Abstract

CO2-EOR is an effective technology for reducing CO2 emissions while enhancing oil recovery in ultra-low permeability reservoir, which has been performed in Shengli Oil Field, China since 2013 with cumulative CO2 injection of 12588 t by 2016. However, the area heterogeneity of reservoir resulted in serious gas channeling and poor production performance. Performance control methods including sweeping area regulation, differential pressure control and real-time producing regulation were proposed to enlarge sweeping area and improve CO2 utilization in areal heterogenous reservoir. 3D physical models of areal heterogeneity and five-spot pattern were utilized in the laboratory. Conventional CO2 flooding, sweeping area regulation, differential production pressure control and real-time producing regulation were conducted respectively in the 3D models, and the flooding efficiency was evaluated through oil recovery increments and changes of performance curves. Corescale numerical modeling was also built to study the profile improvements of CO2 flooding by the performance control methods.

Experimental and numerical simulation results showed that CO2 was displaced unevenly in the areal heterogeneous reservoir, leaving plenty of oil remained in the relatively high and relatively low permeability area. The oil recovery of CO2 flooding in areal heterogenous reservoir can be doubled by performance control methods of sweeping area regulation, differential pressure control or real-time producing regulation. The remaining oil in relatively low permeability area can be effectively displaced by sweeping area regulation, while both larger sweeping area and better CO2 flooding profile can be achieved by differential pressure control and real-time producing regulation. Higher productivity of individual well can be obtained in the early and middle stage of CO2 flooding by differential pressure control, while similar oil & gas production performance and longer displacement period of CO2 injection can be achieved by real-time producing regulation. The performance improvement of CO2 flooding by performance control methods provided a feasible technical strategy for enhancing oil recovery of areal heterogeneous reservoir in the oil field under the condition of a lower oil price.

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