Due to the strong mobility and energizing effect, CO2 can be injected into reservoir as a displacement agent to increase oil production in low permeability reservoir. However, low viscosity of pure supercritical CO2 (SC-CO2) resulted poor sweep efficiency. Therefore, it is essential to thicken SC-CO2 during displacement in low permeability reservoir.

In this study, a novel thickened SC-CO2 system was developed. Its viscosity with different thickener was measured by HP/HT core flooding setup. To explain the EOR mechanism, the oil saturation and sweep area in low permeability core by pure/thickened SC-CO2 flooding system was monitored through NMR on time. Furthermore, the effect of thickener concentration and flooding rate were investigated in detail, respectively.

With the HP/HT core flooding setup, the viscosity of thickened SC-CO2 system can reach to over 0.20 mPa·s while it is 0.04 mPa·s for pure SC-CO2. Increased thickener content leads to enlarged fluid viscosity within system. The increased viscosity resulted in longer breakthrough time of thickened system. After breakthrough, the continuous injection of thickened SC-CO2 system further decreases the remaining oil saturation much more obvious than that with pure SC-CO2. By analyzing the real-time distribution from T2 spectra and NMR imaging, the thickened system can involve the smaller pores which the pure SC-CO2 cannot get through. Meanwhile, thickened system can form a near piston displacement while pure system has an obvious viscous fluid advantage channel. The combination of these two processes results in higher sweep efficiency with thickened SC-CO2. Furthermore, the effect of thickener concentration and flooding rate were shown in detail. The results showed that higher thickener concentration and lower flooding rate all corresponded to higher sweep efficiency.

Due to the high injection pressure of water flooding and low sweep efficiency of pure CO2 flooding, thickened SC-CO2 flooding is proved to be a more applicable flooding agent to develop low permeability reservoir even tight oil. This study will provide new thoughts and insights mechanism about how to develop low permeability reservoir more efficiently and economically.

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