A quantitative method for measuring the wettability of porous media containing brine and crude oil has been developed by the USBM using capillary pressure curves determined with a centrifuge. These measurements were reproducible within a standard deviation of 8.2 percent. The determination of fractional wettability has been compared with a method involving a combination of imbibition and displacement. Results are in agreement qualitatively as to whether a sample was water-wet or oil-wet. When tested with outcrop cores of Torpedo Sandstone, wetting tendencies of oils from widely different fields range from highly water-wet to almost neutral. Silicone-treated cores vary in extent of wettability in a predictable manner, becoming more oil-wet as the concentration of silicone used in core preparation is increased. Wettability, used as a parameter in designing linear mathematical models for predicting recovery efficiency, is equal in significance to viscosity and permeability. permeability Introduction: Wettability is one of the major factors that influences oil recovery. The importance of maintaining a water-wet condition in a field under water drive has been discussed by many authors. These authors have shown that oil recovery, as a function of the water injected, is greater from water-wet cores than from oil-wet cores. A few authors have indicated that recovery from strongly water-wet or oil-wet cores is less than recovery from cores that are at some intermediate wettability. One reason that the significance of wettability to oil recovery is still being evaluated is because a quantitative measurement of wettability has not been available when crude oil is involved. Some qualitative methods for determining the wettability of cores that are discussed in the literature includeuse of relative permeability data,imbibition,displacement pressure,capillary pressure, anda combination of imbibition and displacement. Using these methods, cores can be classified as either water-wet or oil-wet. However, these methods are not satisfactory for intermediate wettabilities between water-wet and oil-wet. Nuclear magnetic relaxation and dye adsorption offer promise of a quantitative method for determining wettability; however, they have not been widely accepted. This paper presents a quantitative technique for determining the wettability of sandstone cores containing brine and crude petroleum oils. The method is rapid and reproducible and requires only a minimum handling of the cores before testing. It also establishes a numerical wettability scale for systems ranging from strongly water-wet to strongly oil-wet. TECHNIQUE: WETTABILITY TEST PROCEDURE: The USBM method for determining wettability is based on a correlation, suggested by Gatenby and Marsden, between the degree of wetting and the areas under the capillary-pressure curves. The method employs the two areas under the capillary-pressure curves obtained by the centrifuge method described by Slobod et al., and extended to determine capillary pressure curves for oil and water. Gatenby and Marsden, and earlier Slobod and Blum, suggested the use of individual points on the capillary pressure curve, but the area is considered representative of the over-all wettability, as it is an integrated value over the practical range of saturations. Capillary-pressure curves obtained by the centrifugal method are not complete descriptions of the capillary pressure vs saturation relationship of a porous medium. The water imbibition curve for pressures greater than zero and the water drainage curve for pressures less than zero cannot be obtained by the centrifugal technique. SPEJ P. 13