Impact of relative timing of low salinity and polymer flooding on mechanisms by which oil recovery is improved

A combination of enhanced oil recovery (EOR) methods, specifically polymer flooding and low salinity (LS) brine injection, has been shown to improve oil recovery beyond what is achievable with either method used alone. However, the optimal sequence and timing of these methods remain unclear, affecting their efficiency. This study investigates the impact of injection sequences and timing of LS brine and polymer to optimize oil recovery by understanding the underlying mechanisms. Six injection scenarios were tested: (1) injecting high salinity (HS) water followed by LS brine (tertiary injection), (2) injecting HS water to intermediate saturation followed by LS brine, (3) injecting LS brine directly (secondary injection), and in each case, (4) polymer injected simultaneously with LS brine, (5) polymer injected after the LS brine, or (6) polymer injected before the LS brine. The results showed a positive synergy between LS brine and polymer in both secondary and tertiary injections. This synergy is highly sensitive to injection timing, sequence, and rock/fluid properties. The combined effect of LS brine and polymer shifts the flow regime by altering the balance between capillary and viscous forces, maximizing oil recovery when both mechanisms are active. Conversely, the effectiveness declines when one mechanism dominates. Therefore, the timing and order of polymer and LS brine injection significantly influence displacement efficiency and oil recovery, with different injection sequences producing varying outcomes, even with the same EOR techniques.