Abstract:

The L low permeability reservoir in Bohai Sea is characterized by medium porosity and low permeability, with reservoir fluids exhibiting low density and low viscosity. Water flooding has been implemented in the early stage to supplement energy. It is therefore urgent to explore the feasibility of different development methods to provide a basis for subsequent rational development. However, the microscopic operation characteristics, mechanisms, and influencing factors of different displacement methods in this reservoir remain unclear. Taking the L low permeability reservoir in Bohai Sea as the research object, two representative sandstone cores from the reservoir were selected. Using online nuclear magnetic resonance displacement technology, indoor experiments were conducted on core samples using CO₂ miscible flooding and water flooding to CO₂ miscible flooding. Real-time scanning of the cores was carried out during the displacement process to identify the differences in microscopic operation characteristics and influencing factors among the different displacement methods. The results showed that under the same displacement conditions with 2.0 PV of injected fluid, CO₂ miscible flooding achieved higher final displacement efficiencies in both the high-permeability core a (33.80×10^-3 μm², 69.31%) and low-permeability core b (2.95×10^-3 μm², 66.18%), compared with those of water flooding to CO₂ miscible flooding (58.07% and 56.97%, respectively). Compared to the low-permeability cores, the high-permeability core had a higher proportion of large pores and better pore connectivity. The displacement efficiency increased by 3.13% and 1.10% for CO₂ miscible flooding and water flooding to CO₂ miscible flooding, respectively, indicating that physical properties had a limited effect on displacement efficiency. During water flooding, the lower pore-throat operation limits were 0.019 7 μm and 0.009 8 μm for the high-permeability and low-permeability cores, respectively. Lower permeability led to higher pressure differentials, resulting in lower pore-throat operation limits. When water flooding was switched to CO₂ miscible flooding, three-phase flow of oil, gas, and water occurred, further increasing the experimental pressure differential and reducing the lower limits of pore throat operation to 0.008 μm and 0.004 9 μm, respectively, which were close to the lower pore-throat operation limits (0.006 9 μm and 0.005 2 μm) of the two cores that had been using CO₂ miscible flooding. CO₂ miscible flooding is recommended as an optimal development method for the later stage of the L reservoir.

Key words: online nuclear magnetic resonance, CO₂ flooding, water flooding to CO₂ flooding, sandstone reservoir, displacement efficiency