Though matrix and secondary porosity permeabilities of a naturally fractured reservoir are generally well-known since the work of Muskat (1949), notions of skin remain extremely vague to a majority of reservoir engineers. This paper aims at alleviating a problem which considerably impairs the understanding of such complex reservoirs.

A detailed mathematical study, similar to Muskat's, is first presented as a reminder of his equation giving the permeabilities of a rock with no primary porosity and a pattern of natural fractures, either conjugated (two parallel systems at an angle) or not. The same tool is then used to demonstrate that a vertical well connected to the natural fractures does show up a negative skin, which is a simple function of the well radius and the spacing of the fractures.

The Hawkins’ definition of (positive) damage skin is then applied to vertical wells. Both a "missed target" skin (well not connected to the fractures) and a fracture invasion damage skin (by drilling mud or heavy ends deposition) are defined. The missed target skin explains why Dennis Beliveau's Productivity Improvement Factor is much better in naturally fractured reservoirs. The mechanism of mud invasion into fractures is reviewed and the related skin calculated as a function of the mud loss volume per unit of drilled length.

Though Hawkins’ skin can also be defined in deviated wells (as Renard and Dupuy did in horizontal ones), the partial completion skin approach (e.g. Brons and Marting's) is shown to be more applicable after production has been initiated. Such a skin is calculated as a function of the number of fractures connected to the well at any time, and of their spacing in the reservoir.

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