How the US Strategic Petroleum Reserve Stores Oil in Salt Caverns
A viral engineering explainer on salt-dome oil storage draws technical corrections and elaboration from Hacker News readers.
What to know
- The SPR holds 714 million barrels across 60 salt caverns at four Gulf Coast sites, each cavern holding about 10 million barrels — more than the entire Navy Red Hill facility.
- Salt domes were chosen over surface tanks (vulnerable to attack, land-intensive) and excavated rock facilities like Red Hill (costly, prone to groundwater contamination that forced Red Hill's closure).
- HN readers corrected and extended the post: one flagged an apparent land-area miscalculation, others detailed how presaturating injection water with salt prevents cavern-wall erosion.
- A recurring open question in the discussion is how much oil must remain in the reserve to keep it operational, and how cavern degradation over repeated cycling is managed.
The dispute Whether the article's 45,000-acre land-area figure for surface-tank storage is accurate, versus a commenter's recalculation putting it near 2,260 acres. · positions read across 22 posts and comments
The engineering explanation is elegant and the piece is a good example of technical writing for non-experts.
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“Makes you appreciate the physical infrastructure challenges, not just software. Bet the data logging and monitoring systems are ancient but robust.”
Ellis_dev · Hacker News ↗
The article leaves out important technical detail on how salt caverns actually preserve their structural integrity, which commenters supply.
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“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.”
anigbrowl · Hacker News ↗
A specific figure in the post (land area for surface tank storage) appears to be a calculation error.
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“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!”
tmellon2 · Hacker News ↗
johnjwang Author of the engineering explainerU.S. Navy Operator of the Red Hill fuel storage facilityanigbrowl HN commenternostrademons HN commentertmellon2 HN commenter
How it unfolded 3 developments, newest first · click a bar or a number to jump articlespostscomments
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Reader flags a math error in the article's land-area figure
Commenter tmellon2 recalculates the acreage needed for 1,130 surface tanks and finds roughly 2,260 acres rather than the 45,000 acres stated in the post, questioning that figure.
“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!”
— tmellon2 -
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…
2 more of the top 3 · 12 posts in this stretch
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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…
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I wondered to what extent they know the size and shape of the cavern at any given time and what they use to monitor it. The answer is apparently sonar, and there are some pretty pictures here:
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Commenters elaborate on how salt caverns actually contain oil
HN users anigbrowl and nostrademons add detail beyond the article on why salt's low permeability and plasticity let it hold oil without a steel lining, and describe how operators presaturate injection water with salt to stop the caverns from dissolving further.
“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.”
— anigbrowl -
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…
2 more of the top 3 · 10 posts in this stretch
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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:
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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.
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Post climbs Hacker News front page
The article is submitted to Hacker News and gains traction, reaching 167 points and generating dozens of comments; a related HN RSS repost registers separately with 20 points.
“The actual solution, like many great engineering solutions, is incredibly elegant and simple.”
— johnjwang, blog author · source -
background
Blogger publishes explainer on SPR salt-cavern engineering — johnjwang publishes a post walking through why surface tanks and excavated rock facilities (like the Navy's Red Hill) don't scale to 714 million barrels, and how the US instead uses solution-mined caverns in Gulf Coast salt domes.
Also covered reported alongside — the timeline has no entry for these yet
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first by HN Best, 8d ago · also HN Frontpage
What people are saying 15 voices from 1 site · best of 22 · verbatim
- Why not simply pump the original brine back into caverns to move oil without dissolving more salt?
- How much oil must remain in the reserve to keep it operationally pressurized, and how close to zero can it realistically be drawn down?
- What happens to the oily water displaced during drawdown — is it treated, evaporated, or reused?
- Sep 17
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Where can I find more of these incredibly clever engineering solutions explained so simply? Love reading about this sort of stuff.
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> Keep the oil secure against attacks by our adversaries. This is an especially hard challenge because petroleum has this tendency to light on fire, so a huge concentration of it is especially difficult to keep safe.> Release oil quickly so it can reach the market during a supply disruption.But what if the adversaries destroy the refineries or…
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Peter Zeihan did a 5m video [1] on this 10 days ago (give or take) - and the history of how they've been used, and why the original design (big draw-downs during major emergencies - perhaps 5 of them, ever) hasn't lent itself well to the way they ended up being used.[1]
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The Wikipedia page needs an update, this map[1] still shows Gulf of Mexico. Thank you for your attention to this matter![1]
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The salt caves were only designed to be used a few times and to be used for maximum draw down, weren't they?
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Underground Natural Gas storage is handled the exact same way interestingly enough
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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.
- Sep 16
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This is so incredibly interesting and one of the best things I’ve seen on HN recently. Thank you for the awesome post!
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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?
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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.
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We can't pump saturated salt solution to prevent more salt from being dissolved?
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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.
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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.
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Makes you appreciate the physical infrastructure challenges, not just software. Bet the data logging and monitoring systems are ancient but robust.
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Very cool, big fan of these bite size explainers. Has there been more cycling recently? Are the sizes of the caverns tracked anywhere?