Abstract: The L low permeability reservoir in Bohai Sea as a whole shows the characteristics of medium porosity and low permeability, and the reservoir has low fluid density and low viscosity. Water drive energy replenishment has been carried out in the early stage of development, so it is urgent to explore the feasibility study of different development methods to provide reference for subsequent reasonable development. However, the micro-operation characteristics, mechanism and influencing factors of different displacement methods in this reservoir are still unclear. Taking L low permeability reservoir in Bohai Sea as the research object, this paper selected two representative sandstone cores in the reservoir, and applied online nuclear magnetic displacement technology to carry out indoor experimental tests of core-CO₂ miscible oil displacement and water-driven to CO₂-miscible oil displacement. Real-time scanning of cores was carried out during the displacement process to identify the differences in micro-operation characteristics and influencing factors of different displacement methods. The results show that after 2.0PV fluid displacement under the same displacement conditions, the CO₂ miscible flooding of high permeability core a (33.8×10^-3μm²) and low permeability core b(2.95×10^-3μm²) (final displacement efficiencies of 69.31% and 66.18%, respectively) is better than that of water drive (displacement efficiencies of 58.07% and 56.97%, respectively). Compared with low-permeability cores, high-permeability cores have more large pores and strong pore connectivity, and the displacement efficiency of CO₂ miscible flooding and water-to-CO₂ miscible flooding is increased by 3.13% and 1.1%, respectively, indicating that physical properties have little influence on displacement efficiency. The lower limit of pore throat utilization in water flooding of high permeability core and low permeability core is 0.019 7 μm and 0.009 8 μm respectively, and the lower limit of pore throat utilization is lower because of the large pressure difference caused by low permeability. When the water flooding is changed to CO₂ miscible flooding, the three phases of oil, gas and water flow, which increases the experimental pressure difference, and the lower limit of pore throat operation is reduced to 0.008μm and 0.0049μm respectively, which is close to the lower limit of pore throat operation (0.008 μm and 0.005 2 μm) of the two cores that have been using CO₂ miscible flooding. CO₂ miscible flooding is recommended as a reasonable development method for L reservoir.

Key words: on-line nuclear magnetic, CO₂ flooding, water drive to CO₂ drive, sandstone reservoir, displacement efficiency