I read somewhere that drilling rate slows down when entering the...

I read somewhere that drilling rate slows down when entering the reservoir so decided to try and find out why. After numerous spurious links I eventually found the following –
http://bittooth.blogspot.co.uk/2009/08/oil-well-pressures-what-brings-out-oil.html

It’s very well written and easy to understand, even if you have no oilfield experience, and there are links to other very interesting topics relating to oil production.

So all the guestimates of where we should be based on past drilling performance are probably very inaccurate.