The paper describes an experimental approach to study the formation damage induced by drilling muds in oil bearing formations. This approach includes i) static and dynamic invasion tests on long core samples performed either under CT Scanning or with specific equipment allowing pressure measurements along the core sample, and ii) characterization of both external and internal mud cakes by means of Cryo-SEM examinations.

A series of seven water-based mud invasion experiments has been conducted on high permeability clayey sandstones. The amount and the extent of formation damage have been quantified. The results show that global oil return permeabilities vary from 44% to 90% of initial permeability depending on oil viscosity and overbalance pressure.

Hysteresis of saturations due to retention of invaded aqueous filtrate was always observed, up to 17 saturation units. This induced an adverse relative permeability effect which may significantly contribute to oil permeability impairment.

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