Joule-Thomson and cooling

[marzolian]

A friend who never studied engineering but is very curious sent me a blog entry, claiming that the methane being discharged from the BP blowout was cold enough to freeze seawater. I think someone is confusing water freezing with hydrate formation.

But I started wondering: What approach would you follow if you wanted to see how cold the methane would get, if it was flowing through an orifice and allowed to expand?

Of course, this does not represent a practical approach to solving that or any other real-life problem. But assume the following:

1. Homogeneous flow (methane only, no oil or other gases).

2. All pressure drop occurs at the wellhead. Ignore the resistance inside the well.

3. The pressure upstream of the restriction is the same as the downhole formation pressure, possibly 16,000 psi.

4. Initial gas temperature is probably between 300 and 400 degrees F, assume 350.

5. The methane flows adiabatically through the wellhead, and expands to the water pressure (approximately 2200 psi). This is another significant assumption, because the pressure at the bottom of the riser / top of BOP stack is undoubtedly higher.

What would be the temperature of the methane, before it mixes with the seawater?

I know that Joule-Thomson is involved, but bear with me, it's been 25 years since I studied this subject and I haven't worked with it since then.

Can anyone give a back-of-the-envelope answer?

Steve Marzuola
Houston, Texas

 

[BigInch]

Why start on a track that will give a wrong answer. First, its no secret that high gas flow expanding to low pressure would undoubtedly go lower than 29F and be able to freeze seawater, but it won't ever happen on a grand scale, because the cold gas would be mixing with what would basically be an unlimited amount of warmer temp sea water around it and the water would never reach but a degree or so below its average temperature at that depth anyway. Otherwise I would assume that the blog assertion is totally reasonable and you could even reach 29F, if the mixed quantity of water was limited to small amounts.

So now, why do a fictional calculation to prove what is most likely the result with the highest probability. At least start with reasonable assumptions before you waste time with the Joule-Thomson calculation.

BTW, hydrates form at higher temperatures at those pressures.

Secondly, 16000 psi at the wellhead is very (too) high. The weight of the oil+gas in the well column is substantial and the static head will reduce the bottom hole pressure considerably as it moves up to the mud line, plus the frictional drop from flow through the well is also significant, with the final resulting pressure just under the BOP, nothing like 16000 psi. I'd guess Perhaps 1/2 that or so. Maybe 8000 psi would be a better starting point. There are likely two combined effects of 2-phase orifice flow; liquid heating plus the gas cooling as they cross the orifice, thereby nulling a considerable amount of the temperature drop you would get with your assumptions.

Now go for a JT solution to the gas flow, add frictional heating for the oil and mix it with only enough sea water to be sure that it will freeze and you'll get the same result as your friend's blog.

**********************
"Being GREEN isn't easy" ..Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com/

 

[marzolian]

BigInch, thanks for your answer. By the way my buddy asked the question, I don't think he's following the news closely and thought that the containment vessel had been plugged with (water) ice. Also, I'm pretty sure that whatever he was referring to was not a technical description. (But he can be cryptic).

Anyway, it's not a real answer I'm looking for, it's a review of how to do a quick-and-dirty Joule-Thomson calculation. I was hoping somebody had done it already, or would have the necessary tools close at hand. If it's an unreasonble request, then never mind.

Since the blowout began I have been thinking about other blowouts, some that I have gone to see in person (I grew up in the oilfield) and others that I have read about. For instance, I once saw a documentary called the "Devil's Cigarette Lighter", about a gas well fire in North Africa in 1962. It's now viewable online, part 1 is here:

http://video.google.com/videoplay?docid=5895560222315361755#

Steve

 

[BigInch]

Sorry, I haven't actually done any JT calcs in a long, long time, so I don't have a spreadsheet template, or anything handy myself... but maybe someone else will. I was really just suggesting you do a little cleaner calc. Its not too hard to make it cleaner and give you something that maybe you could believe.

In the meantime, you can see JT here,

http://en.wikipedia.org/wiki/Joule-Thomson_effect

**********************
"Being GREEN isn't easy" ..Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com/