With increased environmental focus and sustainability, classical water-based EOR with surfactants or polymers meet larger implementation scepticism. This has increased the attention toward water-based EOR methods with low environmental impact and lower costs, such as Smart Water and alkaline flooding. Both methods are based on the establishment of alkaline conditions in the formation. Chemical interactions among the rock minerals, reservoir fluids, and injection brine can be reflected in the pH of the produced water. Thus, the scope of this work is to investigate the development and transportation of pH through porous media during 1) low salinity (LS) Smart Water flooding, and 2) alkaline LS (alk. LS) waterflooding.
Outcrop sandstone cores were used in core flooding experiments. Several pH-screening tests were performed to study the pH development during waterflooding. The ability of LS and alk. LS injection brines to increase the pH in sandstone core material with different mineralogy was compared, and the effect of pH on oil recovery was confirmed in an oil recovery test.
The results of the pH-screening tests by LS brine injection showed a potential for increasing the effluent LS water pH up to 2 units in comparison to its initial pH-value. The oil recovery test performed on the same core material showed almost 10% incremental oil recovery during LS flooding in secondary mode, in comparison with formation water (FW) flooding. pH-screening tests with alkaline LS brine injection showed low potential for extra alkalinity above that obtained by LS brine injection. Transportation of alkalinity through a mineral system with large surface area seemed to be challenging due to pH buffering from brine/mineral interactions as well as from chemical interactions involving inorganic cations from the formation water.
Based on the experimental results, ion exchanges between rock minerals and injected water can influence the reservoir pH and induce the wettability alteration. These chemical interactions can result in both development and consumption of alkalinity depending on the type of injected brine and chemical reactivity of the minerals. It was concluded that an in-situ generation of alkaline conditions at the waterfront seemed to have larger potential for EOR purposes than transferring the alkalinity of the injected brine through the reservoir.