Gas Flow in Unsaturated Condition

[juntao pan]

Dear Glenn,

In our problem, the porous medium (soil) is initially unsaturated (air + water). We then inject CO₂ gas at a specified flow rate from the bottom boundary of the model. As you mentioned earlier, PFLOTRAN cannot model multiple gases. We understand that CO₂ should be introduced as a component in the transport model to flow via diffusion or advection. However, we are unsure how to apply the flow-rate boundary condition for CO₂.

Another problem is that, the gas saturation changes (the upper layer increases while the bottom layer decreases) with an initial condition of only unsaturated strata and no other conditions, while we expect no flow in the strata. Our code is attached.

Thank you so much for your time and support!

Best,

Juntao

[Hammond, Glenn E]

My apologies for the delayed response. See my comments below in red.

Glenn

From: pflotra...@googlegroups.com <pflotra...@googlegroups.com> on behalf of juntao pan <panjun...@gmail.com>
Date: Thursday, September 4, 2025 at 9:57 AM
To: pflotran-users <pflotra...@googlegroups.com>
Subject: [pflotran-users: 8552] Gas Flow in Unsaturated Condition

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Dear Glenn,

In our problem, the porous medium (soil) is initially unsaturated (air + water). We then inject CO₂ gas at a specified flow rate from the bottom boundary of the model. As you mentioned earlier, PFLOTRAN cannot model multiple gases. We understand that CO₂ should be introduced as a component in the transport model to flow via diffusion or advection. However, we are unsure how to apply the flow-rate boundary condition for CO₂.

Use a mass or volumetric flux for the gas phase in the flow condition. To be honest, I am unsure how the transport model will apply its contribution, but my hope is that the gas phase will include the concentrations prescribed in the constraint.

Another problem is that, the gas saturation changes (the upper layer increases while the bottom layer decreases) with an initial condition of only unsaturated strata and no other conditions, while we expect no flow in the strata. Our code is attached.

I believe that the gas saturation is changing due to the liquid saturation changing, which is caused by drainage due to the liquid saturation being greater than the residual saturation for the liquid phase. You might try setting the liquid residual saturation to something large (e.g., 0.8) and see what happens.

Thank you so much for your time and support!

Best,

Juntao

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