What I do know is that to fracture shale a lot of pressure is required. That pressure is sufficient to exploit a weak cement job and find its way to surface (or into aquifers). There have been a number of well documented examples of pressure in a well getting to surface from deep down, including the Maconda well and the Ocean Odyssey disaster some years back where weak cement jobs have been partly to blame. The Maconda well was a lot deeper than any of these proposed frack jobs in the UK.
If Fracking is handled well it can and will work but safe procedures are required to ensure that.
Another area for concern is the local geology in any region. Faulting will also provide potential conduits to surface.
Its actually great for me as Fracking will provide more work in the hydrocarbon industrybut that doesn't stop me from forseeing fun and games for some folk in the future.
I am glad I am not living on top of the Bowland Shale formation.
3 areas for potential failure:
Poor cement jobs.
Faulted geology.
Poor procedures.