The Question That Everyone Gets Wrong (Including Me)

I will be at Open Sauce The 14th-16th of June!

[humanadam]

I am a drilling engineer, we have to get certified in well control before doing any operations. This one is a huge topic.

[stevenjones]

A good analogy is to think about standing on a spring, compressing it. You are pressing downward with exactly your weight, and the spring is holding you up. Place a ceiling directly above your head, so your head is almost touching it. Now step off the spring, and you're still pushing against the ground with exactly your weight, same as before. And now, to simulate the pressurized air being at the top of the tube, we put the spring on your head. The spring isn't compressed by your weight anymore, so it expands and presses against the ceiling, and thus pushes down on you. So now you're pushing against the ground with your weight PLUS the force of the spring pushing you downward by trying to expand against the ceiling.

[egycg]

This is a really well established fact for any petroleum engineer because the well can receive a gas influx from the formation that we call "gas kick", this gas rises up the well which ((increases)) the pressure on the bottom formation which can cause it to fracture, so the most dangerous kick is the gas kick.

[nymalous]

I like that you put several seconds of black screen at the end so that the automatic video suggestions don't block anything important.

[maxie]

Petroleum engineer here. This is very easy to understand with mathematics, perhaps with the help of pressure gradient chart (P vs depth). But seeing it visualized in real experiment like this is a delight. Thank you.

[brainwashingdetergent]

Old oil and gas guy here. I remember leaning this in one of the countless hours of training and education that we had to maintain to keep our field certs. I was in frac, and we didn’t have to worry so much about gas bubbles in the day to day pumping operations in the formations we were pumping into. We just needed to be aware of such phenomenon. Thanks for reminding me of it!

[foogod]

Another way to think about this: When the water is above the air, gravity is pulling the water down onto the air, which exerts a compression force which is opposite to the pressure of the air pushing outwards. Therefore, part of the air's pressure is being counteracted by gravity, and the total net force from the air's pressure is reduced by that amount.

When the water is on the bottom, its gravity is no longer trying to compress the air, so there is no force to counteract the outward air pressure, so the pressure it exerts on the _container_ becomes higher.

It is very counterintuitive, though..

[Oberbremser]

There was one flaw with the question at 0:02 as the right side of the pipe looks closed and the left side (highest) point looks open. Hint would be helpful to mention a closed system. With that I went in wrong direction but finally with recognizing a closed system I couldn't solve correctly as well, but it would have saved some time.

[tomszabo]

Simplest explanation by analogy: what happens if you hold your breath while you ascend from a deep dive? The air in the lungs will want to expand. The exact same thing is happening here. Air is being compressed by water and if that compression is removed without the air being allowed to expand then that air will apply higher pressure against the container that is confining it.

[andrewliang]

For bubble on right side:
Pr = pw * g * h1 = pw * g * h2 + Pb

h1 = height of water measured on the left
h2 = height of water measured from the right
Pb = pressure from air pocket
pw = density of water
g = gravitational acceleration


For bubble on left side:
Pl = pw * g * h3 + Pb = pw * g * h4

Isolate and solve for Pb:
Pb = Pl - pw*g*h3 = Pr - pw*g*h2

Pl = Pr + pw*g*(h3-h2)

Since h3 > h2 (seen visually in the video), we know that the pressure at that point when the bubble is on the left side will be greater.

[alexrvolt]

I found the correct answer with a slight variation of the reasoning of the video: as the air goes up, it "should" undergo an expansion because it "should" be subject to a pressure decrease.
But the volume being fixed, the expansion is constrained: the pressure of the air stays the same: the intuition that the pressure doesn't change is correct, but it applies to the air, not to the whole system.
Therefore, with the same pressure of air, the pressure at the bottom is higher when the air is at the top because you have to include the weight of the long column, while the initial situation only had a small water column added to the air pressure.

[forstig]

The thumbnail, which was at one point also part of the video (3:20) suggests, that it isn't a closed system which is confusing. Without the closed top I would even expect the pressure to decrease.

[jamesblackwell]

I was very surprised. I thought the pressure would stay the same since it's a sealed system.

[jkliao]

The trick is to find the reference point that has the constant pressure. When the vessel is open, it's the atmospheric pressure at the opening that is constant. When it's sealed however, due to liquid being almost non-compressible, the air bubble is now the constant pressure.

[MCHorner]

The movement of the gravitational potential energy was the key for me in fixing this concept in my mind. Great examples to facilitate comprehension. Thanks.

[blarghchan]

My brain wasn't registering that the tube was completely sealed. Which means now we need to see this with the top end open.

[TheElCogno]

I would say it increases. Water is incompressible so its volume is fixed, and therefore the volume of the gas is fixed too. Assuming that the temperature of the gas is fixed as well, then so will be its pressure. So when the gas is moved to the top, it pushes on the water below it with the same pressure it had at the bottom, so the extra water column remaining will increase the pressure at the bottom.

[josephbledsoe]

Chemical engineer here, I think the easiest way to perceive this is to (as always) examine the extreme case. If you were to run this experiment with a column >> 10m then the weight of the water would exceed atmospheric pressure (assuming your gas bubble was injected at atmospheric pressure). You could then imagine at the top, a vacuum would be created, because the force of the water is now stronger than the gas bubble (the gas bubble would compress). This would be an area of zero pressure pushing down on the top of the column of water. Let the bubble rise and now the vacuum has been replaced with a non-zero pressure pushing down on the same column of water.

[jonathanchapple]

I know this from working in the automotive brake industry. A small amount of air in a brake line could dramatically affect the stopping distance ( by several feet). So your stopping force is proportional to the pressure applied, (for tmoc system actually a lot more force than the applied pressure, but at a certain point you get to the knee point and it eventually becomes 1-1 input to output force). Having to bleed brake systems for basically a cubic millimeter of air is very annoying depending on the type/design of the calipers. Some types seem to be prone to getting air stuck. Thats why straight off the assembly line usually has the best brake performance due to manufacturers using a push/pull method to fill the brake line when the system started off dry. That is usually better at getting any air out of the system.

[westonding]

That is very counterintuitive but with the spring example, it makes sense!