Reservoir conditions core flood tests have been carried out to examine the damage arising from overbalanced drilling-in operations. Gannet field cores were exposed to a number of water- and oil-based field muds currently in use by Shell Expro. Results showed that the initial permeabilities to crude oil were dramatically reduced after exposure to overbalanced mud and subsequent back production of crude oil. Thin section petrography and SEM/XRD analysis of the cores clearly showed the source of impairment in all cases to be a thin layer of mud solids plugging pores and throats exposed to the wellbore face, forming an effective permeability barrier that was not dislodged by crude drawdown under realistic drawdown. The results were confirmed by trimming of 1 cm from the wellbore face of the core that led to a restoration of initial permeability.

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