Primary drainage capillary pressure curves are often used to assess the distribution of initial water saturation versus elevation above the FWL in oil reservoirs. This paper focuses on 1) the comparison between oil/water centrifuge tests and mercury injection 2) the effect of wettability alterations due to drilling fluids and laboratory procedures on the oil/water drainage Pc curve 3) relationship between wettability and pore structure. Outcrop low permeability microporous limestones, representative of three different sedimentary facies of the Shuaiba formation in the Middle East have been used for this study. Companion samples were used to assess the influence of the different treatments. Porosity, permeability, X-ray tomography, probe permeametry, and NMR T2 curves were used to ensure that companion samples were very similar. For each facies, the following first drainage capillary pressure curves were compared: • Oil/water at strongly water-wet conditions with refined oil • Oil/water with reservoir oils • Oil/water with reservoir oil after aging with reservoir oil and cleaning • Oil/water with refined oil after flush with oil base mud filtrate and cleaning • Mercury injection For each treatment, wettability tests were also performed to measure the prevailing wettability during the drainage process. The main results are: • Large discrepancies are observed between water/oil and mercury injection Pc curves, particularly in microporous facies. Discrepancies are very significant over the full saturation range. Suggestions based on the very low pore sizes of these microporous rocks are proposed to scale these results to reservoir conditions. • Severe oil-base mud invasion may result in dramatic overestimates of the initial water saturation from first drainage Pc, even after core cleaning. • “Irreducible” water saturation is a poor criterion for rock-typing in carbonates. • Wettability variations within a carbonate reservoir might also be attributed to the topological differences between facies. • The efficiency of cleaning processes in carbonates is dependent on the interplay between the macroporosity and the microporosity, for the same reservoir oil and for the same suite of solvents.