Polymer flooding and low salinity waterflooding are two different but potentially complementary Enhanced Oil Recovery (EOR) techniques. Polymer flooding improves fractional flow and sweep efficiency by improving the mobility ratio for the displacement. Low salinity waterflooding improves pore scale displacement efficiency by changing the wettability of the reservoir rocks toward more water-wet. Reduced salinity water is often used in polymer injection to reduce hydrolysis however the water salinity in this case is typically higher than that needed to obtain a true low salinity effect. This paper describes the outcomes of a systematic study into the potential benefits of combined polymer-low salinity waterflooding versus polymer-high salinity waterflooding, polymer-reduced salinity waterflooding and conventional waterflooding.
Numerical simulation, validated against analytical solutions, was used to evaluate the relative performance of these processes. The impacts of layering and reservoir heterogeneity were investigated using two-dimensional (2D) and three-dimensional (3D) reservoir models. Sensitivity studies of injected water salinity and the start time of injection were carried out in each of these models. Outcomes were compared against the recoveries and water cuts predicted using a one-dimensional (1D) analytic solution for the EOR processes to evaluate the impact of sweep versus displacement efficiency on incremental oil recovery and water cut.
Combined polymer-low salinity waterflooding shows an improvement in recovery and reduction in water cut compared with the other EOR processes in all cases. We show this is partly due to improving the fractional flow (increasing shock front saturation) but is also due to both the leading and trailing shock fronts in polymer-low salinity waterflooding being more stable than in the other EOR processes, reducing the possibility of viscous finger growth and thus increasing performance. The highest incremental oil recovery is observed when the injected water salinity in the combined polymer-low salinity waterflooding is reduced to below the low salinity threshold. It is clearly beneficial to reduce the water salinity to this low level rather than just to a salinity where hydrolysis is prevented. The injection of the combined EOR technique in tertiary mode, particularly at 75% water cut after performing high salinity waterflooding, exhibits an incremental oil recovery of between 15 and 42% and a reduction in water cut of between 11 and 48% at 1.0 pore volume injected (PVI).
This is the first systematic investigation into the performance of combined polymer-low salinity waterflooding compared with conventional waterflooding, low salinity waterflooding, and polymer flooding with reduced salinity water. It provides a clear insight into the benefits of combined EOR process justifying the need for field scale pilots and further laboratory studies.