Quantitative characterization of the extent of crude oil mobilization and the range of pore–throat utilization during the gas flooding stage in low-permeability reservoirs

As low-permeability sandstone reservoirs gradually become the focus of oil and gas development in China, quantitatively evaluating the impact of different gas injection mediums on oil recovery and the range of pore–throat utilization is key to improving recovery rates. This study focuses on the Wutonggou formation low-permeability sandstone reservoir in the Shanan oilfield, using nuclear magnetic resonance (NMR) and computed tomography (CT), combined with high-temperature and high-pressure physical flow simulation experiments, to systematically analyze the dynamic characteristics of oil recovery and pore–throat utilization during CO2 and N2 flooding. The experimental results indicate that when the injection pressure increases from 6 to 26 MPa, CO2 flooding significantly improves oil recovery from 19.95% to 59.47%, with medium and large pore–throat utilization far exceeding that of small pore–throats, and the maximum pore–throat utilization range occurs at 18 MPa. N2 flooding shows lower recovery, increasing from 23.11% to 49.95%, with limited improvement in small pore–throat utilization, and the maximum pore–throat utilization range is achieved at 10 MPa. By combining NMR and CT, a positive correlation between pore–throat radius and relaxation time is revealed, and a mathematical model for pore–throat utilization radius is developed, defining the utilization limits of CO2 and N2 flooding. The study shows that CO2 is more suitable for heterogeneous, small-pore low-permeability reservoirs, providing theoretical support for gas injection optimization and development plans for low-permeability reservoirs.