I didn't fully understand all the info about the 'doomsday scenario' as it was explained on Countdown. I have been looking for info about exactly what is going on. I found a lot of info at this site:
http://bpblowout.weebly.com /
This is a very long post because I have been walking myself through the process and the problems. Unfortunately, I think I understand.
First of all here is a 1.5 minute animation of what happens as they drill into the rock:
http://www.youtube.com/watch?v=U2ms95HX0l4 The Well,or Wellbore: When they start drilling they drill a fairly large hole, I think 36-40 inch in this case. After a certain point, 1000 feet or so they run in pipe called casing and cement around it. They then drill in this 36" pipe with a smaller drill bit and after a certain depth, they run casing again and cement it inside the larger casing, they will continue doing this over and over again until they reach their target formation, I believe on this well there are 7-8 sets of casing in total, with the end casing being around 7 inches or so. So the well when finished is actually a pipe with a 7 inch hollow inner diameter with around a 15 inch wall thickness. The BOP's are attached to this at the top of the well on the seafloor, then in this case a 5000 foot pipe called a riser from the BOP is attached to the drilling platform.
The casing is the steel pipe that was placed in the Deepwater Horizon well hole during the drilling process to prevent the wall of the hole from caving in.
A diagram of the casing that they are worried about:

1. The Well is a pipe that essentially is 36 inches in diameter with a 7 inch hole in the center. The wall of this pipe is essential made of multiple sets of pipe and cement, it is designed and engineered for fairly high pressures. (I have links to pictures and videos of this below)
2. The Blow Out Preventer (BOP) is a big-ass "safety valve" that was supposed to "prevent the blowout" by closing and not allowing anything past, it is on top of the casing. It is designed and engineered for fairly high pressures.
3. The Riser is a pipe that goes on top of the Blow Out Preventer (BOP) and it connects to the platform. It is designed for Moderate pressure and 5000 feet of it hit the ocean floor making the reliability of it's ability to even withstand low pressure is at an extremely low percentage. It also has a nasty bend right at the top of the BOP. Within this Riser is a drillpipe, the end of which is now plugged, there is also a 4" line with big rubber seals inserted into the riser.
All that being said think of it like this. A car pressure washer hose (the well) is hooked up to a high pressure valve (BOP) which has a garden hose attached to it (The Riser)
What is the Goop Coming Out of the Well and Flow Rate Estimation
The stuff that comes out of the well is not 100% crude oil. It contains Oil, Natural Gas (Methane), Formation Particulates and funky stuff called "produced water".
Produced water is usually slightly salty and is essentially ancient seawater. The majority of it, once it is separated from the other stuff that comes out of the well is injected down other wells so that it re-pressurizes the underground oil producing reservoir and eventually produced water replaces a lot of the oil to a point where the oil is not economically viable to recover and the well is plugged, or "abandoned" in oilfield terminology.
Oil bearing formations, or reservoirs are kinda like a giant sponge made of sedimentary material, stuff kind of like sandstone. Some of this crap comes loose and makes it's way to the surface. It is not usually a lot of stuff but an uncontrolled blowout often raises havoc and brings more to the surface then usual.
Misconceptions about plugging the well:
<snip>
You cannot simply put a plug in the end of the pipe to stop the leak, or inflating a balloon inside it as there are a few leaks and the riser is not structurally sound. You plug the big leak the small leak at the top of the blow out preventer will in all likelihood burst or open significantly adding to the problem.
Also plugging this way may burst the riser pipe in other spots that may have been damaged. You also can't cut the pipe off at the top of the BOP (safety valve) and plug it there because it is bent just inside the BOP.
Think of an uncontrolled blowout as if it were a garden hose hooked up to a pressure washer you clean your car off with. You plug it at the end and somewhere the garden hose is gonna start expanding and burst especially if it has been damaged and may burst even if it were 110% perfect, most of the time you have to kill these things at or near the bottom for reasons I will make clear further on.
In my opinion they cannot plug this thing 100% from the top of the BOP without something blowing up. You must also realize that there is drill pipe within the riser pipe making any catheter/balloon plug very ineffective.<snip>
http://bpblowout.weebly.com /
The 'plugging' has been all show. They can't plug it because of the damage to the BOP.
They have no choice but to try to cap it with a device to capture the oil and bring it to the surface.
The choices basically are:
1-Do nothing and just let it flow.
2-Cap the well and bring the oil to the surface so that it doesn't go into the water.
3-If they try to top kill it or plug it, they run the risk of destroying what is left of the BOP and releasing brazillions times more oil.
4-keep drilling relief wells and hope that they work.
When they tried to cap the well at the end of May, they found out that they could not do it without destroying the whole thing. They cannot control the pressure finely enough to do it.
The real problem is the casing below the wellhead that reaches into the reservoir. If that has begun to have fractures, the oil is leaking out to the sides and forcing it's way to the surface through fractures in the surrounding rock.
Imagine a garden hose that is punctured. The water spews from the punctures too. If the pressure of the water that is coming out is high enough, it will continue to flow out of the end as well as the punctures.
From the beginning, they should have also begun a MASSIVE effort to keep the oil off the shores and to try to get it out of the water. It would have to be continuous. Some of it would have reached the areas anyway, but not like this.
1-The relief wells are the only way to stop it that they have on hand. However, even as they say this, they have not explained why other methods won't work. They are not considering out of the box ideas even if they would work.
That whole device is like a bomb ready to go off because of the pressure. Even a new idea might trigger it. They are just hoping everything will not deteriorate a lot until the relief wells are complete.
2-The only way to have really helped the people along the Gulf was to start a massive cleanup effort and keep it going. The dispersants were horrible because of their toxicity and they made it harder to even try to collect the oil. The oil was dispersed with a poison attached which made it a much more dangerous problem for anybody trying to clean it up.
The oil itself is a hazard and would require special equipment and suits to handle it.
3-the only way to still help directly with the spill is to seriously try contain it and clean it up.
The dispersant is the gift that will keep on giving all over the ocean with the oil.
If the plumes were partly a result of the dispersants, that really helped. *sarcasm*
Even if they were going to occur anyway, the toxic dispersants were the last thing needed.
I'm stopping now. I may be wrong on some of this, but I believe a lot of it is correct.
I have only 3 words:
Oy vey!
Screwn