Abstract:

Based on the principle of nuclear magnetic resonance (NMR)，the relationship between NMR relaxation time and pore throat radius of core sample was derived. Combining with the pore throat radius distribution obtained from high-pressure mercury intrusion test，the NMR T₂ spectrum was converted into the pore throat radius distribution by using the numerical difference and the least square methods. Then，according to the law that the oil saturation and NMR signal amplitude gradually decrease synchronously during core displacement process，the changes in the NMR signal amplitude of different pore throat radii under different gas injection rates，gas injection pressures and positions were analyzed，and the lower pore throat producing limits for different development patterns were determined. The results show that the crude oil in micron-sized pores is the main contributor to water flooding recovery. The minimum pore throat producing limit is 0.04 μm at low gas injection rate and 0.01 μm at high gas injection rate. When the gas injection pressure increases from 5.0 MPa to 17.0 MPa，the lower pore throat producing limit decreases from 0.05 μm to 0.01 μm. When the gas injection pressure is high，the overall pore throat producing degree of the core is relatively uniform. With the increase of gas injection pressure，the remaining oil in the pores with the radius less than 0.01 μm is mainly produced.

Key words: natural gas drive, water flooding, NMR, high-pressure mercury intrusion, pore throat radius, producing limit, gas injection rate, gas injection pressure