Fluid migration, Fracture propagation

“Dear Aunty, can the induced fractures reach the water table?”

Oh, goodness me, no my dears – not at all.

Fracking for shale gas in the UK takes place many thousands of feet below the ground, cherubs, a very long way indeed from the aquifer near the surface.

The idea that the tiny fissures created by fracking could extend this far is very misguided.

To understand it properly, it’s back to the classroom again.

Firstly, a quick lesson in fracking:

Once the well is drilled and constructed (with all those steel tubes and cement I’ve mentioned previously) the ‘production’ casing – that extends all the way from the surface to the shale rock itself – is perforated to create a series of tiny holes. Then, my lovelies, the fracturing fluid is injected from the surface and travels down the well at high pressure. When it reaches the shale rock, the fluid escapes through the holes punched in the casing and out into the surrounding rock, forcing its way into pre-existing, natural fractures.

The fracturing fluid extends these natural fractures, but there is a limit to how much it can do so, and that’s defined by those pesky laws of physics again sweeties!

You see, the energy required to extend the existing fractures is transmitted by water molecules, darlings.

When they first flow into the hairline fractures, you can imagine them being all concentrated and bundled together, but as the fluid is pushed further into the rock, those water molecules effectively spread out to form a sort of chain. Eventually, when the well bore itself is full of fluid, those chains of water molecules laid end-to-end simply become so stretched that the molecules can no longer bump into one another, and are therefore unable to transmit energy any further into the rock.

It’s a point not many people recognise, yet an important one: despite the volumes of fluid and pressures used, fracking actually operates at a molecular level, my little pumpkins.

The maximum length of induced fractures observed so far, from what I gather, is about 300 feet from the well because of the limitations of physics.

That’s a long way short of the thousands of feet they’d need to extend from the shale rock all the way to the water table,my dears.

And so the risk of fractures creating a direct means of communication between the shale rock and the aquifer for fluids and any liberated gas is none existent, and not something you need worry about darlings.

Until next time xxx

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