Method for automatically extracting inelastic gamma rays in pulsed neutron logging

Abstract

The invention discloses a method for automatically extracting inelastic gamma rays in pulsed neutron logging, and belongs to the technical field of oil field logging. The method comprises the following steps: a, carrying out pulsed neutron logging to obtain a gamma ray energy spectrum during a time gate period; b, deducting a gamma ray energy spectrum in an early capture time gate period and a gamma ray energy spectrum in a late capture time gate period according to a certain proportion from the gamma ray energy spectrum in the pulse time gate period, and fitting to obtain optimal coefficients under different porosity conditions; and c, establishing a relationship between the coefficient and the counting rate measured by the detector, so that the inelastic spectrum counting rate can be obtained as long as the counting rate of the detector is obtained. According to the method, the epithermal neutron capture is considered, that is, the capture spectrum is considered to be epithermal neutron capture and thermal neutron capture, so that the net inelastic gamma rays are extracted more accurately, and a good foundation is laid for realizing a high-precision controllable source density logging method.

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 pulse neutron logging. Background Art

[0002] The pulsed neutron source emits high-energy fast neutrons that travel into the formation for the first 10 -8 ~10 -6 In the time interval of 10 s, the inelastic scattering reaction mainly occurs, and there is also a part of epithermal neutron capture reaction; -6 ~10 -3 In the s time interval, elastic scattering reactions, thermal neutron activation reactions, and capture reactions are the main reactions. If you want to perform high-precision density measurement and element content analysis, you need to extract accurate inelastic gamma spectra and capture gamma spectra from the measured gamma rays.

[0003] The commonly used method to extract the net inelastic gamma spectrum from the total gamma spectrum during the pulse is the double time gate subtraction background method. The principle of this method is to use the subsequent time gate after the pulse ends to obtain the capture spectrum, and then subtract this capture spectrum from the total gamma spectrum measured during the pulse to obtain the net inelastic scattering gamma spectrum.

[0004] The hydrogen peak reduction method is another commonly used method for extracting inelastic gamma. The hydrogen peak is the contribution of prompt gamma rays produced by the capture reaction between thermal neutrons and formation elemental hydrogen, which does not exist in the inelastic spectrum. Therefore, the inelastic spectrum can be obtained by subtracting a coefficient from the total spectrum and the product of the capture spectrum.

[0005] Both methods obtain the inelastic spectrum by subtracting the thermal neutron capture spectrum from the total spectrum, and neither method considers the effect of epithermal neutron capture on the capture spectrum. Although the obtained inelastic spectrum is not much different from the net inelastic spectrum in some cases, there are also cases where the error is large, so a more accurate extraction method is needed. Summary of the invention

[0006] Based on the above technical problems, the present invention proposes a method for automatically extracting inelastic gamma rays in pulsed neutron logging. The method takes epithermal neutron capture into consideration and can more accurately extract net inelastic gamma rays, laying a good foundation for realizing 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. The formula is obtained based on the relationship between the inelastic spectrum and the gamma-ray energy spectrum during the early capture time gate and the gamma-ray energy spectrum during the late capture time gate:

[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 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 inelastic 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, 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, and the relationship of formula (1) is expressed as:

[0017]

[0018] in, is the epithermal neutron count rate, and is α and β about Function of

[0019] That is, as long as the epithermal neutron count rate is obtained, α and β can be determined, and then the inelastic 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, then the relationship of equation (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 10us and a capture time of 25us, wherein the first 5us is an early capture time gate and the last 20us is a late capture time gate.

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

[0025] d1. According to the structure of the pulsed neutron logging instrument, the corresponding instrument and geological three-dimensional model are constructed, 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. Use the Monte Carlo simulation method to perform numerical simulation on the density logging while drilling, simulate different energy cutoffs and no energy cutoff, and obtain the gamma time spectra of 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 without 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 proportion of the energy spectra of the early and late capture time gates from the energy spectrum during the pulse time gate, obtains the best coefficient by fitting, and finally establishes a relationship between the coefficient and the count rate measured by the instrument detector, that is, as long as the count rate of the instrument detector is obtained, the inelastic spectrum can be obtained, and the automatic extraction of inelastic gamma rays can be realized. The present invention takes epithermal neutron capture into consideration, that is, the capture spectrum is considered to be epithermal neutron capture plus thermal neutron capture, thereby more accurately extracting inelastic gamma rays, laying a good foundation for realizing a high-precision controlled source density logging method. 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 It is a comparison chart between the net inelastic spectrum and the calculated inelastic 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 diagram of the structure 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] The existing inelastic gamma ray extraction methods do not consider the influence of epithermal neutron capture on the capture spectrum, resulting in a large difference between the obtained inelastic spectrum and the net inelastic spectrum, and a large error. The present invention considers the capture spectrum as epithermal neutron capture plus thermal neutron capture, and proposes an automatic extraction method for inelastic gamma rays in pulsed neutron logging. The method can extract inelastic gamma rays more accurately and obtain the inelastic spectrum count rate, laying a good foundation for realizing a high-precision controlled source density logging method.

[0038] Thermal neutrons: After fast neutrons enter the formation, they undergo inelastic scattering, emit gamma rays, and then become epithermal neutrons. Epithermal neutrons continue to slow down and become thermal neutrons. Thermal neutrons undergo capture reactions and also produce gamma rays. The inventors of this application believe that epithermal neutrons also undergo capture reactions and emit gamma rays. Therefore, the capture spectrum is considered to be epithermal neutron capture plus thermal neutron capture, so that inelastic gamma rays can be extracted more accurately.

[0039] The non-elastic spectrum count rate is calculated because it cannot be eliminated 100% like simulation. The net non-elastic spectrum count rate is simulated.

[0040] The present invention will be further described below in conjunction with the accompanying drawings and specific implementations.

[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: According to the structure of the pulsed neutron logging instrument, construct the corresponding instrument and geological three-dimensional model, including setting various instruments and geological parameters such as drill collars, detectors, borehole dimensions and formation dimensions. This embodiment uses dolomite formations with different porosities; the pulse timing is set to a pulse excitation time of 10us and a capture time of 30us. Set the energy cutoff to 0.1MeV, that is, neutrons with energies below 0.1MeV will be directly killed, and a net inelastic spectrum without capture can be obtained. Similarly, setting the energy cutoff to 0.1eV can obtain a time spectrum without thermal neutron capture, preparing for the next simulation.

[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 transient pulsed neutron source at the nanosecond level with high calculation accuracy, the Monte Carlo simulation method is used to perform numerical simulation of density logging while drilling, simulating different energy cutoffs and no energy cutoff, and obtaining the gamma time spectra of different time gates.

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

[0045] See also Figure 2 , it can be seen in the figure that 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 comprises 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 of different time gates.

[0049] Step 3: Deduct a certain proportion of the energy spectra of the early and late capture time gates from the energy spectra 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 get the formula:

[0051] CR net =CR burst -αCR ec -βC R 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 spectrum coefficient during the early capture time gate; β is the 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. In the figure, the inelastic spectrum obtained by only considering thermal neutron capture is significantly higher than the net inelastic spectrum and also contains hydrogen peaks. The inelastic spectrum obtained by the method proposed in the present invention is basically consistent with the net inelastic spectrum. Figure 4 It is a comparison chart of count rates under all porosity conditions. It can be seen that the inelastic spectrum obtained by calculation in the present invention is basically consistent with the net inelastic spectrum.

[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, the inelastic spectrum can be obtained as long as the epithermal neutron count rate is obtained. The relationship can be expressed by the formula:

[0058]

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

[0060] In addition, the count ratio of the near and far gamma detectors can also be used to establish a relationship with the coefficients α and β, which can be expressed as:

[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 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 are substituted into the above formula to calculate the non-elastic spectrum count rate.

[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 protection scope 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. The formula is obtained based on the relationship between the inelastic spectrum and the gamma-ray energy spectrum during the early capture time gate and the gamma-ray energy spectrum during the late capture time gate: 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 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 inelastic 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-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.

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 of That is, as long as the epithermal neutron count rate is obtained, α and β can be determined, and then the inelastic 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 equation (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 10us and a capture time of 25us, wherein the first 5us is an early capture time gate and the last 20us 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; specifically, the steps include: d1. According to the structure of the pulsed neutron logging instrument, the corresponding instrument and geological three-dimensional model are constructed, 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. Use the Monte Carlo simulation method to perform numerical simulation on the density logging while drilling, simulate different energy cutoffs and no energy cutoff, and obtain the gamma time spectra of 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; 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 killed directly, 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.

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