There are many constraints in offshore reservoirs such as lack of fresh water, limited space and short life of platform, and unstable working environment when polymer flooding is implemented to enhance oil recovery. In order to develop offshore oilfields in an efficient and economical way, a study on optimal polymer flooding injection timing is presented, and mechanism analysis is discussed.
Eight polymer injection timing programs were conducted by using both artificial homogeneous cores and heterogeneous ones. Results showed that recovery factor increased with injection timing advanced, and the earlier injection timing, the fewer PV was needed. The final recovery enhanced 2.96%–11.16% and PV decreased 0.41–0.48 with injection timing ranged from at water cut 95% to direct polymer injection, which can save cost economically and speed ??up the development of oil fields; polymer injection timing was more influential in heterogeneous reservoir than in homogeneous ones which can be observed from the change of oil recovery and decline of water cut.
According to theoretical analysis, polymer can improve sweep efficiency with mobility ratio M<1. M is a function of viscosity ratio and relative permeability. When the viscosity ratio is a constant, early relative permeability rate Krp/Kro<<1which can be seen from polymer flooding relative permeability curves, leading to M << 1, so the effect of the flooding is increased even using lower concentration of polymer at the early injection timing.
However, injection pressure increased with polymer injection timing advanced, leading to injection issues. Water flooding for a period of time was considered to reduce the issues, so water cut of about 60% is recommended as the optimal polymer injection timing in the offshore fields based on practicality, performance and economic viability.
