2D Well Model Not Converging

[Taylor W]

Hello all,

I am trying to make a simplified model of a vertical drilling wellbore in granite. I can't get the geostatic step to converge due to excessive distortion. Am I doing something wrong?

Many of the values I am given are in imperial units, but I'm running the analysis in SI units for simplicity. Here's my model:
[ul]
[li]I am using a 2D, axisymmetric rectangle with half of the well in the upper left corner since it's symmetric. I'm only modeling the bottom section of the well, and the drilling depth at the top of the model is 8000 ft.[/li]
[li]I have two steps: first a geostatic one, then a soils step where I remove all the material in the well at once and apply a pressure to the well bottom and walls to simulate the mud being pumped back up to the surface.[/li]
[li]I am using CAX4P elements.[/li]
[li]For ICs, I have the initial geostatic stresses, initial pore pressure, and initial void ratio.[/li]
[li]For loads, I have gravity (as a gravity load) and the pressure from the mud, applied during the last step.[/li]
[li]For BCs, I have the bottom left corner fixed, x-symmetry on the left side, rollers on the bottom edge, and a set pore pressure for the whole model.[/li]
[/ul]

I've attached my CAE file. Does anyone have any ideas?

I'm also wondering how I can model the mud better, because there is also some of the mud seeping into the granite. How would I model that?

 

[FEA way]

It doesn't seem to fail due to excessive distortion. It just diverges around 77% of the first step. Try simplifying the analysis (removing features one by one) to see what causes this problem. I would suspect Mohr Coulomb plasticity though.

 

[Taylor W]

Removing the Mohr-Coulomb plasticity definition does make it converge, but I need that as part of the analysis and I'm fairly sure the Mohr-Coulomb values are correct. If I do use it, there's too much plastic strain, which causes it to diverge. Is there anything else it could be?

 

[Taylor W]

I'm wondering if the problem is using total vs. excess pore pressure. The pore pressures I've used are in terms of total pore pressure, which the documentation says I should do since I have a gravity load, but is there anything else related to pore pressure I did wrong?