A method for tracking the spatiotemporal transport of non-aqueous phase contaminants in low permeability pore-fracture multiscale structures

Abstract

The application discloses a kind of low permeability pore-fracture multiscale structure in non-aqueous phase pollutants space-time transmission tracking method, comprising: integrated C-ERT technique simulates the long-term migration process of NAPLs in low permeability pore-fracture multiscale structure formation, real-time continuous acquisition of the resistivity space-time distribution of contaminated soil;Study the influence of NAPLs concentration on the resistivity of contaminated soil, establish the mathematical correlation model of the resistivity of contaminated soil and NAPLs concentration;Obtain the theoretical value of the space-time distribution of NAPLs concentration;Obtain the measured value of NAPLs concentration, correct mathematical correlation model, and as the quantitative evaluation basis of space-time transmission tracking process.The application can simulate the long duration, large scale migration and transformation process of NAPLs in low permeability pore-fracture multiscale structure formation, and real-time continuous tracking of pollutant trace, realize the visualization analysis of the space-time distribution of pollutant concentration, and it is of great significance to the targeted repair and management of pollutants.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of organic pollutant monitoring, and particularly relates to a method for tracking the space-time transmission of non-aqueous phase pollutants in a low-permeability pore-fracture multi-scale structure. BACKGROUND

[0002] Non-aqueous phase liquids (NAPLs) are common pollutants in oil-contaminated sites. In a pore-fracture multi-scale structure formation, especially in a low-permeability formation, the migration of NAPLs has problems such as long time consumption and unclear rules. It is a great challenge to realize long-time and large-scale continuous monitoring of NAPLs in a pore-fracture multi-scale structure formation.

[0003] The high-density electrical resistivity imaging technology (ERT) can track the trace of pollutants in real time and continuously. The centrifuge model test can simulate the long-time and large-scale migration and transformation process of pollutants. However, how to combine the centrifuge and the ERT technology to develop a space-time transmission tracking technology of NAPLs, and accurately and quickly identify the transmission mode of NAPLs in a pore-fracture multi-scale structure formation under complex conditions, is still a key technical problem to be solved. SUMMARY

[0004] The technical problem solved by the application is that the application provides a method for tracking the space-time transmission of non-aqueous phase pollutants in a low-permeability pore-fracture multi-scale structure. The method can realize long-time and large-scale real-time continuous monitoring of the migration process of NAPLs in a pore-fracture multi-scale structure formation with low permeability by using the centrifuge-high-density electrical resistivity imaging technology (C-ERT), and realize the visualization analysis of the space-time distribution of the concentration of pollutants, which is of great significance for the targeted remediation and treatment of pollutants.

[0005] The technical solution is a method for tracking the space-time transmission of non-aqueous phase pollutants in a low-permeability pore-fracture multi-scale structure, which comprises the following steps:

[0006] Step 1. Integrate the centrifuge model test and the high-density electrical resistivity imaging technology to simulate the long-term migration process of non-aqueous phase pollutants in a pore-fracture multi-scale structure formation with low permeability, and continuously acquire the space-time distribution of the electrical resistivity of NAPL-contaminated soil in real time;

[0007] Step 2. Study the influence of the concentration of NAPLs on the electrical resistivity of contaminated soil through indoor unit tests, and establish a mathematical correlation model between the electrical resistivity of contaminated soil and the concentration of NAPLs according to the test results;

[0008] Step 3. According to the mathematical correlation model established by the indoor unit test, the theoretical value of the space-time distribution of the NAPLs concentration is obtained;

[0009] Step 4. The NAPLs contaminated soil layer simulated in the centrifuge model test is drilled to obtain the measured value of the NAPLs concentration of the sample, to verify the theoretical value of the space-time distribution, and to modify the mathematical correlation model according to the verification result, so as to take the modified model as the quantitative evaluation basis for the space-time transport tracking process of the NAPLs in the low-permeability pore-fracture multi-scale structure stratum.

[0010] Preferably, the specific operation of Step 1 is as follows: firstly, the acceleration of the centrifuge model test, the model box size, the simulated stratum density, the groundwater level, the fracture layout, and the NAPLs leakage position, leakage amount, and leakage speed are determined; secondly, the number of measurement lines, electrode spacing, and electrode insertion depth of the high-density resistivity imaging are designed according to the stratum size simulated by the centrifuge model test; and finally, the resistivity space-time distribution value of the NAPLs contaminated soil is obtained in real time during the operation of the centrifuge.

[0011] Preferably, in Step 1, the simulation method of the low-permeability pore-fracture multi-scale structure stratum in the centrifuge model box is as follows: the clay with a certain dry density is used to simulate the pore structure; the sand is used to simulate the fracture structure, and is arranged in the form of horizontal and vertical sand columns, so as to study the complex migration process of the NAPLs contaminant in the pore-fracture multi-scale structure stratum.

[0012] Preferably, in Step 4, the verification method and the modification method of the mathematical correlation model are as follows:

[0013] (1) The verification method is as follows: the resistivity space-time distribution profile obtained by integrating the centrifuge model test and the high-density resistivity imaging technology is inversed by using the mathematical correlation model, to obtain the theoretical value of the space-time distribution of the NAPLs contaminant concentration, which is compared with the measured value obtained by drilling, to verify the space-time distribution value of the NAPLs contaminant;

[0014] (2) The modification method is as follows: the error analysis is performed on the measured value and the theoretical value of the contaminant concentration, that is, the variation law of the ratio of the measured value to the theoretical value is counted, and the ratio is taken as a correction coefficient; when the theoretical value of the contaminant concentration is analyzed, the approximate measured value of the contaminant concentration can be obtained by multiplying the correction coefficient, and the model modified by the correction coefficient is taken as the quantitative evaluation basis for the space-time transport tracking process of the NAPLs in the pore-fracture multi-scale structure stratum.

[0015] Beneficial effects: 1. The centrifuge model test and the high-density resistivity imaging technology are combined, so that the long-time and large-scale migration process of the non-aqueous phase contaminant in the low-permeability pore-fracture multi-scale structure stratum can be simulated, and the space-time distribution of the non-aqueous phase contaminant can be continuously monitored.

[0016] 2. The combination of the results of the Miller soil box indoor unit test and the centrifuge model test is used to invert the resistivity profile to obtain the concentration distribution of the non-aqueous phase pollutant, and the sampling detection verification is helpful to further improve the resistivity model established by the indoor unit test, and can also quantitatively analyze the space-time distribution of the non-aqueous phase pollutant. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 A flowchart of the space-time transmission tracking method of the non-aqueous phase pollutant;

[0018] Figure 2 A space-time resistivity distribution map of the NAPLs contaminated soil obtained by the centrifuge-high density resistivity imaging technology (C-ERT);

[0019] Figure 3 A flowchart of the application of the resistivity mathematical correlation model to invert the NAPLs concentration;

[0020] Figure 4 A high-performance liquid chromatography analysis result map. IMPLEMENTATION

[0021] The application will be further described below in combination with the drawings and specific embodiments. EMBODIMENT

[0022] REFERENCE Figure 1 The space-time transmission tracking of the non-aqueous phase pollutant in the pore-fracture multi-scale structure stratum is carried out.

[0023] Since the migration process of the NAPLs pollutant in the clay low-permeability stratum is time-consuming, and the migration process is affected by factors such as capillary action, soil particle adsorption, soil fracture distribution, etc., the migration law is more complex than that of sand, therefore, the present application focuses on low-permeability contaminated soil, selects low-permeability clay as the research object, and selects methyl tert-butyl ether (MTBE) as the NAPLs pollutant, and applies the C-ERT technology to study the space-time transmission law of the NAPLs.

[0024] A space-time transmission tracking method of a non-aqueous phase pollutant in a low-permeability pore-fracture multi-scale structure, comprising the following steps:

[0025] Step 1. Integrate the centrifuge model test and the high-density resistivity imaging technology (C-ERT) to simulate the long-term migration process of the non-aqueous phase liquid (NAPLs) pollutant in the pore-fracture multi-scale structure stratum with low permeability, and continuously obtain the resistivity space-time distribution of the NAPLs contaminated soil in real time;

[0026] The specific operation is as follows: first, the basic parameters of the centrifuge model test are set, that is, the centrifugal acceleration is 50g, the size of the model box is 120cm*40cm*70cm, the dry density of the stratum is 1.4g / cm³, the volume water content is 27%, the stratum depth is 60cm, the groundwater level is 50cm below the stratum surface, the horizontal and vertical fine sand column can be randomly arranged to simulate the fracture, and the pore-fracture multi-scale structure stratum is constructed; the NAPLs leakage position is 5cm below the stratum surface, the leakage amount is 3L, and the leakage speed is 0.5L / h, and the actual stratum size corresponding to the above parameters is 60m*20m*35m, and the simulated pollutant leakage time is about 2 years. Secondly, the parameters of the high-density resistivity imaging system are set, small copper rods are used as detection electrodes in the model box, the electrode spacing is 2.5cm, the electrode insertion depth is 2cm below the stratum surface, a total of 4 measuring lines are arranged, and 41 electrodes are arranged on each measuring line; finally, during the operation of the centrifuge, the resistivity spatial and temporal distribution value of the NAPLs contaminated soil is measured every 2h, and the measurement result is shown in the following table. Figure 2

[0027] Step 2. Study the influence of NAPLs concentration on the resistivity of contaminated soil by Miller soil box indoor unit test, and establish a mathematical correlation model between the resistivity of contaminated soil and the concentration of NAPLs according to the test results.

[0028] The expression of the mathematical correlation model is:

[0029]

[0030] Among them, p the resistivity of NAPLs contaminated soil, a the pore micro-uniformity coefficient, p w the resistivity of pore water, n w the water porosity, m the cementation exponent, R w the volume water content (defined as the ratio of the volume of water in the soil pores to the volume of soil particles), p the saturation exponent, b the pollutant influence coefficient (reflecting the influence degree of NAPLs pollutant on the resistivity of soil), R o the volume pollution content (defined as the ratio of the volume of NAPLs in the pore to the volume of soil particles).

[0031] The cementation exponent in the model m The saturation exponent is obtained through the resistivity measurement test of water-saturated soil with different porosities p ​The pollutant influence coefficient b The pore micro-heterogeneity coefficient a The pollutant influence coefficient

[0032] Step 3. According to the mathematical correlation model established by the indoor unit test, the spatial and temporal distribution of NAPLs concentration is obtained by inverting the spatial and temporal distribution of NAPLs contaminated soil resistivity obtained by C-ERT technology.

[0033] As shown in Figure 3 , the specific inversion method is: the spatial and temporal distribution of resistivity, i.e. the resistivity value of NAPLs contaminated soil p , the pore micro-heterogeneity coefficient a , the cementation index m , the saturation index p , the pollutant influence coefficient b value, and the pore water resistivity p w The volume water content R w can be obtained from the formation information in the centrifuge model box. After all the parameters are obtained, the pollutant content R o , i.e. the theoretical value of the spatial and temporal distribution of NAPLs concentration, can be inversely calculated according to the mathematical correlation model proposed in step 2.

[0034] For example: if the resistivity value at a point in the formation is known from the spatial and temporal distribution of resistivity p =140 Ω*m, the cementation index of the test clay is known from the indoor unit test result m =0.34, the measurement result of the saturation index is 1.26, the pollutant influence coefficient of MTBE b =0.7, the pore micro-heterogeneity coefficient a value of the same dry density (1.4 g / cm³) and volume water content (27%) of the centrifugal formation is 0.04, and the measured resistivity of the formation pore water p w =1040 Ω*m, then we can get:

[0035] R o = 140 / (0.04×1040×0.21 0.34 ×0.27 -1.26 ×0.7) – 1 / 0.7 = 0.14

[0036] i.e. the theoretical value of MTBE contaminant concentration at this point R o = 0.14.

[0037] Step 4. Drilling sampling is conducted on the NAPLs contaminated soil layer simulated in the centrifuge model test, and high performance liquid chromatography analysis is conducted, as shown in the following formula, to obtain the measured value of NAPLs concentration at a point in the simulated soil layer, which is compared with the theoretical contaminant content analyzed in Step 3, so as to verify the reliability of the mathematical correlation model; and the mathematical correlation model is corrected according to the verification result, and the corrected model is taken as the quantitative evaluation basis for the time-space transmission tracking process of non-aqueous phase contaminants in the porous-fractured multi-scale structure soil layer. Figure 4

[0038] For example: in the point sampling in Step 3, the actual MTBE contaminant concentration is measured as R o = 0.12, which is close to the theoretical value analyzed in Step 3, indicating that the mathematical correlation model established by the indoor unit test has a certain reliability; analysis shows that the measured value of MTBE contaminant concentration is 0.86 times the theoretical value, and therefore the correction coefficient k R o is introduced for correction, i.e. the final expression of the resistivity model is:

[0039] .​​

Claims

1. A method for spatiotemporal transport tracking of non-aqueous phase contaminants in low permeable pore-fracture multiscale structures, characterized in that, The steps include the following: Step 1. The integrated centrifuge model test and high-density resistivity imaging technology are used to simulate the long-term migration process of non-aqueous phase pollutants in a pore-fracture multi-scale structure stratum with low permeability, and the resistivity space-time distribution of the NAPLs contaminated soil is continuously obtained in real time; Step 2. The influence of NAPLs concentration on the resistivity of the contaminated soil is studied through indoor unit tests, and a mathematical correlation model between the resistivity of the contaminated soil and the NAPLs concentration is established according to the test results; Step 3. The mathematical correlation model established according to the indoor unit test is used to obtain the theoretical value of the space-time distribution of the NAPLs concentration; Step 4. The NAPLs contaminated stratum simulated in the centrifuge model test is drilled and sampled to obtain the measured value of the NAPLs concentration of the sample, the theoretical value of the space-time distribution is verified, and the mathematical correlation model is modified according to the verification results, and the modified model is used as the quantitative evaluation basis for the space-time transmission tracking process of NAPLs in a pore-fracture multi-scale structure stratum.

2. The method of claim 1, wherein, The specific operation of step 1 is as follows: firstly, the acceleration of the centrifuge model test, the size of the model box, the simulated stratum density, the groundwater level, the fracture layout, and the NAPLs leakage position, leakage amount, and leakage speed are determined; then, the number of measurement lines, electrode spacing, and electrode insertion depth of the high-density resistivity imaging are designed according to the size of the stratum simulated by the centrifuge model test; finally, the resistivity space-time distribution value of the NAPLs contaminated soil is obtained in real time during the operation of the centrifuge.

3. The method of claim 1, wherein, In step 1, the simulation method of the pore-fracture multi-scale structure stratum with low permeability in the centrifuge model box is as follows: clay with a certain dry density is used to simulate the pore structure; sand is used to simulate the fracture structure, and is arranged in the form of horizontal and vertical sand columns to study the complex migration process of NAPLs pollutants in a pore-fracture multi-scale structure stratum.

4. The method of claim 1, wherein, The verification method in step 4 is as follows: the mathematical correlation model is used to invert the resistivity space-time distribution profile obtained by the integrated centrifuge model test and high-density resistivity imaging technology to obtain the theoretical value of the space-time distribution of the NAPLs pollutant concentration, which is compared with the measured results of the drilling sampling to verify the space-time distribution value of the NAPLs pollutant.

5. The method of claim 1, wherein, The correction method in step 4 is as follows: the measured value and the theoretical value of the pollutant concentration are analyzed for errors, that is, the change rule of the ratio of the measured value to the theoretical value is analyzed, and the ratio is used as a correction coefficient; when the theoretical value of the pollutant concentration is obtained, the approximate measured value of the pollutant concentration can be obtained by multiplying the correction coefficient, and the model corrected by the correction coefficient is used as the quantitative evaluation basis for the space-time transmission tracking process of NAPLs in a pore-fracture multi-scale structure stratum.

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