http://en.wikipedia.org/wiki/Drilling_fluid
I'm more interested in how exactly can the density/gravity work against pressure, as I assume this works. So, they will pump in the fluid into a gushing stream against it's own pressure...? Shouldn't that cement pump after it work really fast or something? I don't see how fluid itself can stop the pressure from gushing out everything, and possibly damaging the equipment further. Then again, I'm no expert on any of that.