You've just repeated OP's point verbatim, though. The whole point is that in an idealized system, you have the well bore travelling down through a groundwater l

You've just repeated OP's point verbatim, though. The whole point is that in an idealized system, you have the well bore travelling down through a groundwater later which by definition is permeable by water. If your bore lining fails (which it is likely to do, given the hydraulic pressure required for fracking lower layers) you then have an open circuit to the groundwater layer. It doesn't matter how much further down the production layer is. Furthermore, it's unlikely that a small leak into a groundwater layer would be A. Easily detectable or B. Cause the producer to immediately abandon pumping anything else into a well which has already taken a significant investment to drill and line.