Oil spill: Water pressure VS. oil pressure

1. May 30, 2010

proculation

Hello all,

It's my first post. I'm french canadian so I may make grammar mistakes in English :-)

This is a question I'm asking me since yesterday. It's about the oil spill in the Gulf of Mexico.

The oil and gases are less dense than the salted water. The breach is at about 1500 meters deep. So, by rounding, about 150 atm of pressure.

It's sure the pressure in the underground oil cavity is much more than that. It even blew up the valves and put the rig on fire.

My question is: Does the water pressure at that depth make a difference in the release of the oil ? The quantity ?

If that same oil reserve was discovered at 150m depth and the same accident happened, would it mean the oil would spill ~10 times more since there's less water pressure ?

My instinct would tell me "yes" but i'm wondering about the difference the buoyancy makes.

Thank you,

Olivier Gagnon

2. May 30, 2010

pallidin

Sure. The water pressure restricts, to a degree, the amount of the oil release.

3. May 30, 2010

proculation

But does it 'compensates' for the buoyancy of the oil at that depth ?

If you put a ball with air at that dept, the force will be very strong to get it to the surface. Is is the same thing for the oil ?

4. May 31, 2010

pgardn

I would think if the ball was filled with air at the surface and then moved to 1 mile beneath the ocean you would greatly reduce the density of that ball as it would be smashed a bit assuming you dont fill it with air that has the same surrounding pressure as water at 1 mile deep. This is interesting though as I dont know how much oil can be compressed at higher pressures. Obviously it is much more difficult to compress liquids.

Clearly in this situation not all of the oil/methane is getting to the top. So some chemistry is going on at these high pressures and low temperatures (as reported about the slushy hydrates forming preventing the first "dome" from flowing smoothly). Throw in the dispersants they were using at depth and it gets very complex I would imagine.

I think BP would agree that its complex anyway.

5. May 31, 2010

proculation

The pressure of the water which is about 150 atm and the pressure of the oil well.

My question is quite simple: does that water pressure help to reduce the spill ?
2nd question: with the same oil reserve at 150m (15 atm), would it spill 10 times more ? cetaris paribus.

6. Jun 1, 2010

pgardn

Does water pressure help reduce the flow rate? I would imagine it must.

No idea. I was just stating that the density of the oil is going to change. How much I dont know. So when you posted about the buoyant force (3rd post) I reasoned that it was more difficult to reason out. You can cetaris paribus the oil density at 1 mile, while changing the depth and I dont know. I guess someone else can wittle away the enormous number of variables and come up with something.

7. Jun 1, 2010

rcgldr

The buoyancy effect is minimal compared to the difference in pressure between the oil and the seawater at 5000 feet down. Using the denser (than water) "mud" into the pipe was supposed to increase the pressure opposing the oil pressure (very tall pipe section), but a BP manager decided to fill the last upper section with sea water instead of the denser "mud", which may have contributed to the accident.

8. Jun 1, 2010

proculation

Like I said earlier, the question is not about BP.

It's about the pressures and the quantity of oil spilled variable to the depth of the sea.

9. Jun 1, 2010

pgardn

Do you have a link for this?

This is all very sad but at the same time very interesting.

10. Jun 1, 2010

stewartcs

The pore pressure of a normal formation is equal to the hydrostatic pressure of water extending from the surface to the subsurface formation of interest. Thus the normal pressure in any area will be equal to the hydrostatic pressure of the water occupying the pore spaces of the formations in that area.

One could certainly argue that since the formation is below the seafloor and the ocean that the pore pressure is greater due to the addition of the seawater hydrostatic head. If the seawater hydrostatic head was not present, then the pore pressure would be reduced thus causing less flow.

However, in the end it works out such that they typically cancel out. That is, the increase in pressure due to the seawater hydrostatic head in the pore pressure is cancelled out by the seawater hydrostatic head at the point of discharge.

CS

11. Jun 2, 2010

notbacktrack

Hey dude I just saw your post and I'm getting ready to go to class, not physics unfortunately, but I can do this calculation so just give me until tonight and I should have it up for you.

12. Jun 2, 2010

rcgldr

A BP employee explained all of this in a 60 minutes episode, 5/30/2010. It's the first (upper left) video on this web page:

http://www.cbs.com/primetime/60_minutes

13. Jun 10, 2010

XLR8

I'm unclear about the nature of several of the factors in play here.

First, is the pressure differential between the well and the surrounding seawater the result of the methane coming out of solution in the oil? If this is the case, it would effectively be the equivalent of opening a carbonated beverage - with an initial boil-off of mostly gas followed by a much slower, buoyancy-driven flow.

The physics of this scenario at that depth don't add up though; the water pressure should easily be enough to keep the methane in solution at that depth.

Furthermore, the symptoms don't seem to match; the initial fizz would have happened way back when the well was initially tapped, and most of the methane would have formed bubbles that ascended through the well to create a layer of gas above the layer of oil within the well - meaning the leak would be primarily methane, not oil.

The other possibility is that the pressure differential is just caused by the buoyancy of the oil. If this is the case, there are two possibilities. One is that water is flowing into the well somewhere to fill the void left by the escaping oil. The other is that the rock surrounding the well is physically flexing to push the oil out. If the latter is the correct explanation, that's bad news.

In the scenario involving a fizz-off and the one involving water flowing in, I can think of a bunch of decent sounding solutions. In the final case, I've got nothing.

Clearly, I need to take some geophysics classes, so someone step in an enlighten me here.

14. Jun 10, 2010

rcgldr

Before the well was drillled, that oil was under very high pressure, unrelated to methane gas content or buoyancy. The drill is housed in a very tall pipe designed to contain the high pressure oil, as well as the "mud" being piped down during the drilling operation.

I assume that the compressability for oil is less than that for water, but not sure. If oil was more compressable, then there could be some depth where compressed oil would have higher density and sink in water.

15. Jun 10, 2010

XLR8

The question is why the oil is at higher pressure than the surrounding water and earth. Local pressure differentials don't usually just happen spontaneously. Something caused it.

The deep ocean water is at high pressure too, but if you stuck a water-filled pipe down there you wouldn't create a gusher.

The interesting bit is that the oil comes shooting out even though it has a column of denser stuff sitting on top of it.

So the oil pressure differential relative to water must be due to methane solubility, buoyancy, thermal influences or something along those lines. What I'm looking for is which one we're dealing with.

16. Jun 10, 2010

Andy Resnick

Why does it only have to be one factor? And besides, the properties of crude+methane at depth (and seawater at depth) are not the same as the near-surface situation. Plus, the dispersants used alter the properties of the oil.

Newsweek had a good article recently about 'deep water plumes'. I'm not an expert, but it appears that the oil spurts out of the well and is entrained in the deep water currents- think large planar 'tongues' of fluid- that migrate laterally. The oil concentration is small, but the plumes are several miles in lateral extent and hundreds of feet thick.

http://gulfblog.uga.edu/

http://www.nytimes.com/gwire/2010/0...ed-by-size-density-of-undersea-oil-98517.html

http://www.washingtonpost.com/wp-dyn/content/article/2010/06/08/AR2010060801850.html

Of course, BP claims differently:

http://www.energyboom.com/policy/no...p-ceo-tony-hayward-denies-scientists-research

This has some interesting information as well, but nothing quantitative:

www.evs.anl.gov/pub/doc/ProducedWatersWP0401.pdf

17. Jun 10, 2010

XLR8

You're right. It doesn't have to be just one factor. Regardless though, I want to understand exactly which factors are at play in causing the pressure build-up within the well.

In my mind, there is good reason to believe that we can rule methane solubility out (see my first post.) If that's the case, then we're down to a few possibilities - some of which could be solvable; such as if water is flowing into the well to fill the void left by the leaked oil (in which case, buoyancy is a contributor to the pressure.)

And you make a good point about dispersants, but unless they're injecting them into the well rather than applying them to plume, they shouldn't affect the dynamics of the inside of the well.

18. Jun 10, 2010

Andy Resnick

According to the Newsweek article, 185,000 gallons of dispersant were pumped directly into the wellhead. 800,000 gallons were applied at the water surface.

And IIRC, the methane is not in a gas phase at depth- it forms hydrates (ice) when it exits the well.

http://en.wikipedia.org/wiki/Methane_hydrate

19. Jun 10, 2010

XLR8

Whoa - good info. I had no idea. It's amazing to me that they can inject directly into the well, but haven't come up with a way to inject something that will gum up the leak. From the inside, the pressure would be an advantage.

20. Jun 10, 2010

Yankus

There will also be pockets of methane inside the reservoir as the local geology is never usually round, most wells naturally vent for a long time, the wells in saudi arabia didn't need water injected into them until the late 90's to force the oil out.

Dont forget takes alot of time and pressure to form oil.

Injecting something into the well to stop the flow is alot more complicated than pumping something in you have to overcome the outward pressure first and have something that will withstand it and it cant be a solid it has to be a liquid or a gel which will harden later

21. Jun 11, 2010

stewartcs

The reservoir is at a higher pressure than the surrounding seawater due to the process by which the reservoir was formed. The reservoir is underneath not only seawater, but also rock. The rock creates a hydrostatic pressure (so to speak) on the hydrocarbons in the reservoir. During this compaction process (over thousands of years) the reservoir can become abnormally pressured if the porosity of the formation doesn't allow the pressure to escape. Thus, if the rock that was above the reservoir has been removed by some natural process such as erosion then the pressure in the reservoir is high since it was trapped due to the porosity of the formation. If the rock that was removed due to erosion was replaced with seawater the pressure of seawater at that same depth will be lower since the density of seawater is lower than that of rock.

CS

22. Jun 11, 2010

XLR8

Money. That makes sense, and that's what I was looking for. I wonder if the layers of rock that contain the reservoir physically move together as the oil drains out of the well. That would mean that a well of just crude (no methane) would flow with relatively constant pressure throughout its life. Having natural gas in the mix complicates things a bit.

23. Jun 11, 2010

Andy Resnick

It's also important to realize the oil is not contained within some large cavity; the rock is porous and the oil contained within the small (microscopic?) pores of the rock. This is why water is sometimes pumped into the rock to push out the oil.

24. Jun 11, 2010

XLR8

Good point. So it stands to reason that the volume of the cavity (or the volume of the network of pores) doesn't decrease as the oil escapes. Therefore, in a well that just has oil, the pressure should drop off very rapidly. A well with a mix of oil and natural gas would maintain pressure for a much longer period of time - especially if the methane isn't in solution. If the methane isn't in solution though, just venting the gas would remove ALL the pressure.

25. Jun 11, 2010

Yankus

ALL oil wells have some form of gas in them, its another byproduct of the process.

It would be impossible to vent all of the gas and if you could there would still be pressure present.

As for the pressure dropping rapidly that would depend on the volume of oil. As I mentioned before the saudi wells vented naturally for something like 20 years before water had to be pumped in.

When you've got a well that contains billions or hundreds of millions of barrels of oil it can take a long looonngg time for the pressure to drop. Eventully it will reach equilibrium and it will be a case of the remaining oil floating out of the well with no pressure behind it then it will be easy to stop. But noone wants to wait for eventully.

Biggest hurdle to overcome is depth, on normal offshore rigs divers can go down and weld the wellhead shut to stop the flow. Everything thats been tried works perfectly fine for normal depth wells. But theres no technology that they know will work so deep. They are trying to invent a solution as they go.