Studies On Polymer Solution Dissolved By Nanofiltration Softened Seawater For Oil Displacement

The most convenient source of water for polymer flooding on offshore oil field is seawater, but it can't be used directly because the high salinity can make the viscosity of polymer solution lower. Nanofiltration (NF) membrane has distinct advantages in removing divalent ions and multivalent ions such as Ca2+, Mg2+ and SO42-. Comparing with other softening processes, for example lime softening and ion exchange, NF has a bright future in application on the space-limited platform with the advantage of smaller land occupation and stable water quality.Based on the viscosity requirement of polymer solution for the offshore oil field, several kinds of water softened by NF membrane and polymers were studied and compatibility between polymer and softened seawater, viscosity property were studied. This work will provide experimental references for design of polymer flooding engineering on offshore platform using softened water.In this thesis, two types of polymer solutions prepared by polyacrylamide (PAM) numbered as PAM-D and PAM-S were studied. Influencing factors on the apparent viscosity of the PAM solution, including the dissolved and use temperature, stirring rate, shear rate, solution pH and salinity, were discussed. The results showed that the viscosity of PAM solution declined with the increasing temperature and stirring shear; and declined sharply in strong acid or alkali environment, which is suitable for use at the pH range of 4-10. The viscosity was seriously affected by salinity, especially by multivalent cation. For example, low concentration of Al3 + and Fe3 + (at 50mg/L) caused the precipitation of polymer from solution; Ca2 + and Mg2 + which are of high concentrations in seawater also caused the viscosity of polymer solution decreasing significantly, so these two ions in softened seawater for polymer flooding should be controlled below 200mg/L.After mastering the impact factors of polyacrylamide viscosity, the preparation of polymer solution using softened seawater, including dissolution experiments at different softened seawater, viscosity experiments and displacement experiments, were studied, then NFI membrane and PAM-D were chosen. The results showed that there was a significant reduction in Ca2+, Mg2+ and salinity of seawater after nanofiltration softening. The NFI softened water had an obvious viscosity enhancement at the reservoir environment, viscosity retention rate still maintained at 70% after 90 days. The core displacement experiments also showed that the solution of NFI softened water and PAM-D had good compatibility with the core.