Positive skin is a restriction: drilling damage, a crushed perforation, scale, or a completion that penetrates only part of the interval. Negative skin means the near-well rock has been improved, as after a fracture or a successful acid job. Zero skin is the ideal open hole used as the reference, not a description of a real completion.
Skin is not a permeability. A large positive skin in a thin zone near the wellbore can cut the rate sharply even when the reservoir permeability is good, which is why a small acid job or a short fracture can restore a well. It is estimated from a pressure-transient test, not from the pressure drop alone.