MINC and low permeability case problem in Tough2

Hi All,

I set up a case using the MINC with 2 continua.  The first continua represents the fracture and the second one represents the matrix.  For this case, the permeability is the same in fracture and matrix. The case runs when the permeability is larger (e.g 14md), however, if I use a smaller permeability, the case doesn't run well. Basically I run it for almost one day and it only went a very small time step, but it does not showing crashing though.

Does anybody know the cause for this and give me some hint?  Thanks  a lot!