What if the model does't have boundary setting?

Hallow everyone!

I'm a beginner of TOUGH. All the time, i always use the dirichlet boundary by giving a 1E50 volume. Today, an idea come to me : What if the model does't have any boundary settings? If i make gas injection into aquifer, will the water run out? What is the boundary setting closest to the truth?

Forgive my foolishness.