I don't get it. Is it not supposed to take it from the grid section when left blank
I have a porosity of 0.2 for the whole grid.
7 Porosity
• DEFAULT: Grid cell value
From eclipse manual.

So how should the aqunum keyword look like?

Something like this
1 50 18 1 1.00E+05 40000 0.2 1* 500 2* 1 1/