This is quite a specific question, however, I was wondering if anyone has a possible solution to this, I will try to provide as much detail as possible, if further parameter values are necessary please let me know.

I’m modelling salt caverns using a porous flow model (1% porosity, 1x10^-19 permeability) in two parts, first using my own model to calculate the cavern pressure and heat transfer to the rock, and secondly using OpenGeoSys THERMO-HYDRO-MECHANICAL module to determine the strain in the salt rock. As my simulation found little gas transfer, the pores are occupied solely by water.

I use my model first to calculate the pressure and heat flux, this is then applied as boundary conditions to the cavern roof, floor and wall using the linear curve function of OpenGeoSys. The boundary conditions I have applied to other boundaries seem to be standard practice for axisymmetric models, the bottom boundary is constrained in the y-axis, the centre line constrained in the x-axis, and the top and the lateral boundary conform to the lithostatic stresses calculated in the OpenGeoSys documentation. Pressure (fluid) and temperature boundaries (for all other than the cavern itself) are applied such that zero fluid and heat transport occur through them.

For the initial conditions, the temperature and pressure are applied from my previous simulation (I understand this could also be achieved from variable parameters within the .prj file). The initial rock stress follows the lithostatic conditions discussed in the OpenGeoSys documentation, while the initial displacement is set to zero.

I have attached the boundary conditions and several different results from my simulation which are confusing to me. The concerns I have with this are,

- why am I getting a pore pressure as a negative value? could this be due to the deformation of the rock?
- The displacement In my x-axis is all in the positive x direction, should the boundary conditions applied not negate this?
- Why is the displacement in the y-axis positive at the top? surely this should all be in compression (-ve)?
- Why is stress in the xx plane so high along the centreline? and in some instances, it reduces to zero? is this due to the shape of the cavern or the fact that the displacement is initiated to zero?
- Why does this also occur in the zz plane? I have tried applying the lithostatic stress in both instances but a similar pattern is shown

I understand this is quite specific, but if anyone has any experience with this the help would be greatly appreciated.