I should also note that the idea of blocking the hole with gravel isn't exactly a bad one. You're right to point out that if you use big pebbles for your gravel the oil will just squeeze through the abundant empty spaces. So... use finer gravel!

The pressure will try to push the gravel out. So maybe make it really sticky gravel, so that it glues itself together.

Now there are three things to say about this plan:

(a) we call this sticky, very fine gravel concrete and we use it all the time;

(b) a fun educational point: Even with really fine gravel, oil can still diffuse through it. Oil diffuses through "solid" rock all the time. The oil underground is actually trapped inside rock. The rock that holds the oil is relatively porous; the oil is kept underground because above that rock is a layer of denser rock that oil can't diffuse through very well.

(c) In case it isn't obvious yet: The reason your idea still doesn't work very well is that the first cupful of wet concrete you pour onto the hole will promptly get flushed away by the pressure of the leaking oil. ;) As will every successive cupful of wet concrete. Unfortunately, a giant slab of solid concrete must necessarily start out life as a thin layer of wet concrete, so the only way to put a concrete seal over a hole that is actively leaking is to cast it someplace else and then drop it on the hole.

>You can't "diffuse the pressure".

I must be missing something, the parent is talking about wrapping the 21" (inch) diameter pipe in a 25' (foot) outer pipe, this is effectively 200 times the area. The force applied currently by the oil coming up is going to provide 1/200th of the pressure when applied over the end of the pipe.

It's like blowing out through a straw - you can feel the pressure easily. Versus blowing out through a dustbin with a hole in the bottom.

Mind you it's 4:30am ... perhaps I should go to bed and think about this another time.

I understand that, and apologize for my loose terminology. There are two reasons I suggest employing a larger size.

The first is to do with flow, as you mention. While all of a large cap needs to be able to withstand high pressure, that is not in itself an especially tricky engineer problem. But size doesn't hurt; in my experience it's rather easier to stop up a large pipe than a small one with the same pressure, simply because it's a less delicate maneuver. In the case of a domestic plumbing crisis, it's nice to be able to exert your arm rather than just your fingers.

The other reason is simply to do with bulk. The larger the cylinder, the greater it weight when finally sealed, the more room you have to anchor it, and the farther from the (presumably fragile) ground in the immediate vicinity of the gusher the circumference will be, if one is trying to bore into the ground with restraining pegs.

The surgical approach has not been very successful so far. Given the urgent nature of the problem, I feel it's worth trying the crude but frequently effective approach of large mass + gravity.

up voted you because the opportunity to learn more about this situation was one of the main reasons I asked this question