New wettability test for chalk based on chromatographic separation of SCN− and SO42−

Wettability, water-wetness or oil-wetness, is usually defined in terms of the method used to determine the wetting state of a porous medium. In the Amott test, which is most frequently used for porous media, the wetting indices of water and oil are determined by the extent of spontaneous imbibition of water and oil from the state of the corresponding residual fluid saturations of the core. Thus, the Amott fluid wetting index reflects the potential capillary energy associated with the spontaneous imbibition process, which results in a very insensitive way to detect wetting alterations close to neutral conditions. Unfortunately, the natural wetting state of many carbonate reservoirs is close to neutral conditions or preferential oil-wet. In this paper, the wetting state of porous chalk is characterized by the fraction of surface area covered by one of the two fluids, i.e. water or oil. As the new wettability test is based on the chromatographic separation of two water-soluble components, i.e. a tracer, SCN−, and a potential determining ion towards chalk, SO42−, the fraction of the surface area covered by water was decided to represent the new wetting index. Using a chalk core at residual oil saturation, the area between the effluent curves for SCN− and SO42− is proportional to the area contacted by water during the flooding process. The ratio between this area and the corresponding area obtained from a completely water-wet core will give a water index between 0 and 1, representing completely oil-wet and completely water-wet conditions, respectively. The method is excellent to be used close to neutral condition, which will give a wetting index of 0.5. Experimental results are presented for two different chalks at different wetting conditions. Furthermore, the change in the Zeta-potential of the chalk surface over the dispersion zone of SO42− is visualised by a gradient in the Ca2+ concentration in the same region.