Today oil and gas wells are drilled using mostly water-based drilling fluids. The introduction of both the mud solids and polymers into the formation, the changed values of fluid's saturation in the pore spaces, the reactions between the filtrate and the pore contents and/or the matrix materials result in a reduction of the formation's original permeability; thus the formation is damaged.

World wide the range of formation damage has been determined by damage ratio DR. The value of damage ratio is mainly affected by the layer with the lowest permeability (filter cake). Therefore the tested cores have to be evaluated in general as damaged over the whole length, which does not correspond with the real case under reservoir conditions.

In this paper a new defined damage ratio has been used to evaluate the core damage using different drilling fluids. It could be demonstrated, that the new defined damage ratio is an expedient value for rocks with excellent original permeabilities, which allows to determine a reliable extent of the damaged zone of the tested core.

Furthermore, it could be verified that the extent of the damaged zone depends upon the contact time between the drilling fluid and the core, and also upon the stability conditions of the used drilling fluids.

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