Numerical Simulation of Oil Pipeline Leakage Diffusion in Dashagou Yellow River Crossing Section

Abstract

In this study, the ANSYS 2020R1 software simulation is employed to examine the diffusion process of oil leakage and the underground water solute transport law in the Dashagou Yellow River crossing section of the oil pipeline. The simulation results demonstrate that under identical leakage pressure conditions, diesel fuel leakage in powdery, sandy soil is diminished, the emergency window is extended, and the corresponding leakage risk is reduced. In addition, the leakage rate of crude oil is slower than that of diesel oil. After 850 days of downward migration of approximately 190 m, the pollutant reaches quasi-static equilibrium in the big sand ditch. The results of the surface water oil spill analysis demonstrated that the oil film on the river surface migrated for 100 min after the spill, with a thickness that remained between 0.02 and 0.05 mm and a concentration that approached equilibrium.

1. Introduction

With the rapid development of the economy, oil has become the main energy and raw material for all walks of life [1], occupying an important position in the development of the national economy. It is key to the petroleum and petrochemical industry coping with the high-quality and high-speed development of the country [2] and is an important strategic resource. However, in the process of oil production, transportation and storage, especially in oil exploitation areas and petrochemical product production areas, leakage accidents occur frequently, resulting in soil pollution. In recent years, petroleum hydrocarbon pollution has gradually become one of the main forms of soil pollution in the petroleum and petrochemical industry [3,4]. After oil leakage, it migrates to the soil under the action of gravity and capillary force. Once it enters the environment, it will undergo physical migration processes, such as volatilization and convection-dispersion, and chemical migration processes, such as adsorption–desorption, dissolution and precipitation in the soil [5,6], which will not only destroy the original structure of the soil but also change the physical and chemical properties of the soil, reduce soil permeability and affect the growth of microorganisms [7]. In addition, the leaked oil very easily penetrates into the groundwater through precipitation, surface runoff and other ways [8], causing pollution to the groundwater, having a serious impact on animals and plants and greatly increasing the mortality rate. In addition, the oil volatilized into the air will be ingested by the human body through breathing, skin contact, diet and other ways, which will affect the normal function of liver, kidney and other organs to varying degrees and even cause cancer, teratogenicity and mutagenicity, causing great harm to the human body [9,10,11]. Under the action of water circulation, the atmosphere will cause serious pollution, and a part of the evaporated substance will fall to the ground and deposit after chemical changes. Every year, about 8 million tons of oil in the world will enter the environment through various ways, such as running, bubbling, dripping and leaking [12,13], while about 600,000 tons of oil in China, of which about 70,000 tons will flow into the soil, causing pollution to China’s ecological environment [14,15]. In 2004, thousands of tons of crude oil leaked from a pipeline in Yan’an, Shaanxi Province, polluting hundreds of acres of farmland [16,17]. In 2015, the oil spill of Changqing oilfield polluted tens of kilometers of river, causing serious soil pollution in Wuqi County, Yan’an City. The list of soil pollution risk control and remediation of Beijing–Tianjin–Hebei construction land shows that chemical pollution sites account for 44.1%, among which petroleum hydrocarbon, one of the main pollutants, accounts for 42.10% [18]. According to the investigation and statistics of Russian scientists, 7% of the global crude oil extraction will enter the environment every year and then pollute the soil and groundwater [19]. It can be seen that the harm caused by oil leakage has a huge impact on animals and plants and the ecological environment. In recent years, soil restoration and ecological environment management have received extensive attention. The existing soil remediation technologies include physical remediation, chemical remediation and biological remediation. However, due to the complex structure and properties of petroleum, these remediation technologies all have certain limitations [20]. Up to now, the remediation technologies of oil-contaminated soil are still being improved [21,22]. There are many such oil pipeline leakage events, so it is very important to study oil pipeline leakage.

After the oil spills into the river due to pipeline damage, its diffusion law will be affected by velocity, river bed topography, groundwater ring and other factors, changing the diffusion speed and causing varying degrees of damage and pollution to the water quality, microorganisms, animals and plants of the river. Many scholars have conducted a series of studies on oil pipeline leakage. Wang YJ et al. [23] proposed a grey correlation method for assessing the impact of oil pipeline leakage on river pollution so as to improve the accuracy of the assessment. Based on FLUENT, Wang XY [24] analyzed the diffusion process of crude oil in the water body during the floating process when the underwater crude oil transportation pipeline leaks and studied the diffusion shape and diffusion law of the leaked crude oil when it reaches the water surface under conditions of no wind or wind. Considering the corresponding parameters of pipeline leakage, Gu XH et al. [25] proposed a method of oil pipeline leakage detection based on dynamic nuclear independent component analysis, aiming at the problems of time-series autocorrelation of pipeline parameters and low leakage detection accuracy. Zhao LQ et al. [26] proposed a pipeline leak detection method based on second-generation wavelet transform and multistage hypothesis testing to solve the difficult problem of accurate judgment and location of slow leakage in oil pipelines. Li JH et al. [27] used the nonlinear time series prediction method based on BP neural network to establish the new information model and the leak detection model of the pipeline leak monitoring system, which can effectively discover leaks online and in real time. Liu W et al. [28], from the perspective of signal processing, proposed a complete automatic identification scheme for oil pipeline leakage signals based on the characteristics of dynamic pressure wave signals. Okpare, O.A [29] and other wireless sensor networks use pressure changes to detect and locate prototype oil pipeline leaks. Wireless sensor networks will be able to monitor oil pipeline leaks, thereby reducing environmental degradation and economic losses caused by oil spills. Garima Srivastava et al. [30] realized intelligent monitoring and predictive maintenance of oil pipelines by using collaborative cloud services. Liang W et al. [31]. optimized the extraction method through the benchtop study of SPME. They selected the right fiber material and exposure time for each BTEX compound to improve analysis efficiency and accuracy while minimizing environmental impact, resulting in more reliable pipeline leak field study data.

Based on the work of many scholars, this paper takes the Dashagou Yellow River section oil pipeline as the research area. Numerical simulation based on ANSYS software is used to analyze the leakage diffusion process of the oil pipeline across the river under different leakage models as well as the solute transport law of the groundwater in order to provide the theoretical support as well as the scientific basis for the pollution of the groundwater by leakage from the petroleum pipeline.

2. Materials and Methods

2.1. Overview of Dashagou

The Dashagou is located in Lanzhou City, on the north bank of the Yellow River and the left bank of the first-class tributaries. The geographical location of the study area is illustrated in Figure 1. Gansu Province and Lanzhou City are employed to determine the “protection of the Mother River” activities in the focus area of the ditch. It originates from the area of Wuyi Mountain, and the main axis is northwest to southeast. The average width of the ditch basin is 3.40 km, is about 34.130 km long, with an area of 100.65 square kilometers, and the total population of the basin is 11,000 people. The riverbed elevation at the entrance is 1511.31 m, and the average specific drop of the river is 14.20%, which is a typical loess plateau hilly gully geomorphology, and the soil type is IV.

The Dashagou River crossing section of the pipeline, located in China’s inland, is the heart of the Asian continent, with a typical north temperate continental arid climate zone, namely, with sparse precipitation, wind and sand on the whole; evaporation is strong; cold in winter and hot in summer; the temperature difference between day and night; sunshine time and other characteristics. The topography is complex and diverse, with Gobi plains, deserts, alluvial plains, mountains, mountain valleys, hills, stripped hills, saline and alkaline land (wetlands), etc., distributed in a haphazard manner along the pipeline. The ground surface is covered by powdery clay sand (gravel, gravel, pebbles, breccia), pebbles, rocks, chalk, etc. The apparent resistivity of the soil is mostly 28%. The soil’s apparent resistivity is mostly in 28 Ω·m~>1570 Ω·m, and most of the sections are medium corrosion or weak corrosion; its gap local sections are strong corrosion, with apparent resistivity 5 Ω·m~28 Ω·m. The section where the pipeline crosses the Yellow River is located in the upper reaches of the Yellow River, with a wide and shallow riverbed, and the section is basically straight. The flow direction of the Yellow River is NW~SE. Ⅰ and Ⅱ terraces are developed on both sides of the river, and the terrain is stepped down to the riverbed. The terraces are relatively flat and open, with vegetation developed on the left terrace, while the right terrace is densely populated with private houses.

2.2. 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 Figure 2, Figure 3 and Figure 4.

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 Figure 5 and Figure 6.

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 Figure 7 and Figure 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 Figure 9 and Figure 10.

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. For the Yellow River crossing section, due to the poor permeability of bedrock layer, powdery sand during directional drilling and the installation of casing (DN1400/DN1000) in the section entering the bedrock layer, the possibility of oil leakage and spillage into the Yellow River is low in this section, so the main concern of the Yellow River crossing section is the spillage of oil from the pebble layer; based on the as-built section of the Yellow River crossing section, the bed of the Yellow River is a pebble layer, and once oil enters into the pebble layer, the oil spillage will spread to the downstream along the flow of the Yellow River. According to the as-built profile of the Yellow River crossing section, the riverbed of the Yellow River is a pebble layer, and once the oil enters into the pebble layer, it is very easy for the oil to enter into the Yellow River directly from the pebble layer and spread further on the Yellow River.

3. Geometry of Computational Area

Liu E, Zhou L, Tang P, et al. [32] employs numerical simulation to conduct preliminary studies of the concentration distribution of components under a range of operational conditions, including wind speed, temperature, pipe diameter, leakage direction and leakage aperture ratio. The geometric modeling of this simulation is constructed using ANSYS 2020R1 Spaceclaim software, ANSYS 2020R1 Fluent fluid simulation software is used for the flow calculation of oil leakage, the simulation work is carried out using ANSYS Workbench 2020R1 version, and the simulation is carried out through two stages of simulation work to calculate and analyze the oil leakage of the pipeline.

In order to simplify the model calculation, the geometric model is generalized to a cube, which is the porous medium fluid domain of the simulation, a round pipe is hollowed out in the cube, and the surface of the round pipe is the wall boundary of the calculation domain (i.e., the leaking oil will not enter into the inside of the pipeline). At the same time, a circular leakage opening is outlined on the surface of the round pipe, in which the X-axis is the perpendicular pipeline direction, the Y-axis is the direction of the pipeline, and the Z-axis is the direction of gravity, i.e., the pendant direction, as shown in the figure below. Direction of gravity, i.e., vertical direction, is shown in Figure 11.

In order to facilitate the observation of the subsequent simulation results, when the display interface axis shows XZ, the observation direction is the direction of the pipe section, at this time to observe the circular section of the pipe and the soil porous media calculation domain; when the display interface axis is YZ, the observation direction is the direction of the pipe side, at this time to observe the side of the entire pipe and the soil porous media calculation domain.

This numerical simulation only simulates the representative leakage port location, leakage port radius and leakage shape. The representative locations of the leakage ports are categorized as (1) the leakage port is located directly above the pipe (top), (2) the leakage port is located directly to the side of the pipe (side) and (3) the leakage port is located directly below the pipe (bottom), as shown in Figure 12.

Considering that the diameter of the crude oil pipeline in the Dashagou crossing section is Φ711 mm and the diameter of the refined oil pipeline is Φ508 mm, the size of the leakage opening is usually not larger than 10% of the pipeline diameter; therefore, the radius of the leakage opening in the small leakage scenario is divided into two cases, i.e., (1) 10 mm and (2) 20 mm, and the radius of the leakage opening in the large leakage scenario is 50 mm. In order to examine the change in flow pattern after an oil spill, the flow pattern settings of this simulation work are 30 s after the spill, 15 min after the spill, 30 min after the spill and 1 h after the spill, totaling four flow times. The simulation model of the Dashagou crossing section is buried at a depth of 12 m. Diesel oil is used as the simulation fluid in this simulation model. The simulation flow chart is shown in Figure 13.

4. Results and Discussion

4.1. Resistance Coefficient Analysis

The Miller soil box [33] was employed as the measuring instrument to ascertain the resistance coefficient of the soil samples gathered from the Dashagou region in the field in accordance with the Wenner device. The measuring devices are shown in Figure 14.

It was utilized as the background parameter to simulate the leakage in the Dashagou section, with the objective of obtaining a leakage flow pattern that is more consistent with the actual conditions. This was performed with the objective of obtaining a leakage flow pattern that more closely aligns with the actual conditions. Combined with the fact that most of the buried depth of the pipeline in the crossing section is located in the layer of pulverized sandy soil (sandy chalky soil), the leakage simulations in the Dashagou section are calculated by the resistance coefficient value of pulverized sandy soil (sandy chalky soil), which is assumed to be the soil of the Dashagou section. Therefore, the simulation of the leakage in the Dashagou section is calculated using the value of the resistance coefficient assuming that the soil is sandy sand (sandy chalk). The flow pattern views of the leakage for 30 s, 15 min, 30 min and 1 h under different conditions are shown in Figure 15, Figure 16 and Figure 17.

According to the leakage simulation results, the leakage flux is about 0.05 kg/s for a leakage opening radius of 10 mm, 0.13–0.15 kg/s for a leakage opening radius of 20 mm and 0.6–0.62 kg/s for a leakage opening radius of 50 mm, which indicates that the leakage flux is smaller and the leakage time is longer in powdery sandy soil/sandy silt, and the corresponding leakage risk and emergency window time are relatively small. The risk and emergency window time are relatively small.

Under the same radius of the leakage opening, the overall effect of the location of the leakage opening on the leakage flux is not significant. For the same leakage pressure, the size of the leakage port has a more significant effect on the leakage volume. The crossing section of the pipeline in the study area is in a sandy chalky soil (chalky sandy soil) environment, where the oil leakage is subjected to stronger resistance and a smaller range of influence, which indicates that for the examination of the oil leakage flow process in the porous media system, the influence of the soil lithological properties on the distribution of the fluids is crucial: usually the finer the particles, the higher the resistance, the stronger the oil leakage pressure can be attenuated and the smaller the distance and range of diffusion.

4.2. Apparent Flow Rate Analysis

In the simulation using the resistance coefficient values measured in the laboratory, the simulation results are shown in Figure 18, the differences in the apparent flow rates of oil spills for the scenarios with a leak radius of 10 mm and 20 mm were not significant; the apparent flow rates of oil spills for the 10 mm leak radius scenario ranged from 1.04 to 1.32 m/h one hour after the spill, and the apparent flow rates of oil spills for the 20 mm leak radius scenario ranged from 1.48 to 1.87 m/h one hour after the spill 1.87 m/h. For the scenario with a 50 mm leak radius, the apparent oil leakage flow rates ranged from 2.31 to 2.93 m/h one hour after the spill, which was nearly one time higher than the other two scenarios with different leak radii. At the early stage of the leakage, the leakage flow velocities at different leakage outlets had large differences, but as the leakage continued, the apparent flow velocities of the leakage showed little difference in the X, Y and Z axes, indicating that the oil leakage diffusion process was more uniformly distributed in the space without any significant flow tendency or direction, which was possibly due to the fact that the viscous resistance coefficients and inertial resistance coefficients of the sandy chalky/powdery sandy soils were high as determined by the laboratory. The reason may be that the spilled oil cannot spread quickly under the higher resistance condition in the soil; during the oil leakage in the crossing section of the Dashagou, when the equivalent amount of spilled oil becomes extremely large, its flow trend may still be affected by gravity, and its final flow trend is still dominated by the downward movement in the vertical direction. Taking the leakage scenario with a 12 m burial depth of pipeline, lateral leakage opening and a 10 mm leakage radius as an example, the apparent velocity of Y-axis of the experimentally measured value in 30 min is 1.78 m/h, which indicates that the difference of the soil lithology on the flow of the leakage is very significant in terms of the blocking effect. Because of the pebble layer below the sandy silt layer in the Dashagou crossing section, the resistance coefficient experiment of this soil layer cannot be carried out at present due to the unavailability of specific information and pebble samples of this pebble layer. However, considering that the pebble layer has a better permeability, if a leakage occurs in the pipeline crossing this soil layer, the apparent velocity of the leakage corresponds to the leakage diffusion in the sandy soil layer, and the corresponding leakage environmental risk will be higher.

Taken together, the analysis of the simulation results about the leakage flow pattern in the crossing section of the Dashagou shows that the diffusion velocity is extremely high in the short time at the early stage of the leakage, but with the continuation of the leakage, the apparent flow velocity of the leaked oil is rapidly decaying, so the soil, as a kind of porous medium, and its blocking effect on the distribution of the flow pattern cannot be neglected. As the oil continues to leak, the oil will continue to migrate to the soil perimeter at a much smaller rate. Therefore, if the pipeline is buried in a shallow layer of silty sandy soil or sandy chalky soil, the degree of diffusion of the leaked oil is slower, which indicates that the silty sandy soil (sandy chalky soil) under the condition of a traversing section produces a natural blocking effect on the leakage of the pipeline’s micro-port.

For the leakage point located in the pebble layer, the simulated leakage flux and apparent flow rate derived from the reference resistance coefficient for sandy soils with a particle size between 0.8 and 1 mm, it can be inferred that the leakage in the pebble layer will spread faster than in the fine sand layer, resulting in a higher potential environmental risk. A comparison of the apparent flow velocities for all flow regimes for the 12 m burial depth pipeline described above is shown in Figure 19.

4.3. 24 h Simulation Results and Analysis

Considering that the flow time of the simulated flow pattern does not exceed 1 h in the above simulation, the 24 h leakage flow pattern simulation is carried out in this project to provide a certain reference to deal with the oil leakage in the pipeline in the crossing section when the pipeline leakage occurs but fails to trigger the pipeline monitoring system due to the small leakage port, slow flow rate and small pressure change in the pipeline. The 24 h leakage flow simulation is only for the pipeline leakage scenario in the layer of pulverized sandy soil (sandy silt) with high resistance in the crossing section of the Dashagou. It is assumed that the buried depth of the pipeline is 12 m, the soil around the pipeline is saturated, the radius of the leakage opening is 20 mm, and the leakage opening is located directly below the pipeline, i.e., at the bottom.

The simulation results of the 24 h long-time leakage flow pattern in the crossing section of the Dashagou are shown in Figure 20 and Figure 21, and the comparison of the characteristic lengths and apparent velocities of the leakage flow pattern at different moments are shown in Figure 22.

Combined with Figure 20, Figure 21 and Figure 22, it can be seen that the buried depth of the pipeline is 12 m, the soil around the pipeline is saturated, the radius of the leakage port is 20 mm, the leakage port is located directly under the pipeline, the leakage flux in 24 h ranges from 0.131678 kg/s to 0.0131656 kg/s, and the rate of change in the leakage flux over time is not significant, which is about 0.1316 kg/s. The total amount of leakage in 24 h is 5670.22 kg, i.e., about 5.67 tons, and the affected soil volume is 22.19 m³. The total amount of leakage in 24 h was 5670.22 kg, i.e., about 5.67 tons, and the volume of affected soil was 22.19 m³. The maximum distance of leakage diffusion in 24 h appeared in the vertical direction, which indicated that the trend of oil leakage was mainly downward; according to the simulation results of 24 h, the maximum distance of oil diffusion in the vertical direction was about 8.14 m, the range of diffusion in the lateral direction was 21.99 m² in 24 h, and it could be seen that the range of diffusion in the lateral direction was not significant. The range of lateral diffusion is 21.99 m² in 24 h, from which can be seen that the range of lateral diffusion is not significant. Considering that the distance of vertical diffusion is larger, it can be inferred that the main trend of oil leakage in the Dashagou area is vertical downward transport and diffusion. The apparent flow rate of oil leakage is gradually decaying, the volume of soil affected by the leakage is 22.19 m³ in 24 h, the oil leakage is 5.67 tons, and the apparent flow rate of Z-axis is 0.34 m/h. Due to the resistance of the sandy chalk layer, the apparent flow rate is 1 cm/h after 14 h of leakage, and the flow is stable for a long period of time.

4.4. Analysis of Groundwater Solute Transport

4.4.1. Analysis of Benzene Migration and Diffusion

Since the source strength for the groundwater solute transport simulation is conditioned on the substitutes of oil products and their solubility and since oil products are mixtures and their physical and chemical properties are very complex, consequently, benzene is employed as a substitute for the dissolved phase of diesel fuel, while petroleum hydrocarbons are utilized as a substitute for crude oil.

The aquifer parameters, initial conditions and boundary conditions were substituted into the water quality model, and MODFLOW 2005 and MT3D99 software were used to jointly run the water flow and water quality models to obtain the prediction results of the pollutant transport after the oil spill by using the saturation degree of benzene, 1790 mg/L, as the continuous source concentration. Figure 23 shows the comparative distribution of benzene transport and diffusion at different time periods after the oil spill.

According to the results of groundwater solute-phase transport prediction, the following conclusions are summarized: the process of oil migration through the groundwater, its flow rate is the same slow decay; groundwater solute-phase pollutants exceeding the concentration in about 100 days will migrate to the online monitoring sites installed in this project; the groundwater pollution migration rate of the pebble layer in the crossing section of the Dashagou is faster than that of the sand layer in the bed of the Shule River, which is mainly due to the better permeability of the pebble layer and the faster migration rate of the sand layer in the Shule River. The main reason is that the pebble layer has better permeability, and the groundwater flow rate is faster. If the pipeline leakage occurs about 350 m away from the entrance of the Dashagou into the Yellow River, due to the extremely small leakage opening, the groundwater is still undetected after a long leakage time, and the leaked oil has not been subjected to effective leakage pollution source removal measures, the solute-phase pollutants in the groundwater can enter the Yellow River after 730 days and will have a more significant environmental impact on the Yellow River’s water quality.

4.4.2. Analysis of Crude Oil Migration and Diffusion

The aquifer parameters, initial conditions and boundary conditions were substituted into the water quality model, and MODFLOW (version 6) and MT3D software (version 3) were used to jointly run the water flow and water quality models to obtain the prediction results of pollutant transport after oil spillage with total petroleum hydrocarbon (TPH) saturation of 10 mg/L as the continuous source concentration. Figure 24 shows the comparison of the range of petroleum hydrocarbons transported and dispersed in different time periods.

According to the simulation results of groundwater solute-phase pollutant transport, the following conclusions are summarized: When crude oil leakage occurs in the pipeline, the solute phase of crude oil enters into the pebble layer and arrives at the on-line monitoring site after about 300 days. Under the condition of continuous source strength of 10 mg/L, the solute-phase pollutant of crude oil reaches quasi-static equilibrium when it migrates downward about 190 m after 850 days of groundwater transportation, and the pollutant does not enter into the Yellow River but forms a stable equilibrium state in the Dashagou; due to the low solubility of crude oil in water, the solute-phase pollutant of crude oil is significantly weaker in the migration ability in the groundwater as compared with that of benzene in the groundwater. The solute phase of crude oil is significantly weaker in groundwater than benzene due to the lower solubility of crude oil in water.

4.5. Surface Water Oil Spill Simulation Analysis

4.5.1. Simulation and Analysis of Surface Water Oil Spill from Crude Oil Leakage

Regarding the process and trend of oil spills in the Yellow River crossing section during the dry water period in the simulation with continuous 6 kg/s oil spill flux, it takes about 35 min for the crude oil to spill into the Yellow River section in the range of the backup water source of Yingmen Beach during the dry water period; it takes about 70 min to arrive at the downstream Yintan Wetland Park. From the beginning of the oil spill to 40 min after the oil spill, the area of the oil film with the thickness of more than 0.1 mm is maintained in the range of 59,300~61,000 m². The change in the oil film area did not show continuous growth but fluctuating changes in the trend; the reason for this is likely to be affected by the distribution of the river flow rate. The river surface oil film migration since the spill after 100 min and the thickness of the oil film are maintained in the range of 0.02 to 0.05 mm, indicating that the concentration of the oil film migration began to approach a state of equilibrium, and the thickness of the film no longer exhibit significant changes; due to the dry water season and due to the slower water flow and smaller flow, the environmental capacity of the river itself is smaller, and the turbulence of the oil cutting, dilution and other roles relative to other seasons is also weaker. The oil spill on the river surface of the rate of attenuation is also correspondingly slower, so the oil can be migrated along the surface of the river to a more distant area without being completely diluted by the river attenuation. Figure 25 is Effect diagram of oil film diffusion range of crude oil spill.

4.5.2. Simulation and Analysis of Surface Water Oil Spill from Diesel Oil Leakage

From the analysis of Figure 26, during the dry water period, the diesel oil spills on the river surface and reaches the Yellow River cross section in the range of Yingmantan Alternate Water Source after 35 min; it reaches Yintan Wetland Park after 70 min. Since the oil film began to leak and drift on the river surface, the thickness of the oil film remained at 0.02–0.05 mm for 100 min until the end of the simulation time of 200 min, and the thickness of the oil film did not change significantly and continued to move downstream, which indicated that the oil film migration and diffusion on the river surface had reached a certain degree of quasi-static equilibrium state. The oil film area with a thickness greater than 0.05 mm was maintained at 200,500–252,800 m² after 70 min from the beginning of the leakage, and the change in the oil film area did not show a continuous growth but a fluctuating trend, which may be due to the influence of the distribution of the flow velocity in the river. Due to the dry water period, the current speed is slower; the flow rate is smaller; the environmental capacity of the river itself is smaller; the turbulence of the oil cutting, dilution and other roles relative to other seasons are also weaker and the oil spill on the river surface of the rate of attenuation is also correspondingly slower so that the oil can be migrated along the river surface to the more distant areas without being completely diluted by the river attenuation.

4.6. Uncertainty Aspects

This simulation assumes that the oil diffusion in the soil after the spill is laminar because the soil as a porous medium is in a state of compaction and solidification under natural conditions, and there is not enough space for turbulence to occur in the flow of oil after the spill, so the simulation model assumes that the flow of the spill is laminar, which indicates that the oil is maximized to be retained in the soil medium to assess the environmental impacts that can result from maximizing the retention of the oil in the soil. In this way, the environmental impact of maximum oil retention in the soil can be assessed. The simulation process assumes that the oil flow is laminar, but under the actual spill conditions, due to the strong heterogeneity of the soil, it is possible that high-pressure fluids in the shallow stratum under actual natural conditions may cause surge scouring of the soil area with low compaction, which in turn generates preferential flow, and due to the strong uncertainty of the preferential flow, the simulation does not include simulation calculations of the preferential flow that may be generated. In addition, if the preferential flow occurs in the leakage process, with the jet-scouring effect of high-pressure fluid, it is possible to locally cause a large collapsed space or culvert, and the oil flow in the space is very likely to be in the form of turbulence or with a higher flow rate overflowing on the ground or the riverbed and other phenomena due to the fact that such scenarios are prone to cause a large leakage, which is more easily detected on the ground or in the surface water. Accordingly, the simulation process is solely concerned with the oil leakage scenario, which is presumed to be a gradual and minor leakage that is not readily discernible by the management and does not simulate the more complex phenomena, which may be caused by the heterogeneity of the soil and the mobility of the soil particles, as mentioned above. It is assumed that there is no displacement of soil in the porous fluid domain, i.e., there is no fluid-driven loss or movement of soil particles, nor is there any change in the porosity of the porous fluid domain due to the migration of fine powder particles during the fluid-driven process.

Since the source strength for the groundwater solute transport simulation is conditioned on the substitutes of oil products and their solubility and since oil products are mixtures and their physical and chemical properties are very complex, using substitutes (e.g., benzene as a substitute for the dissolved phase of diesel fuel, petroleum hydrocarbons as a substitute for crude oil) as the source strength condition itself has certain limitations, which may have certain differences and impacts on the results of the predictions of the groundwater solute transport simulation. This surface water oil spill simulation is based on the assumption that the source strength is set to occur at the water surface, which is based on conservative conditions, because the vertical movement processes such as oil leaking from the riverbed to the riverbed surface and floating from the riverbed surface to the river surface will significantly weaken the fluxes of the spilled oil; for example, soil particles in the riverbed will adsorb the oil, the organic matter at the bottom of the riverbed will adsorb part of the oil, and the oil may be swept up by the oil during the vertical movement process. For example, soil particles in the riverbed will adsorb the oil, organic matter at the bottom of the riverbed will also adsorb some of the oil, and the vertical movement of the oil may entrain sediment and make it denser, all of which will reduce or attenuate the source strength so that the time of the oil spill, which is derived by not taking these processes into account, is more conservative for management.

Surface water simulation is used as a numerical experiment to numerically predict the drift distance of oil spills and the thickness of oil film on the water surface. As the surface water oil spill process contributes most to the oil drift in the river flow rate, the flow rate and the water depth, at the same time, the surface wind speed, oil volatilization, emulsification and other factors also have a certain contribution to the oil drift due to the average flow rate and the flow rate of the hydrodynamic conditions used in this simulation, the oil film migration process can reach a certain quasi-static equilibrium, while in the actual situation, the change in hydrodynamic parameters may have a greater impact on the oil spill. In the actual situation, the change in hydrodynamic parameters may have a large impact on the oil spill area, and the prediction results of the surface water oil spill simulation must be different from the actual situation, so this surface water simulation can only be used as a reference tool for management, not the actual situation of the oil spill status.

5. Conclusions

Through numerical simulation of the river crossing pipeline in the Yellow River section of the Dashagou, the following conclusions were obtained from the analysis of resistance coefficient, apparent velocity, groundwater solute transport and surface water oil spill.

The leakage flux is 0.05 kg/s, 0.13~0.15 kg/s and 0.6~0.62 kg/s when the radius of the leakage opening is 10 mm, 20 mm and 50 mm, indicating that the leakage flux is smaller and the leakage time is longer in the pulverized sandy soil/sandy pulverized soil, and the corresponding risk of leakage and the emergency response window time are relatively small.

Under the same radius of the leakage opening, the overall effect of the location of the leakage opening on the leakage flux is not significant. Under the same leakage pressure condition, the size of the leakage port has a more significant effect on the leakage volume. Combined with the conclusions reached by Guo YH, Zhang DW and Qi YM et al. [34], who used high-density resistivity imaging to investigate the light non-aqueous phase liquid transport patterns in low-permeability clays as a reference, the following conclusions are drawn: The effect of soil lithology properties on fluid distribution is crucial; usually the finer the particles, the greater the resistance, the stronger the oil leakage pressure energy attenuation and the smaller the diffusion distance and range.

During the oil leakage in the crossing section of the Dashagou, when the oil equivalent of the leakage becomes extremely large, its flow trend may still be affected by gravity, and its final flow trend is still dominated by vertical downward movement. The difference in the soil lithology’s blocking effect on the leakage flow is very significant.

The simulation results for a 24 h period indicate that the maximum distance of vertical upward oil diffusion is approximately 8.14 m, while the range of lateral upward diffusion over the same period is approximately 22 m². These findings suggest that the range of lateral upward diffusion is not significant. Given that the distance of vertical diffusion is larger, it can be deduced that the main tendency of the oil leakage in the Dashagou region is vertical downward transport and diffusion. The apparent flow rate of oil leakage decreases gradually, the soil volume affected by the leakage is 22.19 m³ in 24 h, the oil leakage is 5.67 tons, and the apparent flow rate of Z-axis is 0.34 m/h. Due to the resistance of the sandy chalk layer, the apparent flow rate is 1 cm/h after 14 h of leakage, and the flow rate is stable in a longer period of time.

Regarding the process of diesel oil migration through the groundwater, the flow rate is the same slow decay due to the leakage port being extremely small. After a long time of leakage into the groundwater, it is still undetected, the leakage of oil has not gone through the effective leakage of pollutant removal measures, the groundwater solute-phase pollutants can enter the Yellow River after 730 days, and the Yellow River water quality can produce a more significant impact on the environment.

The groundwater solute-phase pollutants of crude oil can enter into the Yellow River after 850 days of groundwater transport under the condition of a continuous source strength of 10 mg/L and can reach quasi-static equilibrium when migrating downward about 190 m. The pollutants do not enter into the Yellow River but form a stable equilibrium state in the Dasha Gully; due to the lower solubility of crude oil in water, compared with the groundwater migration of benzene, the migration capacity of crude oil solute-phase pollutants is significantly higher than that of benzene. Compared with the groundwater transport of benzene, the solute-phase pollutants of crude oil in the groundwater dynamics process have a significantly weaker migration ability.

The oil spill occurred in the Yellow River crossing section during the dry water period from the beginning of the oil spill to 40 min later. An oil film area with a thickness greater than 0.1 mm was maintained between 59,300 and 61,000 m², the change in the oil film area showed a fluctuating trend, and the oil film migration on the river surface was maintained between 0.02 and 0.05 mm after the spill for 100 min, which indicated that the concentration of the migrating oil film was approaching the equilibrium state, and the oil film thickness was no longer in a state where benzene was migrating. It remained close to the equilibrium state, and the oil film thickness no longer showed significant changes.