I also think I've also replied this to you before, but +1 for the GAO report! I use the full report as my personal model yardstick on how I document technical decisions/issues for non-technical people. The PDF is attached here:

https://www.gao.gov/assets/gao-26-106918.pdf

People like to be dismissive about civil servants without looking at how much extremely high quality work is actually produced on complex subjects. Personally having interacted with them a couple of times I'm also a fan of the UK Parliamentary briefings: https://commonslibrary.parliament.uk/

(unfortunately they can't make people read the briefings ..)

Usual problem, people don't understand what they do so they undervalue what they do or think it is "simple" - almost nothing involving millions of people and their interests is ever "simple".

I get somewhat annoyed when people blanket attack Whitehall (the UK Civil Service) universally i.e. the bureaucracy, yes they have problems but you don't run a country with 70 million people without some bureaucracy and ripping apart a system that has worked for centuries means throwing out a lot of good with the "bad".

It is funny seeing how the estimates of the lower end stability vary drastically. Have seen numbers between 70 and 150 million barrels. I have just called it half way between that and think 110 million barrels is probably the lowest they should go.

I think there's a statuary limit at 150 million because there must be an "emergency military reserve" but the caverns start collapsing at 70 million.

Thank you for this, that would explain why difference field are talking different numbers.

This is the most detailed breakdown of the reserve contents, updated weekly. the sweet/sour differential isn't listed on any other data sources for some reason, and if you want a longitudinal record you'll have to make your own.

https://www.spr.doe.gov/dir/dir.html

We can't pump saturated salt solution to prevent more salt from being dissolved?

Seawater is 10% saturated IIRC, so you'd have to make the saturated solution first, and if the easiest way to do that is to dissolve another cavern then using brine doesn't have much of a cost advantage over "use this one until it collapses, then dissolve the rubble or make another."

I disagree that there is no cost associated with making another. Externalising environmental and rectification cost should not be the default approach.

If you're gonna build a bunch of tanks on the surface to store that saturated salt solutions, you might as well store the oil in those tanks. The point of this salt cavern system is how little infrastructure it needs to store the huge volume of liquid.

Yes but the risks associated with storing brine are far less than oil. Brine leaks out? It might kill the flora nearby but rain will eventually wash it away. Someone blows up the brine tank? Divert some freshwater until you can rebuild.

But in reality, storing all of that brine would be a massive undertaking on its own. The largest salt cavern is 37M barrels which equates to a circular pond that's 600m across and 20m deep. That's huge. And of course you don't want the brine seeping into the ground water so it needs a good liner. But I suppose you really don't need a pond that big, how often are we completely draining the cavern anyway? We're at half capacity right now and that's the lowest it's been in 50 years. Normally it's been much much higher. So you really don't need all that brine, just enough for the normal ebb and flow of the reserves, maybe like 10%. In the event the reserves get drained, divert some freshwater to compensate.

Do they really have brine storage tanks for enough brine to pump out all the oil? If true, they would have just exectly the right volume of above-ground tanks to just store the oil in the first place and not mess with all these caverns and brine and whatnot.

That doesn't solve the someone-might-blow-it-up design problem cited in TFA.

They most likely truck in the brine from a mixing facility to reclaim the oil and truck out the brine to disposal when refilling the oil stock.

You could store the salt. You go up far beyond saturation. Salt sediments on the floor (which also might seal it). When you need brine you pump in seawater into a tunnelsystem in the sediment. The saturated brine should be lighter and further up. Overflow to second basin, where it's pumped into the cavern. Thus size comes down to whatever size is needed to store enough salt and or, be big enough to guarantee a certain rate of volume of fully saturated water per time; so basically dimensions that guarantee a phase separation and high enough diffusion rate. And some bad ass pumps (or have the basin be below sealevel and a dam with gates; probably the pumps for reasons of security>efficency in that case)

> as well store the oil in those tanks

Salt does not burn, unlike oil. It would be sufficient to store dry salt, and prepare the brine during the pumping. Still a lot of hassle, of course.

Please, literally every other country in the world store their petroleum reserve in surface tanks. We have fifty years of experience in designing the safety systems around oil tanks, and the only recent major incidents are from drone strikes in Russia and Saudi Arabia. Just because the US luck out in having the specific geological formation to store oil cheaply doesn't mean the alternative is untenable.

> It would be sufficient to store dry salt

Absolutely impractical. Storing and moving around solids in the megatons is very difficult compared to liquids that can be pumped. For comparison, the absolute largest solids moving operation in the world is a copper mine in Chile, it moves 300,000 metric tonne of mineral per day. The east-west pipeline in Saudi Arabia can move three times that in liquid, 1,000,000 metric tonne per day.

This method appears to be, in nearly every way, far better suited for purpose.

Just spitballing here, can't you just store the salt alone and make the hyper-saturated salt water on the fly? That seems reasonable.

How do you get the salt to make the brine? You'd need to evaporate the water from the brine which means you either need very large evaporation pools or lots of energy to boil the water (and money to maintain the immense amount of scale you create).

Build a water desalination plant next door. Make it a twofer

Agreed, considering that sor.say future wars will be around (drinking) water it seems reasonable to at least link some cost to the brine or the water required.

Salt mines.

Here’s one in the USA:

https://youtube.com/shorts/zXzvUKXhMBo

Here’s a list of salt mines in the USA:

https://en.wikipedia.org/wiki/Category:Salt_mines_in_the_Uni...

They used an even easier method than what's described in the other comments: the salt comes from the initial construction of the SPR caverns.

When they first built the SPR, they filled it with water to wash away any easily-dissolvable salt features as well as any impurities inside the cavern. The result of filling the cavern and letting it sit for a while is saturated brine: eventually all the salt that can be dissolved is dissolved. Then they pumped that brine to aboveground brine ponds.

Those brine ponds contain exactly the amount of salt water needed to completely fill the cavern with saturated brine. So future drawdowns just use the same water that has already been saturated. Some of the water will evaporate on the surface, leaving behind salt crystals at the edges of the brine pond, but the crucial thing to remember is the quantity of salt in the newly created brine pond is equal to the amount of salt needed to generate enough saturated brine to completely fill the cavern. Because that's where it came from in the first place. So if you need to replenish it with more water, you just pump in fresh water, it dissolves the extra salt in the brine pond, filter out any surface guck, and then pump it back into the cabin.

Periodically you might want to add a little extra salt if say the cavern expands or wind erosion carries salt away from the dry brine pond bed. But this quantity is much less, it's the delta in cavern size, not the full initial size of the cavern.

Amazing -- appreciate the explanation.

You don't need tanks to store the brine, just a huge pit/pool. If you look at satellite imagery, that giant pit with brine in it is exactly what they have next to the storage facilities.

They do; https://news.ycombinator.com/item?id=49735585

Hard to find that

I don't know, the article mentions having to deal with brine. Let it evaporate in a pool and you've got plenty.

It would be cool to see what these look like in a photo but I guess they're never empty of liquid. I wonder what happens to the oily water after we use it? Do we boil it until it evaporates?

There’s a 3D model of the caverns here https://www.spglobal.com/energy/en/news-research/blog/crude-...

Wow that is very very cool. Thank you very much for digging that up & sharing the link! Sometimes visuals like that really help me process information.

There was a good comment on r/oil by someone who worked on the engineering for the SPR. Unfortunately it's fallen off the front page and I can't find it now, but:

> Also, the more water we pump in, the more salt leeches into the water and the more the bottom of the chamber deforms, creating shear forces in the upp walls that lower its overall structural integrity.

It turns out that it's trivially easy to prevent the water from leeching out the salt. They presaturate the water with salt, so that it already contains the maximum amount of salt that can be dissolved for a given temperature. When they pump the brine out, it goes into aboveground brine pools that can easily be seen on satellite imagery. The salinity of this brine can easily be controlled: if you want to expand the cavern, you mix it with freshwater so the brine dissolves more of the edges, if you don't you pump it back in as brine.

Edit: Here, found some satellite imagery of one of the SPR sites:

https://maps.app.goo.gl/B8XJ9TVhQfcdErky5

You can clearly see the brine pond, which is as big as the aboveground portion of the SPR itself.

Are the round things brine ponds? They look like floating roof tanks

They look like above ground pits. Empties are showing the white paint and ones with brine look dark.

They’re oil tanks. The ponds are open air ponds.

I was unaware of this; that's important information and I'll keep it in mind going forward.

> the brine pond.

Then why not store the oil there directly instead of going all the trouble of pumping it in and out of the caverns? Is it so it doesn't rain on it?

Crude oil is sketchy stuff. All sorts of VOCs, etc evaporate from it. Some valuable, some, flammable, some will merely kill you. Keeping it confined makes all that more manageable.

TFA lays it all out, oil out in the open is exposed to attack/lightning strikes/etc.

Are you advocating for long term crude oil storage in open air oil lakes?

If the sides of the pond collapse (which happens a lot with mine tailings), then you have something resembling a oil pipeline rupture (which also happens a lot, but some states make it illegal to get video footage of it) which is a rather toxic situation needing soil to be bulldozed and either buried or burned.

Sun, rain, hurricanes, wind, debris is not friendly to long term oil storage.

Huh, of course. Like in salt farming, after harvesting the salt they wash it with saturated brine so it doesn’t just dissolve again.