The Oil Drum

When one first penetrates the reservoir you do a flow test against pressure (i.e. a certain setting on the choke valve) and measure the flow.  This gives you some sense of the reservoir potential.  And you have the logs of the well that give you more information on the structure, plus the 3-D seismic picture, so you have some idea at this point of how big the pool is, how much oil it has, and how it will flow to the well.  And in each case it is different.  Wells that start out looking very promising can die within weeks, and vice versa.  Internal structures within the reservoir can play a significant but often hidden role, so that only part of the reservoir may deplete.  Often in carbonates you may find a difference between the vertical permeability and the horizontal - so even with the same drive you may get totally different performance from two wells that seem the same and are quite close.

This is some science and still a lot of art,  and a greater chunk of luck, though the science is getting better.