M. Adeel Sohal
Erik G. Søgaard
The accurate determination of the wetting condition of carbonate reservoirs is a prerequisite for the selection of any enhanced oil recovery (EOR) fluid. Most carbonate formations are initially oil-wet to intermediate-wet at reservoir conditions. In order to correctly chose an EOR fluid to alter the wettability, we need to understand the initial wetting conditions and design an ionically modified water (advanced water) to alter wettability and improve oil recovery. If a reservoir has already been reached to the optimum wetting conditions by injecting formation water or any other fluid, then there is no need for ionically modified water. A number of methods have been devised to identify the wetting conditions including contact angle measurements, spontaneous imbibition, and chromatographic separation, etc. But contact angle measurement requires surfaces that lack natural surface roughness, spontaneous imbibition tests take months, and chromatographic separation is feasible only for core flooding in sulfate free carbonates at low temperature. A novel application of the well-established technique known as flotation was used in this study to measure the oil-wet and water-wet percent of pure biogenic chalk (Dan Chalk from Denmark). It is an accurate, fast, and most reliable method to quantitatively measure the water-wet and oil-wet fractions of a reservoir rock. It determines the potential of advanced water to improve wettability within days, instead of measurements that can take months and require expensive equipment. Using this technique we were able to quantify the wettability alteration caused by low salinity and potential determining anions (PDAs) such as SO42–, BO33–, and PO43–. The wettability data show maximum oil recovery by dilution is coincident with maximum wettability alteration. The experiments also show that the presence of sulfate or borate enhances wettability alteration by dilution. Moreover, the combined and individual effect of potential scale forming ions (Ba2+ and Sr2+) on wettability restoration was identified. The effect of interfacial tension (IFT) on the measured wettability alteration and the amount of oil attached to the water-wet percent of rock was also determined.