Investigation into the Displacement Efficiency of LNAPL Under Hydrodynamic Conditions

During the processes of oil extraction, storage, and transportation, pollution caused by frequent fluctuations in the groundwater level and interface changes is complex, and oil leakage has become a major challenge. To effectively identify the water–oil displacement process and the influence of hydrodynamic conditions on displacement efficiency, this paper presents the results of multiple laboratory experiments, focusing on the effects of hydraulic head, medium particle size, displacement direction, and apparatus dimensions on oil displacement efficiency. Three types of media were used: fine sand, medium sand, and coarse sand, simulating two displacement directions—top-down and bottom-up. Displacement efficiency was evaluated by integrating gravity drainage and hydrodynamic displacement processes. The results show that medium particle size has a significant impact on displacement efficiency: coarse sand achieves the highest displacement efficiency (98.74%), whereas fine sand yields the lowest (61.7%). Increasing the hydraulic head significantly improves oil displacement efficiency. The top-down displacement direction exhibits higher efficiency due to gravitational assistance. In addition, apparatus dimensions and sand sample height also significantly affect displacement efficiency. Increasing the apparatus diameter enhances gravity drainage efficiency, while increasing the sand sample height reduces hydrodynamic displacement efficiency. This study provides a scientific basis for remediating LNAPL-contaminated sites and suggests optimizing hydraulic head and displacement direction in practical applications to improve remediation efficiency.