Effect of oil and gas extraction on Earth's crust

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   During some recent conversation with friends, two questions came up for which none of us had answers and I think they are both interesting, so I am posting them in two separate threads to see if any PA members can throw any light on them.
This is the first one:

Our planet used to be full of 'bags' of oil and gas under its surface, which we are emptying. When all those 'bags' will have been emptied, is there no risk that the surface on its top will collapse? (or its emptying may cause major earthquakes etc).

As far as I know the emptied volumes are not replenished with any other substance and yet nobody seems to talk about such potential risks, not even the most fervent environmentalists.

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Gerinski Members Profile Send Private Message Find Members Posts Add to Buddy List Reviewer page Prog Reviewer Joined: February 10 2010 Location: Barcelona Spain Status: Offline Points: 5154	Topic: Effect of oil and gas extraction on Earth's crust Posted: February 14 2013 at 13:34
	During some recent conversation with friends, two questions came up for which none of us had answers and I think they are both interesting, so I am posting them in two separate threads to see if any PA members can throw any light on them. This is the first one: Our planet used to be full of 'bags' of oil and gas under its surface, which we are emptying. When all those 'bags' will have been emptied, is there no risk that the surface on its top will collapse? (or its emptying may cause major earthquakes etc). As far as I know the emptied volumes are not replenished with any other substance and yet nobody seems to talk about such potential risks, not even the most fervent environmentalists.

darkshade Members Profile Send Private Message Find Members Posts Add to Buddy List Reviewer page Collaborator Honorary Collaborator Joined: November 19 2005 Location: New Jersey Status: Offline Points: 10964	Posted: February 14 2013 at 13:39
	Good question, and I never thought of that. I have no answers for you, but I'll be following this thread as I'm curious myself.
	My Last.fm

Dean Members Profile Send Private Message Find Members Posts Add to Buddy List Reviewer page Special Collaborator Retired Admin and Amateur Layabout Joined: May 13 2007 Location: Europe Status: Offline Points: 37575	Posted: February 14 2013 at 15:51
	Since all wells are drilled to depths below the aquifer keeping mines free from flooding has been a major part of mining since we first dug a hole in the ground to extract minerals. Therefore I would imagine that like coal, copper and gold mines, all oil and gas wells fill up with water. However, oil and gas are not formed and contained in vast hollow underground chambers or voids - they are trapped in porous rock, like water in a sponge, (however the density involved varies but you're looking at something that could be in the order of 90% rock - 10% void), so this is more like car engine oil soaking into a concrete garage floor - take the oil away and it's still solid concrete. Unfortunately that does not mean that all depeated oil and gas wells are "safe" from surface subsidence, for example some of this oil-bearing porous rock, such as chalk, can react with the (predominately salt-rich) water or under-go other compositional changes due to temperature and pressure and that can lead to subsidence, as happened in the Ekofisk oilfield in 1987. Edited by Dean - February 14 2013 at 16:38
	What?