Mechanisms Of Near Miscible CO₂ Flooding For Eor In Low Permeability Reservoirs

The proportion of low and extra-low permeability reservoirs in China's prospective reserves is gradually increasing year by year. Characteristics of such reservoirs are low permeability, lacking of reservoir energy, rapid decrement of well productions and inferior development of oil field. With the traditional ways, it is easy to convert to a dissolved gas drive, which is characterized by the poor efficiency and long development cycle.Research indicates that CO₂ flooding, which can be developed as an effective EOR technique for those low permeability reservoirs is a good option due to its superior miscibility effect with oil. As an important criterion to select the suitable reservoirs for CO₂ flooding, it is miscible phase that plays an essential role in the screening process. However, due to the restrictive factors involved formation pressure, the purity of injected CO₂, the properties of oil, formation heterogeneity and so on, miscible CO₂ flooding of the field application has some limitations. Yet non-miscible CO₂ flooding has inferior recovery. Thus researchers proposed the concept of near miscible CO₂ flooding. When the displacement pressure is lower than MMP (minimum miscible pressure), there is a press range where the CO₂ flooding could also attain a good recovery. Although it does not reach the standard of miscible CO₂ flooding strictly, it requires less stringent pressure and oil components and reflects varying miscible effect, such as visbreaking, swell, evaporation and condensation.Based on the mechanisms of immiscible displacement and miscible displacement, combined with approach of physical simulation (PVT analysis, slim tube tests and core displacement experiments), numerical simulation, and theoretical analysis, this paper developed a method for near miscible CO₂ flooding, and determined the press range of near miscible condition. It also announced the influencing factors of near miscible CO₂ flooding effects, besides phase-state features and regularity of oil's PVT, and took initial shape about the theory system of near miscible. Then the analyzed conclusion about these three diverse miscible flooding processes can provide reference optimum proposal for the CO₂ flooding pilot project in low and extra-low permeability reservoirs.