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Fracture flow in a single fracture

Posted Apr 4, 2012, 11:04 a.m. EDT Porous Media Flow Version 4.2a 3 Replies

Alain Nguyen
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Dear Comsol community,

I've been trying to model fracture flow in a porous media and I've encountered a big problem.

I'm trying to simulate flow in a fracture. I've modeled a well pumping at the rate of 1 m^3/s. I suspect the results to be incorrect by looking at the plots.
At first, the hydraulic head plot seems OK, but looking at the velocity plot, we can clearly see that the velocity within the fracture is wrong with my Velocity Cut Line Plot. I also compare the In Flow vs the Out flow. The outflow is 1 m^3/s (which is the correct amount), but the In flow is really low (1e-10 m^3/s)

Is there something I'm doing wrong ? Has anyone encountered the same problems ?

Thanks for the help,

Alain Nguyen

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3 Replies Last Post Mar 21, 2013, 2:48 a.m. EDT
COMSOL Moderator

Hello Alain Nguyen

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Silvia Bersan
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Posted: 1 decade ago Mar 19, 2013, 5:20 a.m. EDT
I also encountered some difficulties using fracture flow as boundary condition in a 2D model. In my model, domain was rectangular, flow was horizontal and the fracture was a fraction of the horizontal upper boundary. To check if the results were correct I drew parallel vertical cut lines and I integrated horizontal velocities along these lines. For continuity to be satisfied I should have obtained the same value of the integral (that is the discharge) along all the cut lines. But this did not happened. Therefore I calculated

Q(x)=integral (dl.u(x) along cut line(x))+dl.u(x,0)*df(x)

where df is the thickness of the fault and dl.u(x,0) is the horizontal velocity at the interface with the fault. I obtained Q=const(x), that is correct.

It's likely that my explanation is a bit confusing, but the conclusion is that if you analyze results along cross sections the fluid flowing into the fracture is not taken into account and you have somkething missing in your mass balance. at least this is what happens if you use fracture flow in external boundaries.
I also encountered some difficulties using fracture flow as boundary condition in a 2D model. In my model, domain was rectangular, flow was horizontal and the fracture was a fraction of the horizontal upper boundary. To check if the results were correct I drew parallel vertical cut lines and I integrated horizontal velocities along these lines. For continuity to be satisfied I should have obtained the same value of the integral (that is the discharge) along all the cut lines. But this did not happened. Therefore I calculated Q(x)=integral (dl.u(x) along cut line(x))+dl.u(x,0)*df(x) where df is the thickness of the fault and dl.u(x,0) is the horizontal velocity at the interface with the fault. I obtained Q=const(x), that is correct. It's likely that my explanation is a bit confusing, but the conclusion is that if you analyze results along cross sections the fluid flowing into the fracture is not taken into account and you have somkething missing in your mass balance. at least this is what happens if you use fracture flow in external boundaries.
Silvia Bersan
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Posted: 1 decade ago Mar 19, 2013, 5:28 a.m. EDT
Tha same happens if the fracture is an interior boundary. Just checked out!
Tha same happens if the fracture is an interior boundary. Just checked out!
Juergen Hansmann
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Posted: 1 decade ago Mar 21, 2013, 2:48 a.m. EDT
Hi Alain, hi Silvia,

I've been working with fracture flow boundaries and remember, that generally cut lines do not (or at least did not) seem to be able to capture the existence of fracture flow boundaries.
In your (Alains) model you can check this, if you try to plot material properties along the cut line. For example, try to plot dl.epsilon (the porosity) along your cut line. It will have the value of 0.001, as you defined for the domain. The value of 1, that you assigned to your model's fracture material, will not appear in your cut line plot.
Hence, cut line plots might not be the best option to check your model's behaviour if it includes fracture flow elements.

Best regards,
Juergen

Hi Alain, hi Silvia, I've been working with fracture flow boundaries and remember, that generally cut lines do not (or at least did not) seem to be able to capture the existence of fracture flow boundaries. In your (Alains) model you can check this, if you try to plot material properties along the cut line. For example, try to plot dl.epsilon (the porosity) along your cut line. It will have the value of 0.001, as you defined for the domain. The value of 1, that you assigned to your model's fracture material, will not appear in your cut line plot. Hence, cut line plots might not be the best option to check your model's behaviour if it includes fracture flow elements. Best regards, Juergen

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