Numerous quantitative and qualitative methods have been presented to measure wettability. The most well-known methods are Amott-Harvey and U.S. Bureau of Mines. The Amott method describes how the wetting phase displaces the nonwetting phase spontaneously; the main problem with this method insensitivity near neutral wettability. Another problem with this method is that imbibition can take several hours to more than two months to complete. The most important benefit of USBM that differs from the Amott method is the sensitivity close to neutral wettability, but the disadvantage is that USBM cannot recognize if the reservoir has mixed wettability or not, though Amott can.
We come up with a method to measure sandstone wettability only by Ca2+ and Br− chromatographic separation according to the method described by Strand et al. (2006) on a chalk core. Three sister cores were pre-aged in formation water without Ca2+ and Br−, and the cores were then aged in oil for three weeks at 95°C. The cores were then flooded with the same formation water until Sor was established. Core#1 was flooded with high salinity water (~117,000 ppm) containing identical concentrations of Ca2+ and Br− (89 μmole). Core#2 was flooded with low salinity water d30HSW. Core#3 was sequentially flooded with HS and LS water to investigate the wettability alteration in the same core. All experiments were conducted at 25 and 70°C to examine the effect of temperature on wettability alteration by the new method. The effluents were collected by a fraction collector for chemical analysis for Ca2+ and Br−, divided by the inlet concentration of Ca2+ and Br−, and then plotted with injected pore volume (PV). The area between the Ca2+ and Br− curves was calculated (Ao). Core#4 was pre-aged in heptane in order to establish water-wet sandstone. The heptane was displaced from the core by the same formation water until residual heptane saturation was reached. The same HS water was injected into Core#3. The effluents were analyzed using the same method as for Core#1, 2. The area between the two curves was also determined using the same method as for Core#1, 2 (AH).
The wettability index was then calculated by dividing Ao by AH. The wettability index ranged from 0 for strongly oil-wet to 1 for strongly water-wet and 0.5 for intermediate wettability. Another core was sequentially flooded by HS and LS water to further investigate the wettability alteration by LS water and to verify our new method.
The divalent cation Ca2+ was considered as the most potential ion towards sandstone when injecting low salinity water to sandstone. An ion exchange occurred between Ca2+ and H+ during flooding which is the key point for wettability alteration, and in turn, increases oil recovery. Bromine is a tracer that has no potential to the sandstone surface area. Thus, the area between Ca2+ and Br− is proportional directly to the water-wet surface site in sandstone (i.e., both Ca2+ and Br− contact the same water-wet surface area).