It has long been recognized that there are synergies between EOR (enhanced oil recovery) techniques, as is the case between low salinity waterflooding and polymer flooding. While polymer flooding is a proven technology which improves waterflood recovery efficiency by increasing the water-oil viscosity ratio, low salinity EOR is an emerging technology which enhances the displacement efficiency by changing wettability. One would expect the combined EOR schemes to be at least additive because they target different things. For other reasons, the combination of these two technologies can expand their range of application.
Polyacrylamide polymers widely used in chemical EOR hydrolyze at high temperatures and precipitate if calcium concentration is above 200 ppm. The use of low salinity EOR can make the use of polyacrylamide polymers feasible up to reservoir temperatures of at least 100°C or enable the use of polymers with larger degrees of hydrolysis. Polyacrylamide solution viscosity increases as salinity decreases so OPEX will reduce due to lower required polymer concentration. One third or less of polymer is required with low salinity waterflooding EOR compared to conventional water flooding.
We demonstrate that addition of polymer enhances recovery efficiency and timing of low salinity waterflood. The incremental oil recovery between the homogeneous and heterogeneous cases is close, especially for more viscous oil. At high oil viscosity, combination of low salinity injection water and polymer gives incremental oil recovery about equal or better than the summation of each if used separately. We demonstrate that both secondary and tertiary combined processes are effective but secondary gives better timing. Synergistic behavior of combined processes is more effective in tertiary flood than the secondary cases. Chemical cost comparison of the cases studied shows that a 5-time reduction in chemical cost per barrel of oil recovered can be expected when polymer is added to low salinity. We show that operational constraints in field application can limit the synergy between these combined processes.