included in the source term. It may not be that difficult to update. The source code for this algorithm is located at

https://bitbucket.org/pflotran/pflotran/src/f0fe931c72c03580e489724afeb8c5451406b942/src/pflotran/patch.F90#lines-4908

Based in the scco2.in input file, I believe that you need "RATE MASS_RATE"

https://bitbucket.org/pflotran/pflotran/src/f0fe931c72c03580e489724afeb8c5451406b942/regression_tests/default/scco2/mphase/scco2.in#lines-202

I am out all of next week, but someone else should be able to answer questions. There are others with more MPHASE experience than me on this list....

Glenn

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