empty oil wells

[gaz_farr]

Once a company has finished extracting oil from a well or underground resevoir what happens to the empty space underneath the ground? Is it naturally filled by rock or water, filled up manually or left hollow?

[Clanad]

It's a general misconception that a 'cavern' is left once an oil field is depleted. Actually, almost all oil pumped in the world comes from sandstone or limestone formations. This rock is reasonably porous and the oil, when discovered, is forced out of the formation to a the well zone by either natural gas pressure accompanying the oil or by pressurized water. The water usually used is highly salty water recovered from the oil producing strata. In the later stages of the life of the oil field, additional water, carbon dioxide or other chemicals are pumped under pressure into the strata to help in secondary or tertiary recovery.
By the way, when the oil well is first drilled and recoverable oil is discovered, the producing zone of rock is fractured, either chemically or hydraulically to assist in the flow of the oil droplets to the pipe stem...

[qapmoc]

Good answer Clanad, also just for interest the fact that the oil is contained in sandstones etc.. limits the rate at which the oil can be extracted from an oil well as the oil has to pass through the structure of the rocks, if the extraction rate is pushed up too high in an attempt to get the oil faster you can in fact damage the field by collapsing the producing strata and then you can get almost no oil from the well.