| Combined chemical flooding has been proven to be an effective IOR technique for reservoirs after water and polymer flooding. There are 36 chemical flooding units in Shengli oilfield. The producing geological reserves is 3.76×108t, of which 29 units for polymer flooding, and 80.6% of the chemical flooding reserves. Polymer flooding has made great contributions to maintain the production and to enhance oil recovery in reservoirs with high water cut. However, due to some restrictions of polymer flooding techniques, the enhanced oil recovery factor is only 6-10%, and there are still about 40-60% of crude oil remaining underground after polymer flooding. Presently 17 polymer flooding units have been returned back to follow-up water flooding, and the water cut of the 10 units back to the level before polymer flooding. Therefore it is necessary to used new enhance oil recovery techniques after polymer flooding. Combined chemical flooding, including foam/polymer flooding and surfactant /preformed particle gel/polymer flooding and other chemical displacement system, has been found to be an effective method to further improve oil recovery for reservoirs after conventional polymer flooding.In this study, the changes of reservoir properties and remaining oil distribution were systematically studied using numerical simulation, core data, new drilling data and reservoir engineering analysis for a targeted reservoir (Zhongyi Block of Gudao Region) in Shengli Oilfield, in which polymer flooding has been applied for many years. After polymer flooding, the porosity, permeability, median grain size and the pore throat radius of the oil formations have increased, while the clay content decreased due to long time water and polymer flood. The hydrophilicity of the formation rock increased and the residual oil saturation decreased. The remaining oil has a wide distribution in the space, and high oil saturations can be found between the oil wells, along flow stream lines, on the top of the reservoir and among the interbeds.An optimized combined chemical system, formulated with 0.3% petroleum sulfonate + 0.1% surfactant +900 mg / L PPG (Preformed Particle Gel) +900 mg / L polymer, has been designed for reservoirs with high heterogeneity after polymer flooding. The chemical agent can reduce the crude oil - water interfacial tension as low as to 6.0×10-3mN/m. The incremental oil recovery achieved can be as high as 13.6% by laboratory experiment, and up to 8.5% was predicted by reservoir simulation for the targeted oil field test. The displacement process and the flow characteristics were studied using a large experimental model and a micro-displacement device for different flooding systems, such as water flooding, polymer flooding, S+P flooding, PPG flooding and the Combined flooding. The polymer flooding can increase the sweeping efficiency comparing with water flooding, while there was still a lot of remaining oil in areas unswept by polymer. The S+P flooding can further improve the displacement efficiency in comparison with polymer flooding, but its sweep efficiency is similar to that of the polymer. PPG flooding can make the remaining oil in porous media re-distribution and cause the oil unaffected by polymer or S+P flooding to flow again, but its displacement efficiency is limited. The combined system of surfactant/preformed particle gel/polymer can improve the sweeping efficiency effectively and also increase the displacement efficiency, and then enhance the oil recovery significantly. The combined chemical flooding technique has been evaluated in a pilot project conducted for the targeted oil reservoir, which has shown to be effective. |
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