Could BP Oil Spill in The Gulf of Mexico result in Subsidence of Ocean Floor ?

May 3, 2010 5:51 PM

Can anyone address the possibility/ likelihood of ocean floor subsidence in the Gulf of Mexico and adjacent continental shelves due to the rapid depressurization of pockets of hydrocarbons as a result of the BP Oil Disaster/ Spill? Will the ocean floor open up to allow sea water to fill the ever-growing vacuum/ void?

I'm reading that there are most likely 50,000 barrels of oil spewing out from the scuttled BP oil rig site per day, not the low-ball 5,000 barrel estimate so commonly quoted in the press. I have also read in an AP story that once the remaining pipe erodes below the ocean floor, a huge chasm will open and the oil spill will be unstoppable. How can this occur without major subsidence and intake of ocean water???

posted by mickeefynn to science & nature (20 answers total)

Can you link to the AP article, so I can give it a read?
posted by Houstonian at 5:56 PM on May 3, 2010

Here's the article link... an AP article quoted in Yahoo News

http://news.yahoo.com/s/ap/20100502/ap_on_bi_ge/us_gulf_oil_spill
posted by mickeefynn at 6:04 PM on May 3, 2010

The oil is not leaping out of the sea floor by itself, leaving some sort of a vacuum behind it; it is being forced out by the weight of the water and earth bearing down on it. There is no chasm to be filled. Or, rather, the space recently occupied by oil is instantly being filled by water and earth exactly as the leak occurs.
posted by jon1270 at 6:07 PM on May 3, 2010

For what it's worth the concept of sea-floor collapse is not mentioned in the AP article. The concept is raised within one of the 10,000+ comments.
posted by southof40 at 6:09 PM on May 3, 2010

Exactly... but I FOUND that article while searching for information/ speculation about the possibility of subsidence due to loss of mass within the earth. I posted a question... not a statement or prediction. Please re-read. Thanks!
posted by mickeefynn at 6:14 PM on May 3, 2010

Makes sense, john1270
posted by mickeefynn at 6:16 PM on May 3, 2010

FWIW, my explanation was probably a little simplistic. Part of the pressure driving the oil out could be coming from compressed gasses, which wouldn't fit my explanation, but I don't know enough about oil-related geology to say. I hope someone else can explain what's going on with more confidence.
posted by jon1270 at 6:29 PM on May 3, 2010

I appreciate any and all attempts!
posted by mickeefynn at 6:44 PM on May 3, 2010

there are lots of interesting bits in the thread of this photo. here's the really simplistic version, as i understand it: the long pipe going up to the rig (the riser) broke off when the rig sank, and is kinked and laying on the seafloor. it has a few leaks in it, like so. at the end of the riser is the BOP (blow out preventer), which is what caused all this trouble in the first place. normally is supposed to shear whatever pipe is in it and lock up, but it malfunctioned and is stuck open, causing all this leakage. below the BOP is the wellhead. this is concrete encased steel pipe, that is currently perfectly fine and will not break. from there, it goes down through probably 1000ft of rock (and also horizontally) before you hit the actual oil well.

what this all means to you is that oil will continue to flow at a high pressure, until the pressure in the well is equalized with the seawater. seawater at this depth is probably something like 2000psi. from there, i would imagine the oil would continue to rise (because of density), albeit at a lower rate, and seawater would slowly mix and enter the oil well. the earth will not open up, as the well is far below the seafloor, and the wellhead pipe is stronger than the rock around it.
posted by Mach5 at 6:50 PM on May 3, 2010 [1 favorite]

it all looks like this
posted by Mach5 at 6:53 PM on May 3, 2010 [1 favorite]

Also, it might help to keep these volumes in perspective by doing a little math.

50K barrels is 287,413 cubic feet.

If the hypothetical chasm being vacated by the oil were a circular shape 16 miles across (pulling this out of air; I don't know how big oil deposits tend to be) then we're talking about 200 square miles of ocean floor becoming unsupported.

A volume of 287 thousand cubic feet gets mighty thin if you spread it over an area of 200 square miles. 200 square miles of sea floor would only have to drop just 0.00062 inches (yep, just over half a thousandth of an inch) to fill in the space formerly occupied by 50K barrels of oil.

Point being: while 50K barrels of oil is enough to make a huge f***ing mess, it's not a wormhole into another dimension.
posted by jon1270 at 6:55 PM on May 3, 2010

Are oil deposits just big open caverns of oil filled with crude? I thought it was more like a layer of sediment saturated with oil.
posted by Think_Long at 7:00 PM on May 3, 2010

You ask an interesting question! In general terms, you see some of this in mining activities but I'm not aware of it happening in drilling in recent times.

Always when we drill, we are removing something from the earth and yet... the Middle East, Texas, and many other places have not collapsed. In general, when a well is finished (we say "abandoned"), they are back-filled in a manner that is called plug and abandon. (The current gossip I've heard is that something went wrong with the casing. I'm not sure what (if any) implications this has for right now, though.

But that is in general, and this is a unique situation. Hopefully a geologist will stop by to explain this in detail. I don't think the Gulf caving in is a significant concern. I think this in part because I work in the energy industry and there have been lots of conversations about what's happening and this hasn't come up in those conversations, and also I think this in part because I don't find the commenters to the articles to be credible.

To help others following along, the comment under the article was this:

James: Pollution is bad enough, but the real threat from offshore oil/gas extraction is uncontrollable subsidence of the coast and adjacent continental shelves due to depressurization of pockets of hydrocarbons. Google "Gulf of Mexico floor subsidence".

I took James up on his request to google that phrase, and got one return. It is in a comment under a NYT article, and sounds very similar in wording:

Goldmankaput: [Commenter shares that he is a lifelong Democrat voted for McCain, and doesn't like nuclear power] Offshore oil/gas extraction is the wrong policy because, by reducing subterranean pressures, extraction will lead to uncontrollable subsidence of coastlines and the adjacent submerged continental shelves. Anyone who doubts this is happening is invited to Google "Gulf of Mexico floor subsidence" and to report back to the general readership what it says.

I also googled for similar comments about ocean floor subsidence and this incident, and got one article which is reposted on several websites. I found it at this website, again as a (very long) response. Since the commenter provides his full name and credentials, I googled him... and got some pretty radical articles from him, as well as various other credentials in everything from marine piracy, CIA covert operations, and operating systems.
posted by Houstonian at 7:19 PM on May 3, 2010

(Sorry for that long, rambling answer. Another way to think about it: We have tons of geologists in Houston. If this was a credible possibility that the seabed would drop, sucking in the seabed and leaving the Gulf a massive cavern spewing oil forever, all freeways out of the city would be jammed right now.)
posted by Houstonian at 7:37 PM on May 3, 2010 [1 favorite]

Many thanks, Houstonian! Your explanation and links helped me a lot.
posted by mickeefynn at 8:31 PM on May 3, 2010

Well, yes, but also:

The Mississippi River now discharges an average of about 200 million metric tons of suspended sediment per year past Vicksburg and eventually to the Gulf of Mexico. This sediment discharge to the ocean ranks about sixth in the world today, being equaled or exceeded by those of four rivers of Asia (the Yellow and Yangtze Rivers of China, the Ganges-Brahmaputra of India and Bangladesh, and probably the Irrawaddy River of Burma) and two rivers of South America (the Amazon River of Brazil and possibly the Magdalena River of Colombia).

Note that this accounts for the modern water-management system.

Anyway, in general I would think of this subsidence as a localized issue, primarily of interest to geologists and hydrologists. I can't imagine it having, in and of itself, a significant impact on humans or even the surface ecology to much extent.

Put another way: World oil production is in the neighborhood of 70M bbl/day, or around 8 billion liters. The total volume of world oceans is somewhere around 1.32 billion trillion liters. We would need to pump oil at this rate for 500 million years to pump out a volume equivalent to the entire ocean (and of course there isn't that much oil). Put another way yet, an entire day's worth of oil pumped (and not all of it is from the ocean floor by a long shot) would reduce the depth of the world's oceans by 22 nanometers; in a year, 8 microns. It's really not as much as you think. The oil is damned valuable even at amounts that a single human can carry, but it doesn't amount to much when placed against the volume of ocean it's displacing -- particularly since Gulf-floor deposits are indeed, as Think_Long suggests, compressed by the sheer weight of the Gulf itself to something more like a sedimentary layer than a cavern.
posted by dhartung at 10:04 PM on May 3, 2010

I don't know the answer, but if it's anywhere, it seems likely to be here.
posted by claudius at 12:06 AM on May 4, 2010

Are oil deposits just big open caverns of oil filled with crude? I thought it was more like a layer of sediment saturated with oil.

Often not even sediment, but solid (albeit porous) rock.
posted by -harlequin- at 2:01 AM on May 4, 2010

Thanks to all for your clarification.
posted by mickeefynn at 6:23 AM on May 5, 2010

Sorry if I'm out of line for asking this mid-question (maybe I should create my own question) but I wonder if this spill can't contaminate the whole Atlantic Ocean. I saw an estimate on the guardian this morning that there may be up to 100,000 barrels/day of oil being leaked. Couldn't this oil get carried by ocean currents through out half the world's oceans? Or would it be too thinly concentrated to make much of a difference?
posted by theNeutral at 9:30 AM on May 20, 2010