Wellbore aperture correction method based on uranium fission instant neutron logging

Abstract

The invention discloses a borehole influence factor aperture correction method for uranium fission instant neutron logging, and aims to solve the problem of neutron counting rate response deviation caused by borehole size change in a uranium mine logging process. Time spectrums of thermal neutrons and epithermal neutrons in different apertures in uranium fission instant neutron logging are obtained, and the rule that the time spectrums of the thermal neutrons and the epithermal neutrons change along with the hole diameter is researched. And based on a numerical simulation result, a fitting formula and a correction coefficient of the aperture influence factors are solved. The correction method can be used for analyzing the influence of the aperture size change on the counting rate of the neutron detector, effectively improves the uranium mine grade evaluation accuracy under the complex borehole condition, and is especially suitable for well logging interpretation of non-standard borehole diameters and diameter-expanded stratums.

Description

Technical Field

[0001] The invention relates to the technical field of uranium fission prompt neutron logging, relates to nuclear detection technology, and in particular to a method for correcting wellbore influencing factors, and in particular to an aperture correction method for uranium fission prompt neutron logging. Background Art

[0002] Currently, uranium fission neutron logging technology is still in the stage of theoretical research and laboratory verification, and its promotion to field engineering applications still faces many technical challenges, among which the establishment of a logging instrument calibration system is particularly critical. According to industry specifications, logging instruments need to be calibrated through a standard model well before engineering application, and a standard model with a known uranium content is usually used as a reference benchmark. However, there are significant differences between the actual drilling environment and the standard model well, which is specifically manifested in the mismatch of engineering parameters such as wellbore size, well fluid medium, and casing material / thickness. These environmental factors will cause distortion of the neutron field distribution, resulting in deviations in the original measurement data. In view of the unique geological conditions of in-situ leachable sandstone uranium deposits in my country, this patent focuses on overcoming the interference problem of borehole aperture changes on neutron logging data, and proposes a borehole aperture correction method with engineering adaptability. Summary of the Invention

[0003] In order to at least solve or partially solve the above problems, a method for correcting wellbore diameter based on uranium fission prompt neutron logging is provided.

[0004] In order to achieve the above object, the present invention provides the following technical solutions:

[0005] The present invention provides a method for correcting wellbore diameter based on uranium fission prompt neutron logging, comprising the following steps:

[0006] Step 1: Based on the uranium fission prompt neutron logging tool UNL-4 developed by East China University of Technology, a 1:1 logging tool numerical model and a standard uranium mine wellbore model were established using Monte Carlo numerical simulation software. The logging tool numerical model includes a neutron tube, thermal neutron detectors, and epithermal neutron detectors. The wellbore model includes uranium-bearing strata and uranium-free strata.

[0007] Step 2: Using the model established in step 1, perform Monte Carlo simulation. Place the probe in the wellbore model, align the epithermal neutron detector with the center of the wellbore model, and use the thermal neutron detector and epithermal neutron detector to detect and record epithermal neutrons and thermal neutron counts respectively.

[0008] Step 3: Use the logging tool to measure the physical standard background model, obtain measurement data, and compare the simulated and measured thermal neutron counts to verify the accuracy of the logging tool numerical model. Use the physical standard model with 1 / 1000 uranium content to compare with the numerical simulation results to verify the accuracy of the numerical model simulation. If the relative error of the total count in the effective time period is less than 10%, the numerical simulation is considered to have passed the accuracy verification and the subsequent aperture numerical simulation results are reliable.

[0009] Step 4: Keep the height and thickness of the wellbore model unchanged, change the inner diameter of the numerical wellbore model, and set n inner diameters R n , respectively measure and record the epithermal neutron and thermal neutron time spectrum, and obtain the total epithermal neutron count N in the effective time period area En Total thermal neutron count N Tn , n≥2;

[0010] Step 5: Calculate the total count rate ratio S of epithermal neutrons to thermal neutrons for n inner diameters n :

[0011]

[0012] Step 6: The obtained ratio of epithermal neutron to thermal neutron count rate S n Composed sequence S[n]:

[0013] S[n]={S1,S2…S n}, n≥2 (2)

[0014] Step 7: Set the inner diameter R of the numerical wellbore model to n Composed sequence R[n]:

[0015] R[n]={R1,R2…R n}, n≥2 (3)

[0016] Step 8: With R[n] as the x-axis and S[n] as the y-axis, construct and analyze the relationship curve between the inner diameter of the numerical wellbore model and the ratio of the total count rate of epithermal neutrons to thermal neutrons;

[0017] Step 9: Use the ratio S1 of the inner diameter R1 of the numerical wellbore model as the correction standard to correct the S1 of the inner diameter of other models. n . Set the correction coefficient μ, then we have

[0018]

[0019] Step 10: Set the correction coefficient μ n Composed sequence μ[n]:

[0020] μ[n]={μ1,μ2…μ n}, n≥2 (5)

[0021] Step 11: With R[n] as the x-axis and μ[n] as the y-axis, construct and fit the relationship curve between the inner diameter of the numerical wellbore model and the correction coefficient;

[0022] In the center case:

[0023]

[0024] When leaning against a wall:

[0025]

[0026] Among them, a, b, and c are the parameters of the fitting function;

[0027] Step 12: Calculate the correction coefficient of the wellbore model with different inner diameters according to the formula in step 11, calculate the relative error between the value derived from the fitting formula and the value derived from the formula in step 9, and form a correction table.

[0028] As a preferred technical solution of the present invention, in step one, the models are divided into two categories: one is a circular tube with an inner diameter of 9 cm to 30 cm, an outer diameter of 140 cm, and a height of 180 cm. The inner skeleton of the wellbore is uranium-containing sandstone, the main components of which are silicon dioxide and uranium, and the uranium content is set to 0.1%. No substance is placed in the pores of the inner diameter of the wellbore, which is a dry hole, and the probe is placed in the center of the wellbore; the second category is different from the first category in that the probe is placed against the wall of the wellbore.

[0029] As a preferred technical solution of the present invention, in step 2, a thermal neutron detector, an epithermal neutron detector and a 14 MeV DT pulse neutron source are sequentially arranged in the probe from bottom to top, and a cadmium shielding layer and a polyethylene shielding layer are arranged from the outside to the inside of the epithermal neutron detector. The diameter of the probe is 6 cm, and the particle number of the DT pulse neutron source is 10 9 indivual.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] By applying the correction method of the present invention, uranium fission prompt neutron logging can directly determine uranium content, eliminating the need for indirect uranium content estimation through radium measurement. This correction method provides technical support for the application of direct uranium logging technology. In areas with uranium-radium imbalance, core sampling and chemical analysis are no longer necessary, reducing uranium resource exploration costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings are used to provide a further understanding of the present invention and constitute a part of the specification. Together with the embodiments of the present invention, they are used to explain the present invention and do not constitute a limitation of the present invention. In the accompanying drawings:

[0033] Figure 1The present invention is a flowchart of the steps of a wellbore diameter correction method based on uranium fission prompt neutron logging;

[0034] Figure 2 This is a model diagram of the simulated wellbore and logging tool position in the present invention;

[0035] Figure 3 This is a simulation accuracy verification diagram of the well logging tool in the present invention;

[0036] Figure 4 This is a diagram verifying the accuracy of the numerical model simulation in the present invention;

[0037] Figure 5 This is a diagram showing the simulation results of the ratio of epithermal neutrons to thermal neutrons under different apertures of the probe in the present invention;

[0038] Figure 6 This is a correction coefficient diagram for different probe diameters in the present invention. DETAILED DESCRIPTION

[0039] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0040] Furthermore, if detailed description of known art is not necessary to illustrate the characteristics of the present invention, it will be omitted.

[0041] Example

[0042] like Figure 1-2 As shown, the present invention provides a method for correcting wellbore aperture based on uranium fission prompt neutron logging, comprising the following steps:

[0043] Step 1: Based on the uranium fission prompt neutron logging tool UNL-4 of East China University of Technology, a 1:1 logging tool numerical model and a uranium mine standard wellbore model were established using Monte Carlo numerical simulation software. The logging tool model includes neutron tubes, thermal neutron detectors, epithermal neutron detectors and other components, and the wellbore model includes uranium-bearing strata and uranium-free strata; the model apertures are 9cm, 10cm, 11cm, 12cm, 13cm, 14cm, 15cm, 16cm, 18cm, 20cm, 22cm, 24cm, 26cm, 28cm, and 30cm respectively, with an outer diameter of 140cm and a height of 180cm. The circular cylinder The inner framework of the wellbore is uranium-bearing sandstone, the main components of which are silicon dioxide and uranium. The uranium content is set at 0.1%. No material is placed in the pores within the inner diameter of the wellbore, making it a dry hole. The probe tube is equipped with a thermal neutron detector, an epithermal neutron detector, and a 14MeV DT pulsed neutron source from bottom to top. The spacing between the 14MeV DT pulsed neutron source and the epithermal neutron detector is 24.5cm, and the spacing between the 14MeV DT pulsed neutron source and the thermal neutron detector is 56cm. A cadmium shielding layer and a polyethylene shielding layer are arranged from the outside to the inside of the epithermal neutron detector. The particle count of the 14MeV DT pulsed neutron source is 109, and the diameter of the probe tube is 6cm.

[0044] Step 2: Using the model established in step 1, perform Monte Carlo simulation. Place the probe in the wellbore model, align the epithermal neutron detector with the center of the wellbore model, and use the thermal neutron detector and epithermal neutron detector to detect and record epithermal neutrons and thermal neutron counts respectively.

[0045] Step 3: Use the logging tool to measure the physical standard background model, obtain the measurement data, compare the simulated and measured thermal neutron counts, and verify the accuracy of the logging tool numerical model. Use the physical standard model with 1 / 1000 uranium content to compare with the numerical simulation results to verify the accuracy of the numerical model simulation. Compare the time spectrum obtained by the numerical simulation with the measured data of the logging tool. Figure 3 As shown in the figure, the simulated time spectrum agrees well with the measured time spectrum within the time range of 128μs to 800μs. The energy spectrum curve below 128μs exhibits some errors, but these errors are negligible as they are not used in the calculations. Comparing the thermal neutron counts, the relative error between the simulated and measured count rates in the time range of 128μs to 800μs is 2.77%, indicating that the numerical simulation parameter settings for the logging tool are generally accurate and can be used for subsequent research. Figure 4The following figure compares the neutron time spectrum curves of numerical simulation and measurement. It can be seen that the neutron time spectrum is in good agreement in the range of 128 to 800 μs. By comparing the count rates of thermal neutrons and epithermal neutrons, the relative errors between the simulation results and the measured results in the time range of 128 to 800 μs are calculated to be 0.01% and 5.09%, respectively, both less than 10%, verifying the reliability of the numerical model simulation.

[0046] Step 4: Keep the height and thickness of the wellbore model unchanged, change the inner diameter of the numerical wellbore model, set 20 inner diameters, measure and record the epithermal neutron and thermal neutron time spectra respectively, and obtain the total epithermal neutron count N in the effective time period of 128μs to 800μs En Total thermal neutron count N Tn , n≥2; record the data of the probe tube being centered and close to the wall;

[0047] Step 5: Calculate the total count rate ratio of epithermal neutrons to thermal neutrons for 20 bores, S n :

[0048]

[0049] Step 6: The obtained ratio of epithermal neutron to thermal neutron count rate S n Composed sequence S[n]:

[0050] S[n]={S1,S2…S n}, n≥2 (2)

[0051] Step 7: Set the inner diameter R of the numerical wellbore model to n Composed sequence R[n]:

[0052] R[n]={R1,R2…R n}, n≥2 (3)

[0053] Step 8: With R[n] as the x-axis and S[n] as the y-axis, construct a curve to analyze the relationship between the inner diameter of the numerical wellbore model and the ratio of the total count rate of epithermal neutrons to thermal neutrons (e.g. Figure 5 shown);

[0054] Step 9: Use the ratio S1 of the inner diameter R1 of the numerical wellbore model as the correction standard to correct the S1 of the inner diameter of other models. n . Set the correction coefficient μ, then we have

[0055]

[0056] Step 10: Set the correction coefficient μ n Composed sequence μ[n]:

[0057] μ[n]={μ1,μ2…μ n}, n≥2 (5)

[0058] Step 11: With R[n] as the x-axis and μ[n] as the y-axis, construct and fit the relationship curve between the inner diameter of the numerical wellbore model and the correction coefficient (such as Figure 6 shown);

[0059] In the center case:

[0060] y=0.1751x-0.4994,n≥2 (6)

[0061] When leaning against a wall:

[0062] y=-0.0047x 2 +0.3208x-1.5717, n≥2 (7)

[0063] Where x is in cm.

[0064] Step 12: Use formula (6) (7) to calculate the correction coefficient of the wellbore model with different inner diameters, calculate the relative error between the value derived from the fitting formula and the value derived from formula (4), and form a correction table.

[0065] Table 1 Aperture correction coefficients derived from simulation of uranium fission neutron logging tool

[0066]

[0067] As shown in the above table, the experimental results are reliable.

[0068] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for correcting wellbore diameter based on uranium fission prompt neutron logging, characterized in that: The following steps are involved: Step 1: Based on the uranium fission prompt neutron logging tool UNL-4, a 1:1 logging tool numerical model and a uranium mine standard wellbore model are established using Monte Carlo numerical simulation software. The logging tool numerical model includes a neutron tube, a thermal neutron detector, and an epithermal neutron detector. The wellbore model includes uranium-bearing strata and uranium-free strata. Step 2: Using the model established in step 1, perform Monte Carlo simulation. Place the probe in the wellbore model, align the epithermal neutron detector with the center of the wellbore model, and use the thermal neutron detector and epithermal neutron detector to detect and record epithermal neutrons and thermal neutron counts respectively. Step 3: Use the logging tool to measure the physical standard background model, obtain measurement data, and compare the simulated and measured thermal neutron counts to verify the accuracy of the logging tool numerical model. Use the physical standard model with 1 / 1000 uranium content to compare with the numerical simulation results to verify the accuracy of the numerical model simulation. If the relative error of the total count in the effective time period is less than 10%, the numerical simulation is considered to have passed the accuracy verification and the subsequent aperture numerical simulation results are reliable. Step 4: Keep the height and thickness of the wellbore model unchanged, change the inner diameter of the numerical wellbore model, and set n inner diameters R n , respectively measure and record the epithermal neutron and thermal neutron time spectrum, and obtain the total epithermal neutron count N in the effective time period area En Total thermal neutron count N Tn , n≥2; Step 5: Calculate the total count rate ratio S of epithermal neutrons to thermal neutrons for n inner diameters n : Step 6: The obtained ratio of epithermal neutron to total thermal neutron count rate S n Composed sequence S[n]: S[n]={S1,S2…S n },n≥2 (2) Step 7: Set the inner diameter R of the numerical wellbore model to n Composed sequence R[n]: R[n]={R1,R2…R n },n≥2 (3) Step 8: With R[n] as the x-axis and S[n] as the y-axis, construct and analyze the relationship curve between the inner diameter of the numerical wellbore model and the ratio of the total count rate of epithermal neutrons to thermal neutrons; Step 9: Use the ratio S1 of the inner diameter R1 of the numerical wellbore model as the correction standard to correct the S1 of the inner diameter of other models. n . Set the correction coefficient μ, then we have Step 10: Set the correction coefficient μ n Composed sequence μ[n]: μ[n]={μ1,μ2…μ n },n≥2 (5) Step 11: With R[n] as the x-axis and μ[n] as the y-axis, construct and fit the relationship curve between the inner diameter of the numerical wellbore model and the correction coefficient; In the center case: When leaning against a wall: Among them, a, b, and c are the parameters of the fitting function; Step 12: Calculate the correction coefficient of the wellbore model with different inner diameters according to the formula in step 11, calculate the relative error between the value derived from the fitting formula and the value derived from the formula in step 9, and form a correction table.

2. The method for correcting wellbore diameter based on uranium fission prompt neutron logging according to claim 1, characterized in that: In step 1, the models are divided into two categories: one is a circular tube with an inner diameter of 9 cm to 30 cm, an outer diameter of 140 cm, and a height of 180 cm. The inner skeleton of the wellbore is uranium-containing sandstone, the main components of which are silicon dioxide and uranium. The uranium content is set to 0.1%. No substance is placed in the pores of the inner diameter of the wellbore, which is a dry hole. The probe is placed in the center of the wellbore; the second category is different from the first category in that the probe is placed against the wall of the wellbore.

3. The method for correcting wellbore diameter based on uranium fission prompt neutron logging according to claim 1, characterized in that: In the step 2, the probe tube is provided with a thermal neutron detector, an epithermal neutron detector and a 14 MeV DT pulse neutron source from bottom to top, and a cadmium shielding layer and a polyethylene shielding layer are provided on the periphery of the epithermal neutron detector from outside to inside. The diameter of the probe tube is 6 cm, and the particle number of the DT pulse neutron source is 10 9 indivual.

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