Oil spill: Water pressure VS. oil pressure

[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

 

[pallidin]

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

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

 

[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 ?

 

[pgardn]

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 ?

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 don't fill it with air that has the same surrounding pressure as water at 1 mile deep. This is interesting though as I don't 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.

 

[proculation]

I'm not talking about the situation but about the physics involved.

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.

 

[pgardn]

I'm not talking about the situation but about the physics involved.

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.

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 don't 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 don't know. I guess someone else can wittle away the enormous number of variables and come up with something.

 

[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.

 

[proculation]

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.

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.

 

[pgardn]

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.

Do you have a link for this?

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

 

[stewartcs]

I'm not talking about the situation but about the physics involved.

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.

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 canceled out by the seawater hydrostatic head at the point of discharge.

CS

 

[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.

 

[rcgldr]

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.

Do you have a link for this?

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

 

[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.

 

[rcgldr]

First, is the pressure differential between the well and the surrounding seawater the result of the methane coming out of solution in the oil? The other possibility is that the pressure differential is just caused by the buoyancy of the oil.

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.

 

[XLR8]

Before the well was drillled, that oil was under very high pressure, unrelated to methane gas content or buoyancy.

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.

 

[Andy Resnick]

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.

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-underwater-oil-plumes-bp-ceo-tony-hayward-denies-scientists-research

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

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

 

[XLR8]

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.

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.

 

[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

 

[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.

 

[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 a lot of time and pressure to form oil.

Injecting something into the well to stop the flow is a lot more complicated than pumping something in you have to overcome the outward pressure first and have something that will withstand it and it can't be a solid it has to be a liquid or a gel which will harden later

 

[stewartcs]

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.

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

 

[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.

 

[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.

 

[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.

 

[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 no one 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 that's been tried works perfectly fine for normal depth wells. But there's no technology that they know will work so deep. They are trying to invent a solution as they go.

 

[pavlograd]

I'm not a formal engineer, but I have a long career in the oil business. Let me explain things this way... If one wanted to pump oil from the surface DOWN to the ocean floor 1 mile deep, they would need to OVERCOME 2400 psi of pressure to do it. The same is true of the the oil coming out of the riser pipe. The analogy of the floating soccer ball is not the same situation. If you wanted to pump AIR down to that depth, it would also need to be compressed to higher pressure than 2400psi.

Now, here is the point you guys are missing. When oil is 'struck' the first thing they look at is the FLOW RATE and WELL HEAD PRESSURE. Oil wells which are considered high pressure come in at 1200-1600psi, so to overcome 2400psi of water pressure, this well has to be more than that. Also, in order for it to blow out all THREE of the safety valves, it has to be MUCH more than that!

Question: Does anyone know how DEEP they drilled the well into the ground? Does anyone know about the Russian well that Lukos Oil drilled which went down to 43000feet deep (on land, not from a floating platform!) that encountered extremely high blowout pressures and Russia had to use a small nuke to 'seal' it?

I believe that we are dealing with this same situation here and that BP SHOULD NOT have been allowed to drill such a deep well (theirs is, I've heard, 35,000 feet into the ground!) and it could be a disaster in the extreme! Consider all the thousands of pounds of 'mud' they pumped in when they tried the 'top kill' - and this was pumped in under very high pressure and the oil pushed right through it!

And, more dangerous than the oil are the hydrogen sulfide, benzene, and methyl chloride gases coming up with it WHICH ARE FATAL TO HUMANS in over 10 ppb (parts per billion). AND, those plumes we are hearing about? I believe that his oil that is finding its way underground and popping up in other places!

THEY ARE NOT TELLING US THE WHOLE STORY, TRUST ME.

 

[stewartcs]

Now, here is the point you guys are missing. When oil is 'struck' the first thing they look at is the FLOW RATE and WELL HEAD PRESSURE. Oil wells which are considered high pressure come in at 1200-1600psi, so to overcome 2400psi of water pressure, this well has to be more than that. Also, in order for it to blow out all THREE of the safety valves, it has to be MUCH more than that!

By safety valves I suppose you mean BOPs. First off, we don't know if all of the BOPs were closed. Most likely they only tried the blind shear ram from the surface before the rig sank (and probably the annular too). The other three pipe rams probably where not shut and are definitely not plumbed into the ROV intervention port (not all of them anyway).

Besides that, they are all rated to 15,000 psi. Therefore if one failed due to a pressure higher than their rating, they all would fail regardless of how many more of the same rating were there.

Question: Does anyone know how DEEP they drilled the well into the ground? Does anyone know about the Russian well that Lukos Oil drilled which went down to 43000feet deep (on land, not from a floating platform!) that encountered extremely high blowout pressures and Russia had to use a small nuke to 'seal' it?

Yes the well was a total of 18,000-ft (~5k water, ~13k earth).

Yes, we know about the russian's nuking that gas well. It's all over YouTube.

I believe that we are dealing with this same situation here and that BP SHOULD NOT have been allowed to drill such a deep well (theirs is, I've heard, 35,000 feet into the ground!) and it could be a disaster in the extreme! Consider all the thousands of pounds of 'mud' they pumped in when they tried the 'top kill' - and this was pumped in under very high pressure and the oil pushed right through it!

30k wells are drilled all the time in the GOM with no problems. This one was due to human error not the physics of the geology.

And, more dangerous than the oil are the hydrogen sulfide, benzene, and methyl chloride gases coming up with it WHICH ARE FATAL TO HUMANS in over 10 ppb (parts per billion). AND, those plumes we are hearing about? I believe that his oil that is finding its way underground and popping up in other places!

How would the oil find it's way underground from a plume?

THEY ARE NOT TELLING US THE WHOLE STORY, TRUST ME.

I think there are a lot of details missing about what happened. But I think for the most part the general specifics about the well is known.

CS

 

[tyroman]

The attached text document is a copy I made of a June 7 post by "bignerd" at TOD (The Oil Drum). Sorry I do not have the exact thread link (should have copied the "permalink") but you may find the post at;

http://www.theoildrum.com

The post is bignerd's compendium of well data which he has gleened from various sources... I offer it as a point of departure for discussion.

.

 

[Yankus]

I also agree somewhat with pavolgrad and somewhat with stewart cs.

And just so everyone doesn't blow me off as someone sticking my 2 cents in, my father is the CEO of an oil campany, has been in the industry 30+ years starting on the rigs in GOM and contracts to my countries government on various projects.

And If anyone wanted exact numbers (depth, pressure, BOP ratings, maybe even gas shows and an estimate reservoir size) I could probably get them.

 

[Yankus]

May I also add for pavolgrad, BP wasn't drilling anywhere. BP owns the rights to the seabed in that area, Trans Ocean was doing all of the drilling. The final decision to check the BOP was Trans Oceans. I don't know if any oil company drills their own wells, the drilling is usually handled by contractors.

So if you wish that Trans Ocean had not drilled (on behalf of BP) a well so deep (and stewartcs is right there's quite a few deepwater wells that are perfectly fine (accidents happen in this industry)) I suggest you petition the US government to stop selling rights to deepwater areas.

 

[stewartcs]

May I also add for pavolgrad, BP wasn't drilling anywhere. BP owns the rights to the seabed in that area, Trans Ocean was doing all of the drilling. The final decision to check the BOP was Trans Oceans. I don't know if any oil company drills their own wells, the drilling is usually handled by contractors.

So if you wish that Trans Ocean had not drilled (on behalf of BP) a well so deep (and stewartcs is right there's quite a few deepwater wells that are perfectly fine (accidents happen in this industry)) I suggest you petition the US government to stop selling rights to deepwater areas.

I agree that Transocean is partly responsible. They are actually the ones who are responsible for the safe operation of the vessel and for the safety of the crew (the OIM or Master has legal authority over the oil company).

BTW, Petrobras owns their own rigs and drills with them (in Brazil at least). I'm sure there are probably others as well...maybe smaller companies though.

CS