Effects of low-salinity waterflooding on capillary pressure hysteresis

In this study, low-salinity waterflooding enhanced-oil recovery mechanisms linked to the water-crude oil interfacial viscoelastic response are investigated considering a distinct interfacial effect, namely the water-crude oil interfacial viscoelasticity response. In contrast, wettability alteration is interpreted through analysis of capillary hysteresis. A capillary pressure experimental setup used in our previous research, is utilized here to capture capillary hysteresis. Moreover, new improvements over the traditional quasi-static porous plate method have been implemented to accelerate measurements. The dynamic-static method, i.e. a combination of continuous injection in drainage and stepwise quasi-static method in imbibition on short (<1″ long) core samples, was found to capture the correct envelopes of the capillary pressure curves in a much shorter time span. Two pairs of experiments were conducted to investigate the interfacial visco-elasticity and wettability alteration effects on capillary hysteresis. One pair is conducted on Minnelusa formation rock samples and diluted TC crude oil at 30 °C and without any significant aging to minimize wettability alteration. Two core plugs were flooded with high-salinity and low-salinity brines, separately. Results show that low-salinity brine can still increase oil recovery even in the absence of wettability alteration. This is attributed to the formation of a more visco-elastic brine-crude oil interface upon exposure to low-salinity brine, leading to a more continuous oil phase. Two additional experiment pairs were conducted on Berea and TC oil at high temperature (70 °C). Two cores were subject to the same experimental conditions except that one was allowed to age with crude oil for 3 weeks. This aging process was found to render the rock more oil wet, resulting in more pronounced capillary hysteresis and less oil recovery. Our study shows effects of two mechanisms, i.e. fluid-fluid interfacial visco-elasticity and wettability alteration, separately. The interpretation of our unique capillary pressure datasets sheds light on low-salinity waterflooding mechanisms.