Yeah, I've been reading about this off and on for the last several months. I'm afraid that if those concrete balls are actually big enough to make any difference at all, they are going to either turn into missiles or just cause the hole to widen. That mud is coming from a formation under several thousand pounds per square inch of pressure. A few 1000-lb balls don't even come close to balancing that force.
The more I think about it, I think they need to consider high-explosives. If they could set off an explosion of sufficient magnitude above the mud layer, they might be able to cave in the hole the mud is rising through. I'll grant you that if that doesn't work, the problem just got a lot worse... I'm convinced that the answer is to collapse the wellbore, though. Somehow. There is enough mass there to counter the pressure from below, but they need to find the right way to leverage it into working for them. Concrete balls aren't going to do that.
@Turambar,
Thank you for that charmingly graphic mental image. If I ever meet you, I'm feeding you to my kittens.
