Many papers have described different techniques and procedures for core floods that measure permeability "return permeability" to evaluate the magnitude of formation damage caused by fluid(s) used in operations (e.g. drilling muds, completion fluids). Often these measurements of permeability are used to make critical decisions for field development and well construction design.

This paper demonstrates with examples, the danger on having used only the permeability measurements to determine the extent of formation damage. Commonly, laboratory core flood tests will have used core cylinders cut to 1.5 inch diameter and short lengths (less then 4 inches) unavoidably limited by core retrieval. Short core samples may be useful if the depth of formation damage was restricted to within the core length, however would have been misleading if any damaging affects extend further than this. Formation damage effects commonly extend further than the length provided by short core samples. The extent of formation damage that occurred would not have been identified by using permeability but requires identification by performing comparative before and after test geological analysis techniques. Subsequently the extent of damage must be scaled from the laboratory to the field by experts in an attempt to extrapolate what those damaging effects have on field injection and/or production. Formation damage that caused alteration(s) to any core sample should be identified by comparing before and after test permeability measurements combined with all other analysis. The nature of any damage mechanism(s) would unlikely to have been identified by just some or any one measurement alone. Scanning Electron Microscopy (Dry and Cryogenic), Energy dispersive x-ray spectrometry (EDS), X-ray Diffraction Analysis and Thin Section Analysis should be incorporated to properly determine the nature and extent of formation damage.

The examples in this paper have never previously been published and they represent a continuing evolution in our understanding of formation damage measurements and mechanisms. Examples of mechanisms such as fines migration, fluid retention and mud body damage will be presented and discussed in terms of their potential impact on return permeability measurements.

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