Science Questions

Does oil extraction leave a cavity?

Sun, 13th Feb 2011

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Yehuda Schonfeld asked:

Hi Chris,


Thank you for your interesting podcast!


My question is, when people pump billions of gallons of fossil fuels out of the earth, does the huge empty hole cause the earth's crust to become unstable and possibly collapse? Does the fossil fuel replenish?


I always wondered about this.





Chris -   That's a really good question and itís one that actually, weíve also had from Ady Yates who said ďwhat do we replace pumped-out oil and gas with?Ē

I guess I can probably help both of you in one go here.  A few years back when I first started doing all this, I thought oil existed underground in these big open caverns, almost like a coal seam - and when you take the coal away you're left with a big cave. But oil's different and, in fact, the best analogy I can think of to explain what itís really like is to imagine sticking a straw into a wet sponge and sucking water out, because that's essentially what the conditions are like underground.

When the oil was formed, it was lots of marine creatures and other organic matter; this got compacted on the sea floor below layers of sediment and the huge pressure heated everything up and cooked all these dead creatures into the soup which became the crude oil that we extract today.  But that means that what that material is trapped inside is a porous rock, and I have to acknowledge on our forum,, we had an answer to a similar question a couple of years ago which a geologist on the forum, JimBob, gave a very elegant answer to.  He points out that if you look at the rock that an oil well is drilling into, the porosity of that rock, in other words, the proportion of holes, is about 13%.  So in other words, if you take the cross-sectional area thatís holes as a whole proportion of the cross-sectional area of the piece of rock, about 13% is just empty space in the rock; and that empty space is filled with the oil.  Now, he also says that that is about the same open porosity as concrete.

So, in other words, when you take the oil away, you're left with something which is equivalently strong already to concrete.  So you're not leaving a big space.  You're just taking the oil out from between all these little holes which are, to a certain extent, in continuity. 

But then the next point to bear in mind is that this oil and gas that's underground is under extremely high pressure and that means that as you take the oil or gas away, largely under its own pressure, then other things will move in to displace it.  And therefore, some water will move in from the adjacent rock and will also take up some of the space that's been vacated. 

In fact, when people call a well spent, in other words, they say that an oil well has become empty, actually, the amount of oil that's left behind can be as much as 90%, because the oil is very hard to get out. 

To come back to Adyís question, what do we replace it with, well sometimes, you can help to get the oil out by pumping something else into the porous rock such as water to help the oil be pushed up to the surface because it floats on the water.  The Norwegians have also got a technique where they pump steam in underground and the steam, being hot, can make the oil become runnier, so itís more likely to consolidate - join together - into big blobs of oil which are easier to get out.  And also, as Dave pointed out earlier when we were discussing this, sometimes they also put surfactants, things like washing up liquid, down underground and that helps the oil to have a lower surface tension so it can flow out of all these little holes more easily. 

So, the bottom line is, you're sticking a straw into a sponge; the porosity of that sponge is equivalent to concrete so itís still very tough rock underground; therefore, you don't get left with a great big gaping hole, and therefore there are probably few seismic consequences as a result.


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Yehuda Schonfeld asked the Naked Scientists: Hi Chris, Thank you for your interesting podcast! My question is, when people pump billions of gallons of fossil fuels out of the earth, does the huge empty hole cause the earth's crust to become unstable and possibly collapse? Does the fossil fuel replenish? I always wondered about this. Thanks, Yehuda What do you think? Yehuda Schonfeld, Sun, 14th Nov 2010

This was discussed in the geology forum a while back...
Bass, Mon, 15th Nov 2010

There is no cave-like cavity. Fossil fuels are extracted from porous rocks or fractured rocks which may have 10-20% microscopic pore space - to the eye the reservoirs appear to be solid rock. Often 30-50% of the oil is unrecoverable. Often water is pumped into the reservoir in order to increase the pressure (replacing the oil). The Earth is perfectly stable as the rock remains - only the fluid is removed. Fossil fuels would not replenish on a human time-scale. aubreywhymark, Sun, 2nd Jan 2011

What about the weight. I know the size of the earth compared to the amount of oil is vastly different. But, could we ever run in to issues with the amount of weight we are re-distributing? Chrwilcoa, Thu, 9th Apr 2015

Hi. If the rock containing oil is about as strong as concrete, isn't that still dangerous? The water you say takes the place of oil is not as dense and sturdy, is it? I ask these because concrete cracks- sometimes easily- and oil is thicker than water............. so, how can it be true that harvesting oil doesn't cause instability or collapse? margaret mary hepker, Mon, 17th Aug 2015

I know this is a little off topic, but I've seen a substantial amount of blogs on the internet claiming that drilling is not bad for the earth. Bottom line is there have been no significant tests to prove this, a test of that magnitude would cost a fortune and we still have a great deal to learn. Some people feel they need to display their degrees even though you can watch a documentary about dinosaurs and learn most of this information, this in no way proves that drilling oil is not harmful to the long-term health of the earth, this is only data taken from an unrelated subject and passed threw a philosophical centrifuge. Mike H, Tue, 19th Apr 2016

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