It's Not a Cave

Before we get into this week's topic, there's an update I need to cover.

The US struck Iran again on Sunday. CENTCOM confirmed American forces hit a pair of rocket launchers on Larak Island — the first publicly acknowledged strike on Iranian positions since late July — and Iran answered by hitting US bases in Jordan. Crude gapped up on the open: WTI +2.5% at $85.46, Brent +2.7% at $90.49.

This was supposed to be the quiet stretch, with Washington pivoting from military operations to economic pressure and Treasury declaring an "economic D-Day".

Five days later, we're here. We've said this many times before: these kinds of start and stop wars don't just end because of one thing. So expect more of this to come, or something breaks.

Now back to how oil reservoirs work and how you can have 100+ year wells…

Most people think an oil reserve is like an underground lake — a black pool in a hollow that you drill into, pump out, and eventually empty like a swimming pool.

There is no pool.

What's down there is rock. Sandstone, limestone, dolomite — solid enough that if I handed you a core sample from a producing reservoir, you'd think you were holding a rock, because you would be.

The oil lives inside it.

A kitchen sponge is a solid object that happens to be mostly connected empty space, and that's your reservoir.

Geologists call the empty space porosity — in a good conventional reservoir, five to thirty percent of the rock's volume is pore space rather than mineral, and the hydrocarbons sit in those pores alongside water.

Space alone gets you nothing, though.

You also need permeability: whether those pores connect to each other well enough for fluid to travel between them. A sponge with sealed bubbles is useless; a sponge with open channels is the entire business.

And the reason any of it is still down there after tens of millions of years is the impermeable caprock sitting on top — dense shale or salt, a lid with no meaningful permeability at all.

Porous rock. Sealed lid. Enormous pressure from the weight of the earth above.

Drilling in doesn't open a tap on a tank — it introduces a single point of low pressure into a formation that's been sitting at very high pressure, undisturbed, for geologic time.

Fluid moves from high pressure toward low. That's the whole mechanism.

Oil in the pore space nearest the wellbore flows in, oil behind it moves up to take its place, and the pressure drop propagates outward through the rock — slowly, unevenly, continuously.

The well doesn't drain a pool. It creates a gradient, and the gradient does the work.

Squeeze one corner of a wet sponge and water arrives from the far side — not instantly, but it arrives.

Which is why hundred-year wells exist.

McClintock No. 1, drilled in Rouseville, Pennsylvania in August 1861 — two years after Drake, while the Civil War was on — went down 620 feet and has produced oil continuously ever since.

Not much, and it's a museum piece now. But it still gives up oil a hundred and sixty-five years later, while the far more famous Drake Well went dry in about two.

The EIA calls a well producing fifteen barrels a day or less a stripper well, and at the agency's last detailed count there were roughly 380,000 of them operating in the US against about 90,000 non-stripper oil wells — together accounting for around ten percent of national production.

Most American oil wells are old wells producing slowly.

They're still producing because the gradient hasn't closed.

Every well declines — production peaks early, when the pressure differential is most extreme, then falls off hard.

And then it flattens.

The curve runs to a long, shallow tail that can extend for decades, because the drainage area keeps widening and the rock keeps feeding. The steep part is what everyone photographs; the tail is what pays the bill for decades.

And you can go back in.

Commonly cited figures put primary recovery somewhere in the neighborhood of ten to twenty percent of the oil originally in place, with secondary and enhanced techniques lifting that meaningfully higher.

Read that again. When a well stops being economic, most of the hydrocarbons are still down there.

Which is why you can re-stimulate: a refrac opens new pathways through rock the original completion never effectively contacted, or restores conductivity in fractures that have closed up over the years.

You're not drilling a new well — you're using the wellbore you already paid for and giving the rock a fresh set of channels.

The result is a production bump, then a decline curve of roughly the same shape from a lower peak.

You've reset the gradient.

Note the conditional, though: refracs cost money, they don't always work, and the decision turns on oil price, service costs, and the specific completion history of the specific well.

A reservoir isn't a tank you empty. It's a sponge you keep coaxing. This is also why some of the prolific basins remain prolific. Not only because of the concentration of logistics and service companies, but firstly, because of the basin's geology.

Your Oil and Gas team, CrownForge Energy.