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
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:
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."
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.
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’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.
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
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.
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.
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
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.
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.
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.
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...
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.
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.
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?
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.
https://www.gao.gov/assets/gao-26-106918.pdf
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.
Unrelated: dang, he wasn’t underselling this being a pretty elegant solution! I never would’ve thought of it, for sure.
And do they use fresh water?
https://www.construction-physics.com/p/how-the-strategic-pet...
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.
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?