How do they keep the ocean water from filling in the oil cavity left from extracting the oil in deep water?

Answer 1

Well, the oil is in rock that is under water, not actually in the water. So the water won't automatically rush in and fill it.

I don't know much about deep-ocean oil drilling, but I read that when they drill for natural gas they usually get brine out of the bragain as well; the more responsible drillers store the brine and then pump it back into the hole left by the gas they took out.

Answer 2

1) There is an impermeable mud, usually bentonite, than fill the interstices of the pipe, that is put there while you are drilling, to prevent the walls from collapsing.
2) The oil is less dense than water, so it floats on water, (it tends to go up).
3) Oil is trap[ed at higher pressures than the surrounding and above ocean water. Flow moves from zones of high pressures to lower pressures. again making oil to go up.
4) If any water is infiltrated, or if you inject water to the oil reservoir it will make the oil go up, to float. This property is used to increase the amount of oil that can be extracted.
5) So the question should be more like: How to prevent oil spills left from the cavities. Good question, btw.

Answer 3

With a big cork, or drain plug

Answer 4

gorilla glue