The Question That Everyone Gets Wrong (Including Me)

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

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.

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!

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

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.

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.

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..

I’m a scuba diver. And with my experience and with playing with soda bottles sealed and un sealed bottles. So I knew there would be a change in pressure. Great job. Thank you 😊

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.

Yup. I got it wrong.

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.

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

Glad to have got it wrong in exactly the same way you did haha.

And that bubble pressure increase was then probably also the reason for the oil rig explosion in some video I cannot locate anymore, where they are drilling the hole and suddenly they scream they hit an unexpected gas pocket and need to abandon the area, seconds later greyish-brown thick liquid erupts from the hole and covers the whole area, including a nearby city. Or was that one of my strange dreams many years back that I now falsely assume having been a video? I even remember clearly how some of the shots in the video looked, for example a street light next to a low, white wall in the dark under a tree in the rain.

Who else laughed when he said "It looks like a bomb or something" haha

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.

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

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

According to the formulas: Case 1: When the bubble is at the bottom Its P = h*d*g ; h= height of vessels or tube, d= density of liquid, g means gravity When the "air bubble" gets on top the pressure becomes P = P(atmosphere) + h*d*g

Also without the bubble at the top, the surface tension of the water will prevent the water from falling (even if the other side is open-ended), that is off-loading the weight of the water to the tube itself, causes a decrease in water pressure overall.