[CIG-SHORT] pseudo plane-strain in Pylith

[Charles Williams]

Hi Lucas,

I've done a similar model, but going the other way (feeding fluid pressures from a flow code into the fault-normal stress for PyLith.  To do this, I used a 2D mesh for PyLith and a 3D mesh (1 element thick) for the flow code.  If you are careful when designing the meshes, you can have elements that are identical, except that the quadrilaterals are extruded to form hexahedra.  The flow code provided identical fluid pressures on the front and back faces, so I was able to select one to use.  Hopefully, you can do something similar for your problem.

Cheers,
Charles


On 6/04/2013, at 9:07 AM, Lucas Abraham Willemsen wrote:

> Hello,
> 
> Currently I am trying to investigate the geomechanical response of a reservoir with a single fault when CO2 is injected. The fluid-flow part of the problem is performed by a code which is coupled to Pylith. The results from Pylith are fed into the flow code, which marches time forward and the  new fluid pressures are then fed back into Pylith, etc.  
> 
> The CO2 sequestration geometry I am considering is plane-strain, but the complication is that the flow-code (currently experimental) only works with hexahedrals. So I turned the 2D problem into a 3D-problem that is 1-element thick in the out-of-plane direction (y-direction). To enforce plane strain conditions I put zero displacement in the y-direction on the front and back plane. But the problem is that the fault intersects these front and back planes. This is a natural consequence of turning the 2D geometry with fault into a 1-element thick 3D geometry with fault. 
> 
> All of the input parameters are independent of 'y' since I am trying to model a plane strain problem. So there should be no reason for slip in the y-direction on any of the fault nodes (intuitively speaking). But Pylith does not allow me to explicitly constrain the y-displacements of the fault where they intersect the front and back plane. Therefore, the only way to run the model is by excluding the nodes on the fault from the front- and back- plane nodesets. But this way the boundary condition on these fault nodes is effectively zero normal stress, and elastic extrusion of material takes places. (Applying Neumann B.C. with normal stress to these fault nodes on the front and back plane to somehow reduce the extrusion does not seem to affect the fault nodes either. It would not be a great solution anyway).
> 
> Is there a way to deal with faults in this 'pseudo plane-strain' geometry I described?
> 
> sincerely,
> Lucas
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Charles A. Williams
Scientist
GNS Science
1 Fairway Drive, Avalon
PO Box 30368
Lower Hutt  5040
New Zealand
ph (office): 0064-4570-4566
fax (office): 0064-4570-4600
C.Williams at gns.cri.nz

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