Salt contains the oil and helps seal small fractures. The rock salt surrounding the SPR’s caverns has extremely low permeability, meaning fluids have very little ability to pass through it. It also doesn’t react with petroleum. Under enormous pressures underground, salt also slowly deforms, which helps close small fractures. The salt itself can therefore contain the oil without a steel-and-concrete tank lining the cavern.
Oil floats on water, which means pumping water into the bottom of the cavern pushes the oil out. As fresh water is pumped into the bottom of the cavern, the oil gets displaced upwards into a delivery system.
The more water you pump in, the lower the quality of the oil that's pumped out; we are basically loaning the oil refiners money to buy SPR oil they don't really want. 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.
I've posted this before; it's a recent report on the status and maintenance of SPR facilities, which is very clearly written and well worth your time if you want to understand the topic better: https://www.gao.gov/products/gao-26-106918
nostrademons 9 hours ago [-]
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:
You can clearly see the brine pond, which is as big as the aboveground portion of the SPR itself.
raducu 16 minutes ago [-]
> 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?
anigbrowl 6 hours ago [-]
I was unaware of this; that's important information and I'll keep it in mind going forward.
jordanb 7 hours ago [-]
Are the round things brine ponds? They look like floating roof tanks
greggsy 11 minutes ago [-]
They’re oil tanks. The ponds are open air ponds.
Kerbonut 5 hours ago [-]
They look like above ground pits. Empties are showing the white paint and ones with brine look dark.
aaron695 3 hours ago [-]
[dead]
nxobject 11 hours ago [-]
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:
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?
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."
seb1204 6 hours ago [-]
I disagree that there is no cost associated with making another. Externalising environmental and rectification cost should not be the default approach.
chabska 9 hours ago [-]
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.
wildzzz 8 hours ago [-]
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.
tedd4u 6 hours ago [-]
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.
fourthark 3 hours ago [-]
That doesn't solve the someone-might-blow-it-up design problem cited in TFA.
Kerbonut 5 hours ago [-]
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.
nine_k 8 hours ago [-]
> 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.
chabska 6 hours ago [-]
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.
dd8601fn 3 hours ago [-]
This method appears to be, in nearly every way, far better suited for purpose.
justinator 9 hours ago [-]
Just spitballing here, can't you just store the salt alone and make the hyper-saturated salt water on the fly? That seems reasonable.
wildzzz 8 hours ago [-]
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
seb1204 7 hours ago [-]
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.
lelandbatey 8 hours ago [-]
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.
sokka_h2otribe 10 hours ago [-]
Hard to find that
mixmastamyk 10 hours ago [-]
I don't know, the article mentions having to deal with brine. Let it evaporate in a pool and you've got plenty.
FlowingRiver 11 hours ago [-]
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.
anigbrowl 6 hours ago [-]
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.
I’m certain engineers have considered this, but it doesn’t seem answered anywhere: why not just pump the original brine back in to move around the oil? Wouldn’t that at least prevent eroding the storage unit and causing it to fail?
Unrelated: dang, he wasn’t underselling this being a pretty elegant solution! I never would’ve thought of it, for sure.
aesthesia 10 hours ago [-]
Wouldn't you then need to solve the similar problem of where to store the brine in the meantime?
kulahan 8 hours ago [-]
Yeah, but that’s probably easier to solve than eventually losing the whole dang structure!
samus 4 hours ago [-]
Yes, that's what they do. The brine is stored in huge pools next to the extraction site.
Ellis_dev 11 hours ago [-]
Makes you appreciate the physical infrastructure challenges, not just software. Bet the data logging and monitoring systems are ancient but robust.
bt1a 11 hours ago [-]
Yea man the federal government is on top of protecting civilian groundwater supplies and communicating discoveries of contamination
walleeee 9 hours ago [-]
I know you jest, but the US govt funds a lot of water science. Whether that's put to use in the public interest is another question ofc and one we should be asking
tmellon2 7 hours ago [-]
Hmm...The Math does not hold up. 300 feet diameter circular area is 1.62 acres. So at max you would need about 1130 x 2 = 2260 acres and not 45000 acres as projected !
From the post :
"300 feet in diameter..."
"To hold 714 million barrels of oil (the full capacity of the Strategic Petroleum Reserve), you’d need about 1,130 of these tanks. If you use the standard capacity of the largest commercial petroleum farms (e.g., in Cushing, Oklahoma), you need about 45,000 acres to store these tanks."
Retric 6 hours ago [-]
You can’t tile circular objects at 100% efficiency and you definitely don’t want these tanks to touch.
You further need space between them with each other and the edge of your property for a host of other reasons such as being able to inspect and paint each tank. Thus looking at actual facilities is going to give you vastly more resources options than just looking at the footprint of each tank.
tmellon2 3 hours ago [-]
You have a point. However, the requirement estimate of 45000 acres is off by 10 times ! I have seen some of these tanks - even accounting for spacing between them; at best one needs maybe a total of 5000 acres.
It doesn't really matter. 45,000 acres for a tank farm, or multiple tank farms, would be basically free by the standards of this project. Yes it's "an area the size of Washington DC" but it wouldn't be in Washington DC, where rent is expensive, it would be on some low-value desert land that the military might otherwise use as an artillery range.
Storing it in underground salt caverns is more secure, not necessarily cheaper.
Retric 3 hours ago [-]
Being realistic is the opposite of being off.
As the article points out fire is a real concern here, which is one of many reasons making salt caverns so appealing. But it’s also a major driver for distance between each above ground tank, you want to isolate risks so a single failure doesn’t propagate across the facility.
LPisGood 10 hours ago [-]
One fun fact I’ve heard is that the strategic oil reserve requires like 100-150 million barrels of oil to maintain operational pressure and even be functional. This means we can’t draw it down to zero; not even close.
Why can't they pump more water down there to create pressure?
zachlatta 8 hours ago [-]
This is so incredibly interesting and one of the best things I’ve seen on HN recently. Thank you for the awesome post!
hankbond 12 hours ago [-]
Very cool, big fan of these bite size explainers. Has there been more cycling recently? Are the sizes of the caverns tracked anywhere?
FlowingRiver 10 hours ago [-]
Funnily enough, I guess the more you cycle them, the bigger they get until they just collapse. So you have a lot of potential storage just before you have none.
bt1a 11 hours ago [-]
id imagine if you have the location of a site that the largest pipes visible from satellite photography would be
consistently scaled in some manner with the tank size. this is guess
analog31 11 hours ago [-]
An interesting aside is that the process for making these salt domes is also used for mining salt. Drill a hole, pump water down, and up comes brine. The brine can be used as a chemical feedstock, e.g., for production of things like chlorine, sodium hydroxide (lye), and sodium carbonate for glass.
What do they do with the brine when they fill it back up with oil?
And do they use fresh water?
cyberax 9 hours ago [-]
One thing about salt domes: they almost by definition are surrounded by very low-permeability dry soils. Because otherwise they would have dissolved long ago!
And a related way to use salt domes is for natural gas storage.
tgsovlerkhgsel 8 hours ago [-]
Another related way is to store nuclear waste. The properties (as described in this article, impermeable/self sealing etc.) sound great until the reality that water dissolves salt comes knocking and further confronts you with the reality that salty brine corrodes nuclear waste barrels...
CamperBob2 9 hours ago [-]
And even helium, which is famous for its ability to leak out of almost any container you put it in.
US has some of the world's largest natural gas deposits, and is a major producer of helium, so it makes less sense to have a strategic helium reserve. Even the SPR was created in the aftermath of the last middle east oil crisis, back when the US was a major net importer of oil, and many people have been saying we should scrap the SPR, as no major oil producer has an SPR.
The problem with the SPR is that it is used to manipulate prices, which was why Biden on the one hand cut oil leases and reduced domestic oil production, but then did major SPR drawdowns before the election to hide the realities of these policies. Now Trump is playing the same game, sanctioning oil producers and creating wars in the middle east, but using the SPR to hide the effects of these policies until after the midterm.
In both cases, we'd probably be better off without an SPR, so all of these actions against oil production would be immediately felt at the pump before the election and voters could respond quickly. It's just too tempting to have this tool to manipulate oil prices that incumbents can use before elections.
qlte 5 hours ago [-]
Huh? The oil price shock in 2022 and resulting SPR drawdown had nothing to do with oil leases. Prices almost doubled worldwide in the span of a week after Russia invaded Ukraine. Look at any chart of petroleum products and it's clearly visible as a huge, almost vertical line.
The explicit purpose of the SPR is to function as a lever to smooth temporary supply shocks. The US increasing oil output by a few percent in either direction wouldn't have made any difference to global disruption of oil markets from a major exporter to Europe starting a war and then getting sanctioned.
ivraatiems 11 hours ago [-]
How long does the crude last uner these conditions? Refined oil products like gas have pretty limited shelf lives (I think 6-12 months for gas).
aqfamnzc 11 hours ago [-]
Under ideal conditions - about 200 million years at least. In the salt, I'm not sure.
nostrademons 8 hours ago [-]
In the salt it's pretty similar. The salt caverns are basically just like the conditions where the early, easily-exploitable oil deposits ("gushers") were.
buckle8017 11 hours ago [-]
well it's been underground in similar conditions for a couple dozen million years, so I think a while
thrawa8387336 10 hours ago [-]
One time I heard the limited shelf life is by design to avoid consumers hoarding. Diesel OTOH fares better
jaggederest 8 hours ago [-]
No, it's because gasoline is not a very thoroughly refined chemical, so it basically has a bunch of aromatic rings, benzene, and all sorts of monomers floating around, that rapidly (~year or two) turn into varnish and other polymerized nastiness when they oxidize and crosslink. I believe gasoline was originally a refinery byproduct that was used because it was so cheap as a fuel.
Diesel and other heavier fuels generally are much closer to aliphatic hydrocarbons (straight chains, not aromatic) and so are much more storable, because aliphatic hydrocarbons have much less opportunity for oxidation (they're like saturated fat, i.e. much more stable).
You can get polymerization inhibitors to put into gasoline if you want it for long term storage, with the caveat that they produce some nastiness at the bottom of tanks that you'll have to avoid when pumping it out, and I think they can also slightly reduce the octane rating.
calmlynarczyk 9 hours ago [-]
This is a pretty popular topic this past month, huh?
Interesting that these articles don't mention China's storage capacity, which is bigger and more crucial these days.
Maybe China publishes less official knowledge?
treebeard901 5 hours ago [-]
I always thought the smartest thing to do since countries chose to accept fiat currency in exchange for oil was to create as many patroleum reserves as possible and print as much money to buy as much oil to store for later use as they would sell.
sh34r 3 hours ago [-]
It was a great plan, but all it took to foil it was one orange man in the Epstein files.
redwood 10 hours ago [-]
Dumb question but why not pump it back into where it was pulled from.. or just leave it in a a reserve paid for by the gov
schainks 9 hours ago [-]
Brine is useful to sell to industry.
AIUI, salt caverns don’t like to be filled with brine because they become more susceptible to temperature changes of the earth and pressure from surrounding rock. Both can heat the cavern, which is bad for structural integrity.
Loudergood 9 hours ago [-]
This way the US Gov can be a buyer of last resort. Backdoor bailouts.
hiring_signals 9 hours ago [-]
[flagged]
recursivegirth 10 hours ago [-]
[flagged]
aesthesia 10 hours ago [-]
This isn't a political article?
recursivegirth 10 hours ago [-]
[flagged]
schainks 10 hours ago [-]
Politics and infrastructure are often intertwined, but the world you and I enjoy nonetheless relies on said infrastructure and how it works.
I don’t see anything political about the article? What made it political for you?
gordonhart 10 hours ago [-]
It’s a fascinating engineering project that most people don’t know much about. That’s what HN is for!
jLaForest 10 hours ago [-]
You complain about politics on HN in the same breath that you implore moderators to stiffle discussion that offends your politics. Do you see the hypocrisy?
Rendered at 10:52:23 GMT+0000 (Coordinated Universal Time) with Vercel.
Oil floats on water, which means pumping water into the bottom of the cavern pushes the oil out. As fresh water is pumped into the bottom of the cavern, the oil gets displaced upwards into a delivery system.
The more water you pump in, the lower the quality of the oil that's pumped out; we are basically loaning the oil refiners money to buy SPR oil they don't really want. 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.
I've posted this before; it's a recent report on the status and maintenance of SPR facilities, which is very clearly written and well worth your time if you want to understand the topic better: https://www.gao.gov/products/gao-26-106918
> 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.
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?
https://www.gao.gov/assets/gao-26-106918.pdf
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.
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.
> 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.
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...
https://www.spr.doe.gov/dir/dir.html
https://www.phmsa.dot.gov/technical-resources/pipeline/under...
Unrelated: dang, he wasn’t underselling this being a pretty elegant solution! I never would’ve thought of it, for sure.
From the post :
"300 feet in diameter..."
"To hold 714 million barrels of oil (the full capacity of the Strategic Petroleum Reserve), you’d need about 1,130 of these tanks. If you use the standard capacity of the largest commercial petroleum farms (e.g., in Cushing, Oklahoma), you need about 45,000 acres to store these tanks."
You further need space between them with each other and the edge of your property for a host of other reasons such as being able to inspect and paint each tank. Thus looking at actual facilities is going to give you vastly more resources options than just looking at the footprint of each tank.
BTW - If everyone followed Elon Musk's projection of 100 x 100 miles of Solar; then the US would not need these fossil fuels. See : https://www.reddit.com/r/Futurology/comments/1vq0uu6/solar_i...
Storing it in underground salt caverns is more secure, not necessarily cheaper.
As the article points out fire is a real concern here, which is one of many reasons making salt caverns so appealing. But it’s also a major driver for distance between each above ground tank, you want to isolate risks so a single failure doesn’t propagate across the facility.
https://www.cnbc.com/2026/08/15/strategic-petroleum-reserve-... "Depleted strategic oil reserve nears level that raises concerns about damage to caverns, operations"
https://fortune.com/2026/08/17/depleted-us-strategic-oil-res... “U.S. strategic reserves are getting so low the drawdown threatens to damage the 60 underground salt caverns storing American oil”
And do they use fresh water?
And a related way to use salt domes is for natural gas storage.
The US formerly maintained a National Helium Reserve (https://en.wikipedia.org/wiki/National_Helium_Reserve), in fact, until we decided that a helium reserve wasn't all that useful or important.
The problem with the SPR is that it is used to manipulate prices, which was why Biden on the one hand cut oil leases and reduced domestic oil production, but then did major SPR drawdowns before the election to hide the realities of these policies. Now Trump is playing the same game, sanctioning oil producers and creating wars in the middle east, but using the SPR to hide the effects of these policies until after the midterm.
In both cases, we'd probably be better off without an SPR, so all of these actions against oil production would be immediately felt at the pump before the election and voters could respond quickly. It's just too tempting to have this tool to manipulate oil prices that incumbents can use before elections.
The explicit purpose of the SPR is to function as a lever to smooth temporary supply shocks. The US increasing oil output by a few percent in either direction wouldn't have made any difference to global disruption of oil markets from a major exporter to Europe starting a war and then getting sanctioned.
Diesel and other heavier fuels generally are much closer to aliphatic hydrocarbons (straight chains, not aromatic) and so are much more storable, because aliphatic hydrocarbons have much less opportunity for oxidation (they're like saturated fat, i.e. much more stable).
You can get polymerization inhibitors to put into gasoline if you want it for long term storage, with the caveat that they produce some nastiness at the bottom of tanks that you'll have to avoid when pumping it out, and I think they can also slightly reduce the octane rating.
https://www.construction-physics.com/p/how-the-strategic-pet...
Maybe China publishes less official knowledge?
AIUI, salt caverns don’t like to be filled with brine because they become more susceptible to temperature changes of the earth and pressure from surrounding rock. Both can heat the cavern, which is bad for structural integrity.
I don’t see anything political about the article? What made it political for you?