why tight reservoirs has negative skin ?

[kochichiro]

Well, yes. Typically mud extremely overbalanced. Moreover, during drilling in shale layers swelling occurs and mud density increasing further. Conventional reagents that employed as shale inhibitors as good as shit. Supervisor’s notifications are usually ignored by drilling team. Therefore, when time comes to DST test of such reservoirs they do not flow naturally and drilling team runs several acid flushes until they receive some, even negligible rate. I know many cases when reservoirs were considered as dry basing on results of such DST tests.

[Tejaswy]

I think what he meant was that the BOP/safety conditions are not so good and hence they use overbalanced technique to make sure there is no blow out. Then they acidize it to undo some of the formation damage.