Thanks. So they must intend to penetrate the shaft with another shaft (same diameter?) then pump in a sealant. Why did they choose the depth they did? Couldn't they do this closer to the seabed floor?
The purpose of the relief wells is to attempt a bottom kill, so it has to be at the bottom.
So that you have plenty of wild well bore above your intercept to contain the plug. Intercept down low and you don't need as much pressure to contain the leak, as the flow of oil helps move the plug mud up the bore. The higher you intercept the more pressure you need in your relief wells, and the greater the risk of splitting the casing.
Basically, if you're 10,000 feet down, you have 10,000 feet of potential plug with a lot lower risk of splitting casing because of the pressure at which you feed the plug mud.
What people are ignoring, is that with a mud weight of between 14.1 and 16.8, depending on sources, the well was under control, even though the plug was compromised.
Put that same hydrostatic pressure on the formation, and it will be shut in again. then the new plug can cure. The rest, once you can put the fluid in the wellbore, is just a math problem.
The same fluid column (drilling mud) will be exerting hydrostatic pressure in the relief well, holding the formation fluid back.
The problem on the Deepwater Horizon came about when the riser pipe was displaced with seawater (despite apparent indications that cutting the hydrostatic pressure on the formation was letting the well flow, indicating a bad plug), flow indicators were ignored, and the well blew out. The displacement with the lighter seawater reduced the hydrostatic pressure on the formation downhole by some 2200 psi, letting the well blow out.
Close as possible to oil bearing formation, just incase a formation/casing leak.