An automatic extraction method for inelastic gamma rays in pulsed neutron logging

Abstract

The present invention discloses a method for automatically extracting inelastic gamma rays from pulsed neutron logging, belonging to the technical field of oilfield logging. The method comprises the following steps: a. performing pulsed neutron logging to obtain a gamma ray energy spectrum during a time gate; b. deducting a certain proportion of the gamma ray energy spectrum during the early capture time gate and the gamma ray energy spectrum during the late capture time gate from the gamma ray energy spectrum during the pulsed time gate, and obtaining the optimal coefficients under different porosities through fitting; c. establishing a relationship between the coefficients and the count rate measured by the detector. As long as the detector count rate is obtained, the inelastic spectrum count rate can be calculated. The present invention takes epithermal neutron capture into account, i.e., the capture spectrum is considered to be epithermal neutron capture plus thermal neutron capture, thereby more accurately extracting net inelastic gamma rays, laying a good foundation for implementing high-precision controlled source density logging methods.

Description

Technical Field

[0001] The invention belongs to the technical field of oilfield logging, and in particular relates to an automatic extraction method of inelastically scattered gamma rays in pulsed neutron logging. Background Art

[0002] The pulsed neutron source emits high energy fast neutrons which are emitted into the formation within the first 10 -8 ~10 -6 In the time interval of 10 s, the inelastic scattering reaction mainly occurs, and there are also some epithermal neutron capture reactions; in the subsequent -6 ~10 -3 The s time interval is dominated by elastic scattering reactions, thermal neutron activation reactions, and capture reactions. For high-precision density measurements and elemental content analysis, it is necessary to accurately extract the inelastic gamma spectrum and capture gamma spectrum from the measured gamma rays.

[0003] A commonly used method for extracting the net inelastic gamma spectrum from the total gamma spectrum during a pulse is the dual-time-gated background subtraction method. This method uses a subsequent time gate after the pulse ends to obtain a capture spectrum. This capture spectrum is then subtracted from the total gamma spectrum measured during the pulse to obtain the net inelastic gamma spectrum.

[0004] The hydrogen peak reduction method is another commonly used method for extracting inelastic gamma rays. The hydrogen peak is the contribution of prompt gamma rays from the capture reaction between thermal neutrons and elemental hydrogen in the formation, and is absent from the inelastic spectrum. Therefore, the inelastic spectrum can be obtained by subtracting a coefficient from the total spectrum and multiplying it by the capture spectrum.

[0005] Both methods obtain the inelastic spectrum by subtracting the thermal neutron capture spectrum from the total spectrum. Neither method considers the effect of epithermal neutron capture on the capture spectrum. While in some cases the resulting inelastic spectrum is comparable to the net inelastic spectrum, there are also cases where significant errors can occur. Therefore, a more accurate extraction method is needed. Summary of the Invention

[0006] To address the above technical issues, the present invention proposes a method for automatically extracting inelastic gamma rays from pulsed neutron logging. This method, which takes epithermal neutron capture into account, can more accurately extract net inelastic gamma rays, laying a solid foundation for implementing high-precision controlled source density logging methods.

[0007] The technical solution adopted by the present invention is:

[0008] A method for automatically extracting inelastic gamma rays in pulsed neutron logging comprises the following steps:

[0009] a. Perform pulsed neutron logging to obtain the gamma-ray energy spectrum during the pulse excitation time gate, the gamma-ray energy spectrum during the early capture time gate, and the gamma-ray energy spectrum during the late capture time gate;

[0010] b. Deduct a certain proportion of the gamma-ray energy spectrum during the early capture time gate and the gamma-ray energy spectrum during the late capture time gate from the gamma-ray energy spectrum during the pulse excitation time gate, and obtain the optimal coefficients under different porosities by fitting; the specific steps are as follows:

[0011] b1. Based on the relationship between the non-elastic spectrum, the gamma-ray energy spectrum during the early capture time gate, and the gamma-ray energy spectrum during the late capture time gate, the formula is obtained:

[0012] CR net =CR burst -αCR ec -βCR lc (1)

[0013] Among them, CR net is the non-ballistic spectrum count rate; CR burst is the gamma ray spectrum count rate during the pulse excitation time gate; α is the energy spectrum coefficient during the early capture time gate; β is the energy spectrum coefficient during the late capture time gate; CR ec is the energy spectrum count rate during the early capture time gate; CR lc is the energy spectrum count rate during the late capture time gate;

[0014] b2. Substitute the net non-elastic spectrum count rate, the gamma spectrum count rate during the pulse excitation time gate, the spectrum count rate during the early capture time gate, and the spectrum count rate during the late capture time gate obtained under different porosities into formula (1), and obtain α and β under different porosities by fitting;

[0015] c. Establish a relationship between α and β and the count rate measured by the detector. Then, as long as the count rate of the detector is obtained, the non-elastic spectrum count rate can be obtained. Specifically, α and β are determined by the count rate measured by the detector, and then the gamma energy spectrum count rate during the pulse excitation time gate, the energy spectrum count rate during the early capture time gate, and the energy spectrum count rate during the late capture time gate obtained by the detector are substituted into formula (1) to calculate the non-elastic spectrum count rate.

[0016] In the above step c: establish the relationship between the epithermal neutron count rate and α and β. At this time, the relationship of formula (1) is expressed as:

[0017]

[0018] in, is the epithermal neutron count rate, and Is α and β about function;

[0019] That is, as long as the epithermal neutron count rate is obtained, α and β can be determined, and then the non-elastic spectrum count rate can be obtained.

[0020] In the above step c: establish the relationship between the count ratio of the near and far gamma detectors and α and β. At this time, the relationship of formula (1) is expressed as:

[0021] CR net =CR burst -α(η)CR ec -β(η)CR lc (3)

[0022] where η is the ratio of the near and far gamma detector count rates, and α(η) and β(η) are functions of α and β with respect to η.

[0023] In the above step a: the pulse timing of pulse neutron logging is set to a pulse excitation time of 10 us and a capture time of 25 us, wherein the first 5 us is an early capture time gate and the last 20 us is a late capture time gate.

[0024] The above method further includes determining the time spectrum of epithermal neutron capture by a Monte Carlo simulation method; specifically comprising the following steps:

[0025] d1. Based on the structure of the pulsed neutron logging instrument, build the corresponding instrument and geological 3D model, including setting the instrument and geological parameters, and setting the pulse timing to a pulse excitation time of 10us and a capture time of 30us;

[0026] d2. Numerical simulation of density logging while drilling is performed using the Monte Carlo simulation method, simulating different energy cutoffs and no energy cutoff, and obtaining gamma time spectra for different time gates, including the net inelastic gamma ray time spectrum, the gamma ray time spectrum during the pulse time gate, the gamma ray time spectrum during the late capture time gate, and the time spectrum containing only epithermal neutron capture and inelastic gamma rays;

[0027] d3. Deduct the net inelastic gamma ray time spectrum from the simulated time spectrum containing epithermal neutron capture and inelastic gamma rays to obtain a time spectrum containing only epithermal neutron capture.

[0028] In the above steps, the energy cutoff is set to 0.1 MeV, that is, neutrons with energy below 0.1 MeV will be directly killed, that is, a net inelastic spectrum with no capture is obtained; or the energy cutoff is set to 0.1 eV, that is, a time spectrum without thermal neutron capture is obtained.

[0029] The beneficial technical effects of the present invention are as follows:

[0030] The present invention deducts a certain percentage of the energy spectra from the early and late capture time gates from the energy spectrum during the pulse time gate, and then obtains the optimal coefficient through fitting. Finally, the coefficient is related to the count rate measured by the instrument detector. Therefore, once the count rate of the instrument detector is known, the inelastic spectrum can be calculated, enabling the automatic extraction of inelastic gamma rays. The present invention also considers epithermal neutron capture, treating the capture spectrum as a combination of epithermal and thermal neutron capture, thereby more accurately extracting inelastic gamma rays and laying a solid foundation for the development of high-precision controlled source density logging methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A flow chart of the method for automatically extracting inelastic gamma rays in pulsed neutron logging provided by the present invention;

[0032] Figure 2 is the time spectrum of inelastic gamma and captured gamma;

[0033] Figure 3 Comparison chart of net non-elastic spectrum and calculated non-elastic spectrum;

[0034] Figure 4 is a graph showing the relationship between the net non-bullet count rate and the calculated non-bullet count rate;

[0035] Figure 5 This is a schematic structural diagram of the well logging instrument used in the present invention.

[0036] In the figure: 1-epitothermal neutron detector, 2-near-thermal neutron detector, 3-near-gamma detector, 4-far-thermal neutron detector, 5-far-gamma detector. DETAILED DESCRIPTION

[0037] Existing inelastic gamma-ray extraction methods fail to consider the impact of epithermal neutron capture on the capture spectrum, resulting in significant discrepancies between the obtained inelastic spectrum and the net inelastic spectrum, leading to large errors. This paper considers the capture spectrum to be the sum of epithermal neutron capture and thermal neutron capture, and proposes an automated method for extracting inelastic gamma rays from pulsed neutron logging. This method can more accurately extract inelastic gamma rays and obtain inelastic spectrum count rates, laying a solid foundation for implementing high-precision controlled source density logging methods.

[0038] Thermal neutrons: After entering the formation, fast neutrons undergo inelastic scattering, emitting gamma rays. These neutrons then become epithermal neutrons. Epithermal neutrons further decelerate into thermal neutrons, which then undergo capture reactions, also producing gamma rays. The inventors of this application have determined that epithermal neutrons can also undergo capture reactions and emit gamma rays. Therefore, they consider the capture spectrum to be a combination of epithermal and thermal neutron capture, enabling more accurate extraction of inelastic gamma rays.

[0039] The non-ballistic spectrum count rate is calculated because it cannot be completely eliminated like in simulation. The net non-ballistic spectrum count rate is obtained by simulation.

[0040] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0041] The present application first obtains the net inelastic scattered gamma ray time spectrum, the gamma ray time spectrum during the pulse time gate, the gamma ray time spectrum during the late capture time gate, and the time spectrum containing only epithermal neutron capture and inelastic scattered gamma rays through Monte Carlo simulation, which specifically includes the following steps:

[0042] Step 1.1: Based on the structure of the pulsed neutron logging instrument, construct the corresponding instrument and geological three-dimensional model, including setting various instrument and geological parameters such as drill collars, detectors, wellbore dimensions, and formation dimensions. This example uses dolomite formations of different porosities; the pulse timing is set to a pulse excitation time of 10us and a capture time of 30us. The energy cutoff is set to 0.1MeV, which means that neutrons with energies below 0.1MeV will be directly killed, resulting in a net non-elastic spectrum without capture. Similarly, setting the energy cutoff to 0.1eV can obtain a time spectrum without thermal neutron capture, preparing for the next simulation step.

[0043] Step 1.2: Perform pulsed neutron logging simulation. Since the Monte Carlo simulation method can analyze the gamma-ray time spectrum generated by the nanosecond transient pulsed neutron source with high calculation accuracy, the Monte Carlo simulation method is used to perform numerical simulation of the density logging while drilling. The gamma-ray time spectrum of different time gates is obtained by simulating different energy cutoffs and no energy cutoff.

[0044] Step 1.3: The time spectrum of only epithermal neutron capture is obtained by deducting the net inelastic time spectrum from the time spectrum obtained by truncating the energy by 0.1 eV.

[0045] See also Figure 2 ,As can be seen in the figure, there is a certain proportion of epithermal neutron capture during the pulse.

[0046] like Figure 1 As shown, a method for automatically extracting inelastic gamma rays in pulsed neutron logging includes the following steps:

[0047] Step 2: Obtain the gamma-ray energy spectrum during the pulse excitation time gate and the gamma-ray energy spectrum during the early and late capture time gates by measuring, which specifically includes the following steps:

[0048] Step 2.1: Perform pulsed neutron logging and set the pulse timing to 10us pulse excitation time and 25us capture time, where the first 5us is the early capture time gate and the last 20us is the late capture time gate, and obtain the gamma spectrum energy spectrum at different time gates.

[0049] Step 3: Subtract a certain proportion of the energy spectra of the early and late capture time gates from the energy spectrum during the pulse time gate and obtain the optimal coefficients through fitting, which specifically includes the following steps:

[0050] Step 3.1: Based on the relationship between the inelastic spectrum and the captured spectrum, we can get the formula:

[0051] CR net =CR burst -αCR ec -βCR lc

[0052] Among them, CR net is the non-ballistic spectrum count rate; CR burst is the gamma spectrum count rate during the pulse; α is the energy spectrum coefficient during the early capture time gate; β is the energy spectrum coefficient during the late capture time gate; CR ec is the energy spectrum count rate of the early capture time gate; CR lc is the energy spectrum count rate of the late capture time gate.

[0053] Step 3.2: Substitute each set of data obtained under different porosity conditions into the formula and obtain the optimal α and β under different porosity conditions through fitting.

[0054] Step 3.3: Calculate the non-elastic spectrum count rate using the fitted α and β, and compare the calculated non-elastic spectrum count rate with the net non-elastic spectrum count rate.

[0055] See also Figure 3 and Figure 4 , Figure 3 This is a comparison of energy spectra under porosity conditions. The inelastic spectrum obtained by considering only thermal neutron capture is significantly higher than the net inelastic spectrum and also includes a hydrogen peak. The inelastic spectrum obtained using the method proposed in this invention is essentially the same as the net inelastic spectrum. Figure 4 The comparison chart of count rates under all porosity conditions shows that the calculated non-elastic spectrum and the net non-elastic spectrum are basically consistent.

[0056] Step 4: Establish a relationship between the two coefficients α and β and the epithermal neutron count rate and verify it on the energy spectrum (e.g. Figure 3 ), specifically including the following steps:

[0057] Step 4.1: Because this instrument has an epithermal neutron detector, such as Figure 5As shown, the epithermal neutron count rate can be easily obtained, and the relationship between the epithermal neutron count rate and α and β can be established. This functional relationship can be obtained through simulation. At this time, as long as the epithermal neutron count rate is obtained, the inelastic spectrum can be obtained. The relationship can be expressed as follows:

[0058]

[0059] in, is the epithermal neutron count rate, and Is α and β about function.

[0060] Alternatively, the count ratio of the near-far gamma detector can be used to establish a relationship with the coefficients α and β, which can be expressed as follows:

[0061] CR net =CR burst -α(η)CR ec -β(η)CR lc

[0062] where η is the ratio of the near and far gamma detector count rates, and α(η) and β(η) are functions of α and β with respect to η.

[0063] Therefore, the non-elastic spectrum count rate can be obtained as long as the detector count rate is obtained; specifically, α and β are determined by the count rate measured by the detector, and then the non-elastic spectrum count rate is calculated by substituting the obtained gamma energy spectrum count rate during the pulse, the energy spectrum count rate of the early capture time gate, and the energy spectrum count rate of the late capture time gate into the above formula.

[0064] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the scope of protection of the present invention.

Claims

1. A method for automatically extracting inelastic gamma rays in pulsed neutron logging, characterized in that: The following steps are involved: a. Perform pulsed neutron logging to obtain the gamma-ray energy spectrum during the pulse excitation time gate, the gamma-ray energy spectrum during the early capture time gate, and the gamma-ray energy spectrum during the late capture time gate; b. Deduct a certain proportion of the gamma-ray energy spectrum during the early capture time gate and the gamma-ray energy spectrum during the late capture time gate from the gamma-ray energy spectrum during the pulse excitation time gate, and obtain the optimal coefficients under different porosities by fitting; the specific steps are as follows: b1. Based on the relationship between the non-elastic spectrum, the gamma-ray energy spectrum during the early capture time gate, and the gamma-ray energy spectrum during the late capture time gate, the formula is obtained: CR net =CR burst -αCR ec -βCR lc (1) Among them, CR net is the non-ballistic spectrum count rate; CR burst is the gamma ray spectrum count rate during the pulse excitation time gate; α is the energy spectrum coefficient during the early capture time gate; β is the energy spectrum coefficient during the late capture time gate; CR ec is the energy spectrum count rate during the early capture time gate; CR lc is the energy spectrum count rate during the late capture time gate; b2. Substitute the net non-elastic spectrum count rate, the gamma spectrum count rate during the pulse excitation time gate, the spectrum count rate during the early capture time gate, and the spectrum count rate during the late capture time gate obtained under different porosities into formula (1), and obtain α and β under different porosities by fitting; c. Establish a relationship between α, β and the count rate measured by the detector. Then, as long as the count rate of the detector is obtained, the non-bullet spectrum count rate can be obtained; Specifically, α and β are determined by the count rate measured by the detector, and then the gamma spectrum count rate during the pulse excitation time gate, the energy spectrum count rate during the early capture time gate, and the energy spectrum count rate during the late capture time gate obtained by the detector are substituted into formula (1) to calculate the non-elastic spectrum count rate.

2. The method for automatically extracting inelastic gamma rays in pulsed neutron logging according to claim 1, characterized in that: In step c: establish the relationship between the epithermal neutron count rate and α and β. At this time, the relationship of formula (1) is expressed as: in, is the epithermal neutron count rate, and Is α and β about function; That is, as long as the epithermal neutron count rate is obtained, α and β can be determined, and then the non-elastic spectrum count rate can be obtained.

3. The method for automatically extracting inelastic gamma rays in pulsed neutron logging according to claim 1, characterized in that: In step c: establish the relationship between the count ratio of the near and far gamma detectors and α and β. At this time, the relationship of formula (1) is expressed as: CR net =CR burst -α(η)CR ec -β(η)CR lc (3) where η is the ratio of the near and far gamma detector count rates, and α(η) and β(η) are functions of α and β with respect to η.

4. The method for automatically extracting inelastic gamma rays in pulsed neutron logging according to claim 1, characterized in that: In step a: the pulse timing of pulse neutron logging is set to a pulse excitation time of 10 us and a capture time of 25 us, wherein the first 5 us is an early capture time gate and the last 20 us is a late capture time gate.

5. The method for automatically extracting inelastic gamma rays in pulsed neutron logging according to claim 1, characterized in that: The method also includes determining the time spectrum of epithermal neutron capture by Monte Carlo simulation method, which specifically includes the following steps: d1. Based on the structure of the pulsed neutron logging instrument, build the corresponding instrument and geological 3D model, including setting the instrument and geological parameters, and setting the pulse timing to a pulse excitation time of 10us and a capture time of 30us; d2. Numerical simulation of density logging while drilling (LWD) was performed using the Monte Carlo simulation method. Simulating different energy cutoffs and no energy cutoff, the gamma time spectra for different time gates were obtained, including the net inelastic gamma ray time spectrum, the gamma ray time spectrum during the pulse time gate, the gamma ray time spectrum during the late capture time gate, and the time spectrum containing only epithermal neutron capture and inelastic gamma rays. d3. Deduct the net inelastic gamma ray time spectrum from the simulated time spectrum containing epithermal neutron capture and inelastic gamma rays to obtain a time spectrum containing only epithermal neutron capture.

6. The method for automatically extracting inelastic gamma rays in pulsed neutron logging according to claim 5, characterized in that: The energy cutoff is set to 0.1 MeV, that is, neutrons with energy below 0.1 MeV will be directly killed, and a net inelastic spectrum with no capture is obtained; or the energy cutoff is set to 0.1 eV, and a time spectrum with no thermal neutron capture is obtained.

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