|
ThatDamnedPanda -> RE: BP Oil Spill : Who's Obama gonna call ? (6/4/2010 2:25:00 PM)
|
quote:
ORIGINAL: SL4V3M4YB3
I'm not an expert in deep sea drilling I just assume that if you drill a hole relieving the pressure, that this pressure existed before the hole was tapped and so could be reinstated much as it was before (if you filled the hole). The pressure hasn't just suddenly increased and are we suggesting that once oil is tapped it must flow or else? What would have happened to that pipe if this oil supply was cut off in normal operation?
You just hit on one of the root causes of this particular event, although you may not have realized it. That's one of the things that make deepwater drilling so tricky - oil that deep is usually under incredible pressures. This was a very high-pressure reservoir, which is exactly why it's so attractive for BP to drill it - all you need to do is tap it, and huge volumes of oil flow out under their own pressure. You don't need to pump water into the reservoir to force the oil up; just hold a bucket under the faucet and add up the revenues.
If I recall correctly, the Horizon deposit has a "bottom hole" pressure of around 25,000 pounds per square inch. That means that at the point where the borehole taps into the top of the reservoir (at about 18,000 feet below the seafloor), the pressure is 25,000 psi. That pressure will decrease as you bring the oil closer to the surface, but the casings in the borehole are still under tremendous pressures, and it's a constant battle to keep the pressures stable. It's a very delicate balancing act, and the initial blowout came because they lost control of the process they were using to balance it, and once they lost control, there was no way to get it back.
Once you tap into that much pressure, you are at constant risk of having your casing and/or your wellhead blow out if you don't keep it in balance. What "Top Kill" was intended to do was re-establish the balance in the borehole by pumping mud into the casing, but the casing had already been damaged by the uncontrolled pressures of the initial blowout, and they didn't know how badly. All they knew was that they were losing pressure at a point in the borehole that should have been holding pressure, and that could only mean a leaking (and therefore damaged) casing. If they had tried to simply cap the pipe at the seafloor without stabilizing the pressure below that point, yes - it would have blown the pipe out completely.
That's the biggest danger of deepwater drilling - once you poke a hole in that reservoir, you'd better be ready to contain whatever is coming out, because sure as hell it's coming out. There are other reservoirs in that region that they can't even consider producing, because they have bottom-hole pressures of 35,000 psi or greater, and nobody has the technology to control that kind of pressure. Deepwater Horizon was close to the edge of existing technology, and as we are seeing this last week or so, it was apparently a lot closer to the edge than anyone realized.
quote:
ORIGINAL: SL4V3M4YB3
How is it we know so much about a well head even though we can't even work to a depth to fix the pipe? We know more about the situation at a level deeper than the pipe we can't fix?
I can't recall the exact details, but in loose terms, they can tell a lot about what part of the pipe is leaking by how much pressure they're losing and at what point in the process they start losing it. In the days and weeks after the initial blowout, they were able to determine from going over their logs and analyzing the pressure readings that a specific joint in the casing (a joint which was located about 1,000 feet below the seafloor) had apparently been blown out during the accident. In simplest terms, I believe they noticed a sudden pressure drop at the time that the gas bubble was passing through that joint. Ergo, that joint failed, and released some of the pressure at that instant.
Now they knew they had a weak point in their casing, so that was their best guess as to why they were unable to hit their target pressure in the casing - it was leaking out through that hole, and there was a risk that if they kept stressing that part of the well, it would blow completely. I may have some of the details wrong on that point, because I haven't been able to get much information on it, but I know that I have the gist of it down.
|
|
|
|
