Displacement-History Control of Low-Salinity Waterflooding in Chalk: Insights from Early Brine Switching

Low-salinity waterflooding is widely recognized as a promising enhanced oil recovery method, yet its reported performance remains highly variable because the observed response reflects not only brine composition, but also the displacement state of the porous medium at the time the injected brine is introduced. While most previous studies have focused on optimizing brine chemistry, the mechanistic role of injection timing and displacement history has received far less attention. In this study, we investigate how the timing of brine switching governs low-salinity response in chalk by introducing an early tertiary flooding design, in which the injected brine is switched immediately after secondary brine breakthrough. This approach enables controlled probing of an intermediate saturation state representative of the reservoir region just behind the advancing flood front. The resulting recovery behavior was interpreted together with ion chromatography measurements to track brine displacement and chemical-front propagation. The results show that the apparent efficiency of low-salinity flooding is strongly governed by displacement history. Conventional tertiary injection produced negligible incremental recovery, whereas early tertiary injection of diluted seawater yielded 11.3% additional oil recovery (11.8% when formation water was used as the tertiary brine), comparable to values commonly reported for secondary low-salinity flooding. Ion chromatography revealed that the onset of tertiary oil production occurred only after the preceding brine had been displaced from the core. This indicates that chemical-front propagation, in situ mixing, and fluid accessibility govern when the tertiary response becomes observable. The data also confirmed transient formation-water banks ahead of the injected brine front. These findings indicate that tertiary low-salinity performance is controlled not only by brine composition, but also by the fluid occupancy and chemical accessibility inherited from prior flooding at the moment of brine introduction. More broadly, the results support viewing secondary, early tertiary, and conventional tertiary flooding as successive stages of flood maturity, each probing a different displacement state of the reservoir. Conventional tertiary corefloods may therefore underestimate the observable low-salinity response by introducing the modified brine only after extensive prior brine invasion and oil displacement. This highlights injection timing as a key mechanistic variable in chalk waterflooding.