Injection of waste water is often accomplished by gravity. The waste water is poured into the well. But when the well is full of water, say two miles deep, the pressure at the bottom is just shy of 6,000 psi and it is unrelenting. Hold that in mind and consider fracking for a moment.

Fracking involves large pumps driven by locomotive-like V12 diesel engines … maybe 12 of these working together. They pump an average of 4 million gallons of water at 9,000 psi. Each fracturing event lasts from a couple of hours of pumping to maybe 4 or 6 hours — so the pressure is higher, but the duration is short relatively speaking. This makes cracks averaging 1000 feet long, with some longer. Maybe half of one percent go 2000 feet. It costs money to run those big diesels, and 1,000 feet is satisfactory, so on to the next frack.

Back to natural fracturing. If the chimney is pressurized by a gravity fed source the pressure is unremitting, other natural processes have time to work eating away at the rock, propping open the crack. Nature has time that the driller does not. That is why chimneys can be so much longer that fracked cracks.

So how long does this take? Well, the same study cited in the article above reports on studies of natural chimneys forming. The time is more than hours and less than years — many days.

The very big question the article does not speculate about is what may happen in a spent gas well that allows water to leak in and continue the process begun by artificial stimulation. Will derelict wells develop natural chimneys extending thousands of feet upward? No on knows. But the bore hole is forever, and future generations will surely find out.