As we have so many oil mines around the world, how large are these

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As we have so many oil mines around the world, how large are these holes left behind, and what is put in place of the oil that is taken out. wouldn't the pressure be different? and what would happen if a oil cavern collapsed?
 
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in pretty much all cases, that I'm aware of, the oil isn't in caverns. :)
its within the pores of the rock. It accumulates at the top of folds of rock layers called anticlines.

Dave
 


Ok I understand that, but these pores within these rocks have to be large and if oil miners are pumping steam into these pores wouldn't that leave for lack of a better word holes? I guess what I'm getting at is possible collapses with the tectonic plates constantly moving. it's hard to believe not a one crack or shift hasn't happened.
 
To the OP. Apparently this is a VERY uncommon occurrence, but has happened.
From: http://Earth'sci.org/processes/struct/subside/subsidence.htm


Oil & Gas
Oil and Natural gas are both fluids that can exist in the pore spaces and fractures of rock, just like water. When oil and natural gas are withdrawn from regions in the Earth near the surface, fluid pressure provided by these fluids is reduced. With a reduction in fluid pressure, the pore spaces begin to close and the sediment may start to compact resulting in subsidence of the surface.

This has occurred recently in the oil fields of southern California. For example, in the Wilmington oil field of Long Beach, California, subsidence was first recognized in 1940 due to withdrawal of oil from the subsurface. The area affected was about 50 km2. Near the center of this area, the surface subsided by up to 9 meters . In 1958 repressurization of the area was attempted by pumping fluids back into the rocks below. By 1962 further subsidence had been greatly reduced, and the area continuing to subside had been reduced to 8 km2. Still, up to this point, very little uplift had occurred to restore the area to its original elevation. This subsidence event has cost over $100 million.
 


You are right to consider this question.

As the oil is extracted from a new drilling it goes through several stages.

First the oil flows naturally because although it is contained within the rock pores and other interstices it is unde pressure. Sometimes this pressure can be enormous as has been seen at the recent Horizon disaster.

As the oil flows the pressure decreases and eventually the natural flow ceases. Further extraction can be obtained by pumping out.

In the last few decades it has been possible to obtain more oil by pumping a replacement fluid - water - in.

Whilst I will not pretend this is done for altruistic reasons, it does have the added benefit of reducing or eliminating the reulting subsidance.

As a matter of interest, this effect has also been observed with mining of solid material, where the subsidence has presented a large problem. In some places concrete has been pumped into fill the voids, even yars or centuries after the mining has finished.
 


Sounds like another good reason to drill off-shore. If a section of ocean floor subsided by a few meters I can't think of any reason anybody would care
 


mrspeedybob said:
Sounds like another good reason to drill off-shore. If a section of ocean floor subsided by a few meters I can't think of any reason anybody would care

That's a great point in and of itself. Though I am somewhat sceptical of continued off-shore drilling these days.
Even still, I think your right.