Experimental Study and Quantitative Evaluation on Oil Recovery from Different Pore Sizes of Tight Sandstone Reservoirs Under N2 and CO2 Flooding

Abstract The low permeability of tight sandstone reservoirs limits the application of water flooding to improve oil recovery. Owing to their properties, N2 and CO2 have been used in gas flooding to increase the oil recovery from these reservoirs, achieving good results. However, the oil-displacement mechanism at different pore sizes in tight sandstones during N2 and CO2 flooding, especially the quantitative characterization and comparison of the displacement effects of gas flooding, remain unclear. In this study, gas flooding experiments were conducted combined with Nuclear Magnetic Resonance (NMR) to simulate the distribution characteristics of crude oil before and after N2 and CO2 flooding in five tight sandstone core samples at five different displacement pressures from 4.0 MPa to 24.0 MPa at 60°C. The fluid distribution and oil recovery rate under CO2 and N2 flooding were quantitatively evaluated at different pore scales under different displacement pressures. The results revealed that the difference between N2 and CO2 flooding in the large pores was considerably higher than that in the small pores. However, the variation trend of the difference in oil recovery from small and large pores under N2 and CO2 flooding at different pressures was almost identical. Consequently, this variation was only slightly influenced by the pore radius and highly influenced by the displacement pressure. Additionally, before CO2 became miscible, the maximum difference in the recovery rate between N2 and CO2 flooding was observed when CO2 reached the supercritical state. After CO2 became miscible, the difference between N2 and CO2 recovery continued to increase with increasing pressure. Furthermore, for similar tight sandstone reservoirs, the total oil recovery rate under CO2 flooding was found to be always 10–20% higher than that under N2 flooding. Subsequently, the larger the pores, the better the oil recovery rate under CO2 flooding than that under N2 flooding. This study may provide significant guidance during field production.