ChatGPT & DALL-E generated panoramic image of a plain with a high-pressure pump, a big pond and a very large pile of gray sand
Let’s step back. In the early days of the oil industry, wildcatters used to drill in hopes of finding a gusher. That meant drilling into an underground body of oil in permeable rock held in place by a capstone of some sort. When they found a pressurized body of oil, physics would make the oil rush up the drill hole to the place of lower pressure, turning into a gusher of oil. This was a common situation and in the early days of oil often involved the gusher accidentally catching fire.
So what was their insight? That just as incompressible oil came out of compressible underground reservoirs at a high rate, water could be forced down into compressible rock reservoirs at high pressure. Drill holes down to the appropriate strata of shale capped by an impermeable upper layer. Put a big reversible pump at the top of the hole, one capable of creating 200 atmospheres of pressure, equivalent to 2 kilometers or 1.25 miles under the surface of the sea. Pump water down the pipe.
If they keep it plugged, the water doesn’t escape, but wants to. That creates mechanical potential energy. It doesn’t involve a lot of heat loss because water is incompressible, unlike. This solution compresses the rock underground and expands the pores a bit instead of compressing gases in rock caverns underground.
What about the basic physics? Turns out 200-atmosphere underground compressible rock is like a hydro dam or pumped hydro head height of two kilometers. Pressure and volume is really just the inverse of mass and height. A lot less mass would be required at the equivalent of two kilometers to create the same power for the same number of hours.
What about volumes from this kind of porous rock reservoir? A normal well might return 5,000 barrels per day, about 800 cubic meters of water. At 200 atmospheres of pressure through a 30-centimeter pipe, the orifice equation — yes, that’s what it’s called — tells us that the flow rate would be 0.0092
Water is a quite a bit less viscous than light oil, and so can move through porous rock more quickly. A bit of research found the Darcy-Weisbach equation, which can be adapted to determine the difference in flow rates for liquids of different viscosities. Assuming I applied it correctly, it suggests that less viscous water would flow just over three times faster, so a 100,000 barrels of oil would equate to about 320,000 barrels of water.