Tem's advice is good.

In many cases the two most common problems are:

a) Low porosity and high permeability cells - these give rise to througput problems. Indentifying and removing these cells ( or lowering the perm there) can help. You need to be able to plot permx vs poro to find these cells.

b) Often too high a vlaue of PERMZ can be a problem. Plotting the histogram of TRANZ or PERMZ and using a MAXVALUE keyword to limit the highest 10% can often improve convergence while not affecting the solutution. ( similarly reducing kv/kh oftne improves convergence - but now the answer will most likley change.

There are many other reasons. I've found these 2 are a good place to start.

itag