Abstract
Polymer flooding has been proven to effectively improve oil recovery in the Bohai Oil Field. However, due to high oil viscosity and significant formation heterogeneity, it is necessary to further improve the displacement effectiveness of polymer flooding in heavy oil reservoirs in the service life of offshore platforms. In this paper, the effects of the water/oil mobility ratio in heavy oil reservoirs and the dimensionless oil productivity index on polymer flooding effectiveness were studied utilizing relative permeability curves. The results showed that when the water saturation was less than the value, where the water/oil mobility ratio was equal to 1, polymer flooding could effectively control the increase of fractional water flow, which meant that the upper limit of water/oil ratio suitable for polymer flooding should be the value when the water/oil mobility ratio was equal to 1. Mean while, by injecting a certain volume of water to create water channels in the reservoir, the polymer flooding would be the most effective in improving sweep efficiency, and lower the fractional flow of water to the value corresponding to $$ \Delta \,J_{\hbox{max} } $$ . Considering the service life of the platform and the polymer mobility control capacity, the best polymer injection timing for heavy oil reservoirs was optimized. It has been tested for reservoirs with crude oil viscosity of 123 and 70 mPa s, the optimum polymer flooding effectiveness could be obtained when the polymer floods were initiated at the time when the fractional flow of water were 10 % and 25 %, respectively. The injection timing range for polymer flooding was also theoretically analyzed for the Bohai Oil Field utilizing relative permeability curves, which provided methods for improving polymer flooding effectiveness.
Highlights
The results showed that when the water saturation was less than the value, where the water/oil mobility ratio was equal to 1, polymer flooding could effectively control the increase of fractional water flow, which meant that the upper limit of water/oil ratio suitable for polymer flooding should be the value when the water/oil mobility ratio was equal to 1
Heavy oils account for about 69 percent of the total of about 17.8 9 108 m3 original oil in place discovered in offshore reservoirs in the Bohai Oil Field
The effect of the water/oil mobility ratio on the efficiency of polymer flooding is studied at different fractional flows of water in heavy oil reservoirs using relative permeability curves, and the polymer injection timing is optimized
Summary
Heavy oils account for about 69 percent of the total of about 17.8 9 108 m3 original oil in place discovered in offshore reservoirs in the Bohai Oil Field. The largest difference between onshore and offshore oil production is that the design service life of an offshore platform is less than 30 years (Rivas and Gathier 2013) It is extremely important for offshore oil fields to select an optimum injection timing to initiate polymer floods in heavy oil reservoirs (Zhang et al 2013) for better ultimate oil recovery. The effect of the water/oil mobility ratio on the efficiency of polymer flooding is studied at different fractional flows of water in heavy oil reservoirs using relative permeability curves, and the polymer injection timing is optimized
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