Oil leakage during the processes of extraction, storage, and transportation poses a significant challenge due to the complex nature of pollution caused by frequent fluctuations in groundwater levels and variations in the water–oil interface. To effectively identify and monitor the position of the water–oil interface and displacement processes, geophysical methods have proven to be an efficient approach. This study utilizes electrical resistivity measurements to analyze changes in medium resistivity during water–oil displacement, enabling simulation of the spatial relationship between groundwater levels and petroleum contaminants based on resistivity characteristics and natural potential responses. After analysis, the following conclusions can be drawn: (1) During the water displacement process, when water forms a connected flow channel between sand and gravel, the resistivity decreases abruptly. Conversely, during oil displacement by water, when oil fills soil pores and creates a high-resistance conductive path, the resistivity increases abruptly. (2) Changes in resistivity are determined by the position of the water–oil interface. By observing characteristic changes in resistivity, it is possible to verify whether soil is undergoing water–oil displacement. (3) The direction of displacement significantly affects changes in resistivity for all three media involved due to gravity effects during water displacement by the oil process. (4) Resistance values during the water–oil displacement process are directly influenced by the size of sand particles used in experiments.

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.