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Common practice, relative permeability curve is used to estimate residual oil saturation (Sor) and irreducible water beside also to indicate wetting condition of the rock. Relative permeability itself comes from laboratorium measurement of special core analysis (SCAL). This study is facilitated by data quality of relative permeability regarding with high Sor and impact to economic of X field Waterflood Optimization project as well as simulation uncertainty. This study is started with collecting all of relative permeability from SCAL, normalize and denormalize. Focus on the best sand Duri Formation that is the main production zone has been waterflood. Relative permeability curve show Sor 0.4 and Swirr 0.22. This residual oil saturation number is too high on this good reservoir having permeability in the range of 200 to 4800 md with average of 800 md. Calculation of maximum displacement and recovery based on above data shows only 0.34. Current cumulative production has shown 0.37 meanwhile X field Waterflood Optimization project predict recovery of 0.46 based on simulation. It may mean unreliable relative permeability data or probably some pitfalls in the laboratorium measurement. Simulation result after history matching show Sor number is 0.32. Oil saturation from new wells log of depleted zone are analized to support that statement of high Sor also cased hole log data have been used as well. This study review existing Sor from relative permeability curve and estimate reliable Sor from the analitycal process using plot of oil saturation and pore volume injected as well as using other data of SCAL such as waterflood susceptibility test, capillary pressure test. Currently other cores from this field are sent to laboratory to re-measured relative permeability as part of SCAL program to reduce this uncertainty. To avoid similar pitfall, every steps are reviewed then new ideas are propose such as CT-Scan, miscible cleaning, and bump rate test. Result of this new SCAL measurement is expected to give more reliable Sor.