The deeper we go the more the penetration rate will slow.
http://www.calculatoredge.com/calc/d-exponent.htm
http://books.google.com.au/books?id=cj166TQwn3YC&pg=PA151&lpg=PA151&dq=drilling+depth+penetration+rate&source=bl&ots=1s6fVfM3LW&sig=5Nuy2ZudmzACjXGmheOnEeDtx64&hl=en&sa=X&ei=Vk56T6f5LM-ziQe50LnfAg&ved=0CDwQ6AEwAw#v=onepage&q=drilling%20depth%20penetration%20rate&f=false
Vertical Well calc.