On the one hand, it is impossible to answer without information on what your well is LIKE. Is it dug or drilled, is it lined or not, if lined is the liner fully-intact versus damaged, that kind of thing.
That said, if your chickens will free range then they will go where they want to anyhow and unless you are really mismanaging the coop it will not be a very large source of nutrient input into the ground or runoff. And 12-15 chickens is not a huge number.
If it were me, I would have a floor inside the coop (as you're going to do), make sure to have my chicken-poo compost pile as FAR away as possible, preferably at least 100' and further would be a lot better especially if there are some question-marks about the well or the drainage patterns. I would make real sure that the run was graded in a way such that runoff goes pretty far in the opposite direction from the well. And maybe truck in some roadbase or coarse sand or whatever to raise the run surface a bit so that it never becomes a "bathtub" with the poo marinating in puddles.
So, if you approach it in a cautious kind of way (and keep testing your water periodically for coliform bacteria, as you should *anyhow* when you're on a well) I think it is pretty unlikely you'd have a problem. And frankly, if you DID have a problem in the absence of any really nasty management lapses, then it'd probably be telling you something important about the *general* safety of your wellwater *anyhow*.
Good luck, have fun,
Pat