(559l) Experimental Investigation of Synergy between Low Salinity and Polymer Flooding in Enhancing Heavy Oil Recovery on Alaska North Slope

The first ever polymer flood pilot has been initiated to develop the heavy oil (100-5,000 cp) on the Alaska’s North Slope (ANS), where thermal methods are not feasible due to various environmental, technical and economic concerns. This study aims to understand the role of salinity during polymer flood. Several striking findings were observed, which are beyond the expectation of the well-established classic polymer flooding philosophy.

Core flooding tests were carried out at reservoir temperature. The crude oil viscosity was 286.3 cp. Residual oil saturation condition was established by flooding the cores with sufficient brine (> 40 pore volumes, salinity = formation brine = 26,700 ppm). High salinity polymer flooding (HSPF, 26,700 ppm) and low salinity polymer flooding (LSPF, 2,500 ppm) were then sequentially carried out. Residual resistance factor was measured after each polymer flooding process. The effluent was monitored to obtain the water cut and oil recovery factor. Microscopy observation was also conducted to examine the effluent. Reproducibility of the results were validated.

Several interesting findings were observed, some of which were unexpected according to traditional polymer flooding theory. 1) The oil recovery was effectively increased (~5 OIIP%) during HSPF despite the oil viscosity was close to 300 cp , which is well beyond the applicable limit according to traditional screening criteria (oil viscosity < 150 cp). It proves that polymer flooding is still valid even beyond the oil viscosity limit. 2) A significant reduction in residual oil saturation by polymer flooding after tens of pore volumes of water flooding. Traditionally, the well-established polymer flooding philosophy believes that the oil recovery is increased by an improved sweep efficiency. There is no change in residual oil saturation (and thus displacement efficiency) compared with water flooding. 3) After sufficient pore volumes of HSPF, further oil saturation reduction and increase in oil recovery could still be achieved by LSPF even with slightly lower viscosity. 4) O/W emulsion was observed at effluent during LSPF, while no emulsion was detected during HSPF. The in-situ emulsification might be one key mechanism to achieve remarkable incremental oil recovery (~13 OIIP%) by LSPF. The results were tested to be reproducible.

This study suggests polymer flood not only increases the sweep efficiency by improving the mobility ratio, it also increases the displacement efficiency. Compared with HSPF, a better performance can be achieved by LSPF, where in-situ emulsification occurs. The study is useful to expand the application limits of polymer flood and find the optimal salinity for the pilot test. More work will be done in the near future to further investigate the EOR mechanisms of LSPF.