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
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...
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.
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.
> 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?
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.
Fuel companies state that untreated fuel will remain usable for only one year, after which fuel contamination ("the diesel bug") begins to appearAnd do they use fresh water?
Unrelated: dang, he wasn’t underselling this being a pretty elegant solution! I never would’ve thought of it, for sure.
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”
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.
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.
https://www.construction-physics.com/p/how-the-strategic-pet...
Maybe China publishes less official knowledge?
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...
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.
Storing it in underground salt caverns is more secure, not necessarily cheaper.
https://www.phmsa.dot.gov/technical-resources/pipeline/under...
[1] https://en.wikipedia.org/wiki/Strategic_Petroleum_Reserve_%2...
> Release oil quickly so it can reach the market during a supply disruption.
But what if the adversaries destroy the refineries or other downstream equipment which is much simpler to do?
