Does the well leak? The answer, surprisingly to some, is yes it does. However, if the drilling engineers know what they are doing, leakage occurs where they want it and only where they want it.
The top hole does not leak simply because it is steel pipes surrounded by concrete with each phase pressure tested before drilling proceeds to the next stage. This is serious stuff because if any stage fails, then the whole well is almost certain to be abandoned and many millions of £ Sterling. However, to proceed and have a failure later on would be even more expensive.
Depending on the strata drilled through and the ultimate predicted depth, the vertical shaft may go down more than 1000 meters before it starts to turn in a gigantic letter “J” and then running as a lateral following the shale seam which may or may not be near horizontal. Now here is another difference between the original drilling in the USA (where each vertical well had only one lateral and maybe only one mile long) and here in the UK (Where we will drill up to a dozen laterals from one vertical well hole and each lateral may be up to 10 kilometers long).
Returning to leak risks, one of the functions of the drilling fluid (with its primary function to lubricate the drill bit that does the cutting of the rock) is to seal the hole left by the drill. Seal, that is, where the drilling engineers want to seal (which is the vertical or top hole). In the laterals, the design wants leakage but into the hole, ie. This is where the gas comes in.