This paper evaluates return permeability of reservoir cores damaged by oil-based-mud （OBM） invasion. A main hole of a highly deviated well in a domestic field suffered from formation damage whereas its vertical pilot hole successfully was showing oil. Both holes were drilled by means of OBM resulting in success from a drilling operation point of view; however, it caused formation damage during completion. Two types of solvents were evaluated to remove contaminants created by mixing of OBM and KCl brine. The candidate remedy fluid was screened through laboratory tests and used in wellbore clean-up operation for the subject well. Consequently, the remedy fluid significantly improved the borehole condition by removing contaminants from the wellbore inside, while we still have remaining damage in the near wellbore region. For further investigation on the near wellbore damage, three types of OBM-invaded cores （i.e. systems of synthetic formation brine, KCl brine, and base oil） were prepared assuming water and oil bearing zones, and they were evaluated from aspects of return permeability under the reservoir conditions. Results showed that OBM-damage was found in oil bearing zone, in particular, whereas a little damage in water bearing zone. It revealed that the magnitude of OBM-invasion was larger in oil bearing zone than in water bearing zone. Effectiveness of the solvent was also investigated for removing the OBM-damage using reservoir cores.
Finally, this paper recommended potential risk-evaluation of formation damages caused by OBM, preparation of remedy fluid, and plans of other remedial options （ex. re-perforation, micro-fracturing, etc.） in case of such damages. All should be done in advance.
