When you drill down to a subsea oil deposit chamber, you don't want all of the oil to leak out of its chamber before you can pump it out. You can't sell that oil, it ends up on a beach, people yell at you. So maybe you drill most of the way to the chamber, maybe at first you just drill partway through there. And then, instead of pumping oil out (which you can't do, you haven't reached the oil) you pump cement in. The cement flows out through the tip and then flows where it can--into the edges of the hole you drilled. The cement fills up the places where the oil might otherwise leak out. You don't let the cement stay in your pumping system, but you make sure it's all out there, sealing up the area around your drill path. And then you drill more.
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