Abstract
Restricted by the short life of the oil development platform and its high investment, offshore oilfield needs higher oil recovery rate in the early developmental stage so as to recover its investment fast. Therefore, polymer flooding method carried out in medium water cut period was proposed. The production performance improved through the use of polymer flooding in Bohai H oilfield, but became worse when the polymer flooding reached the end stage. The oil recovery increased by 10.5%. It is important to find a way to improve production performance other than polymer flooding. Therefore, this article optimizes surfactant-polymer flooding as the subsequent technology that enhances oil recovery. By using laboratory experiment to compare different experimental schemes, two important parameters including number of injection pore volume and injection timing were optimized. The research result was applied in field development of Bohai H oilfield. Based on the research results, this article concludes innovatively that surfactant-polymer flooding can enhance oil recovery significantly in offshore oilfield, and the number of injection pore volume and injection timing can affect the surfactant-polymer flooding obviously. It is reasonable when the number of injection pore volume is 0.3 PV for Bohai H oilfield; oil recovery can further increase if surfactant-polymer flooding is carried out in higher water cut period. The field application from 2013 to 2018 shows that the oil production performance of Bohai H oilfield can be improved significantly by using surfactant-polymer flooding; the daily oil production increased by about 1.25 times for average single well; and the oil recovery increment forecasted is 4.6% points. The research result can be used as an important reference for similar offshore oilfields to enhance oil recovery. Key words: Surfactant-polymer flooding, enhancement of oil recovery, injection pore volume number, injection timing, offshore oilfield.  
Highlights
Tapping the potential of the remaining oil is an important research work for the development of offshore oilfield, the geological conditions are usually complex and very heterogeneous, and this is a common phenomenon in offshore oilfields
Due to great distance between the injection and production well and the irregular well pattern of the offshore oilfield, the proven methods for enhancing oil recovery in onshore oilfield cannot be used in offshore oilfield directly
With the progress of technology and needs of technicians (Salehi et al, 2013; Falode and Ojumoola, 2015; Aifen et al, 2011; Dongdong et al, 2016; Hongying et al, 2012), polymer flooding has become an important method for enhancing oil recovery, but this method will get worse if the scheme reaches the end of the stage
Summary
Tapping the potential of the remaining oil is an important research work for the development of offshore oilfield, the geological conditions are usually complex and very heterogeneous, and this is a common phenomenon in offshore oilfields. S/P flooding method can improve the production performance besides polymer flooding in Bohai H oilfield. The capability of single polymer flooding to enhance recovery comes from the improvement of low permeability core production.
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