Does oil extraction leave a cavity?
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 it's one that actually, we've also had from Ady Yates who said "what do we replace pumped-out oil and gas with?"
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 it's 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, nakedscientists.com/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 that's 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 Ady's 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 it's 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 it's 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.