# Numerical Simulation of Oil Pipeline Leakage Diffusion in Dashagou Yellow River Crossing Section
## 2 Materials and Methods
### Research Methods  
For the Dashagou Yellow River crossing section of the pipeline, leakage patterns include four main types of scenarios. Scenario 1: Pipeline buried position is located above the groundwater table line, leaking oil passes through the pipeline defect position or leakage port under pipeline high-pressure conditions and enters into the groundwater after leaking in the air-packed zone, and the oil solutes migrate downstream under the groundwater dynamic conditions, as shown in Figures 2-4. Figure 3: Schematic diagram of oil migration after oil leakage into groundwater under scenario 1. Figure 2: Schematic diagram of oil leakage into groundwater under scenario 1. The process can be summarized as the movement of oil from the air-packed zone to the saturated zone. When the cut-off valves on the upper and lower sides of the pipeline are opened and the additional pressure (i.e., the pipeline oil delivery pressure) disappears, the pipeline is located above the groundwater line, and when the oil leaking to the soil loses the additional pressure, the movement of the oil is mainly due to the combined effect of gravity and capillary force, and it is transported vertically downward to the groundwater line and arrives at a certain depth of the aquifer. After reaching a certain depth of the aquifer, it reaches the balance of force, stops penetrating vertically downward in the aquifer and migrates laterally downstream under the action of groundwater power. In addition, the migration of the solute part of the oil will be faster than that of the pure phase oil. Scenario 2: The pipeline buried position is located below the groundwater table line, leaking oil enters into the groundwater system through the pipeline defect position or leakage port under pipeline high-pressure conditions, and oil solutes migrate downstream under groundwater dynamic conditions, as shown in Figures 5 and 6. Figure 4: Schematic diagram of continuous migration along groundwater of solute dissolution after oil product enters groundwater under scenario 1. Figure 5: Schematic diagram of oil leakage in groundwater system under scenario 2. Scenario 3: The Shuanglan line oil leakage in the Dashagou area is due to the high pressure action of the overlying strata fracture, the formation of leakage in the bottom of the ditch in the Dashagou leakage port and oil from the leakage port and along the bottom of the ditch trips oil overflow as shown in Figures 7 and 8. Scenario 4: The Shuanglan line in the Yellow River crossing section through the pebble layer of the pipe section leakage enters into the Yellow River in the Yellow River river surface oil spill diffusion, or oil from the Dashagou within the pipeline flows into the pebble layer, long term, along the pebble layer transport into the Yellow River slopes and then from the pebble layer profiles into the Yellow River in the Yellow River river surface oil spill in the hydrodynamic conditions of the downstream migration as shown in Figures 9 and 10. Figure 6: Schematic diagram of solute continuous migration of oil in groundwater system under the action of groundwater under scenario 2. Figure 7: Schematic diagram of oil squeezing the soil layer in the vadose zone to form cracks and flow to the bottom of Dashagou under scenario 3. Combined with the hydrogeological conditions of the Dashagou Yellow River crossing section, the pipeline is buried in the pulverized sandy soil layer of the Dashagou crossing section, and the most probable scenario after the oil spill is as follows: the main tendency of the movement of the oil after the oil spill is to be transported downward, and after arriving at the saturated zone of the pebble layer, the oil will be transported to the downstream under the action of the underground hydrodynamic force; if the oil leaks into the pebble layer and is not recognized for a long time, then the oil may enter into the Yellow River directly through the pebble layer and eventually spread downstream along the flow of the Yellow River oil spill.