What I've read is that, the SPR was designed for a 25 year lifetime and 5 purge/fill cycles. It's now at 40 years and dozens of purge/fills, so it's way out of it's design parameters.

The problem is that as you pump oil and water in and out of it, in the intervening layer between the salt water and oil there forms a water/oil/impurities sludge which is useless (it ruins the pumping equipment). The thing is, they have no way of measuring it, nobody knows how much good oil is left and how much is sludge (or if they do, none of that information is public). And the closer you get to the sludge, the harder the good oil is to extract, slowing down the extraction pace.

I asked Claude to research for me so don’t take this as truth but it sounds like

(a) as the levels drop below 300M barrels, pumping out of the reserve is much less efficient, so we can’t necessarily draw it down as quickly as when it’s full. So the lower it gets, the less useful it is

And (b) there are a lot of varying estimates for the real effective floor, anywhere from the 180ish to 300M barrels is within the range of a safe minimum level.

Read up on the why, I didn't get it either. Its fascinating though, basically the caverns would collapse.

Also, the bottom of the reserve is mostly sludge.