Deuterium-deuterium neutron porosity correction method based on capture cross section

Abstract

The invention discloses a deuterium-deuterium neutron porosity correction method based on a capture cross section, and the method comprises the following steps: obtaining a neutron time spectrum of a near-far detector according to the logging response difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity under different scale well conditions; calculating a capture cross section through a sliding time window method according to the neutron time spectrum; and acquiring a relation among the capture cross section, the deuterium-deuterium neutron porosity and the compensated neutron porosity, and establishing a linear relation among the capture cross section, the deuterium-deuterium neutron porosity and the compensated neutron porosity for correction. And correcting the deuterium-deuterium neutron porosity based on the capture cross section, so that the corrected deuterium-deuterium neutron porosity is consistent with the traditional chemical source compensation neutron porosity. According to the method, the capture cross section is calculated based on the neutron time spectrum of the pulse mode, the deuterium-deuterium neutron porosity is corrected through the capture cross section, the corrected deuterium-deuterium neutron porosity is consistent with the traditional chemical source compensation neutron porosity, and development and application of the deuterium-deuterium neutron porosity logging instrument are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of well logging, and in particular to a deuterium-deuterium neutron porosity correction method based on capture cross section. Background Art

[0002] Conventional neutron porosity logging instruments use americium-beryllium chemical sources for porosity measurement, which can cause irreversible damage to the surrounding environment and operators. During the transportation and logging of the instrument, once the source is lost or the instrument is stuck, it may cause serious problems such as regional radioactive contamination and oil well scrapping. Compared with the americium-beryllium chemical source, the controllable neutron source has the advantages of safety, environmental protection, and controllable radioactivity. The main application of the controlled neutron source is the deuterium-tritium neutron source. The deuterium-tritium neutron generator can emit fast neutrons with an energy of 14.1MeV. Although the deuterium-tritium neutron generator has no radioactive hazards when not working. However, there is a possibility of tritium contamination of the formation, and there is a radiation hazard of activated gamma when working. The deuterium-deuterium neutron generator can emit fast neutrons with an energy of 2.45MeV. The neutron source has a low energy, no activated gamma is produced, and there is no possibility of formation contamination.

[0003] However, there is a response difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity using the pulse mode in the prior art, especially the later the acquisition timing is, the greater the formation capture effect is, and the greater the response difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity is, so that the deuterium-deuterium neutron generator cannot be used in the porosity logging instrument. Summary of the invention

[0004] In order to solve the problems existing in the prior art, the present invention provides a deuterium-deuterium neutron porosity correction method based on capture cross section, which corrects the deuterium-deuterium neutron porosity based on the capture cross section so that the corrected deuterium-deuterium neutron porosity is consistent with the traditional chemical source compensated neutron porosity.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A deuterium-deuterium neutron porosity correction method based on capture cross section includes the following process:

[0007] According to the difference in logging response between deuterium-deuterium neutron porosity and compensated neutron porosity under different calibration well conditions, the neutron time spectrum of near and far detectors is obtained;

[0008] The capture cross section is calculated based on the neutron time spectrum by the sliding time window method;

[0009] The relationship among the capture cross section, deuterium-deuterium neutron porosity and compensated neutron porosity is obtained, and a linear relationship between the capture cross section, deuterium-deuterium neutron porosity and compensated neutron porosity is established for correction.

[0010] Preferably, the process of acquiring the neutron time spectrum of the near and far detectors is specifically as follows: in wells calibrated with different lithology, porosity, and mud content, the deuterium-deuterium neutron porosity and the compensated neutron porosity logging response of the americium-beryllium neutron source are obtained; the compensated neutron porosity logging of the americium-beryllium neutron source measures the near and far neutron counts, and the deuterium-deuterium neutron porosity logging based on the pulse mode is used to obtain the neutron time spectrum of the near and far detectors.

[0011] Furthermore, the neutron ratio of near and far detectors is obtained by setting the time window for neutron time spectrum acquisition, and the neutron porosity is calculated by the neutron ratio of near and far detectors.

[0012] Preferably, the capture cross section is calculated using a sliding time window method, including: setting an initial time window, obtaining an initial sigma value, determining a time window for calculating the capture cross section based on the size of the initial sigma value, and calculating the sigma value based on a single exponential fitting.

[0013] Furthermore, if the initial sigma is less than 38, the calculation time window is t=816-1258 channels; if the initial sigma is greater than 45, the calculation time window is t=611-683 channels; for other initial sigmas, the time window is t=611-683 channels.

[0014] Preferably, the capture cross section is calculated based on the count rate, and the count rate formula is:

[0015]

[0016] Among them, C i is the gamma count rate of the ith channel, in Cps; t i is the time of the ith channel, in μs; t0 is the initial time for calculating the formation capture cross section; τ F is the gamma decay time constant related to the formation capture cross section, in μs; Σ is the formation capture cross section, in cu; A is the amplitude, and the initial value can be determined by the counting rate corresponding to time t0.

[0017] Preferably, the capture cross section calculation formula is:

[0018]

[0019] Where, Σ is the formation capture cross section, in units of cu; τ F is the gamma decay time constant related to the formation capture cross section, in μs.

[0020] Preferably, the deuterium-deuterium neutron porosity and formation capture cross section of different acquisition time windows are obtained, and the relationship between the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity is compared and analyzed to obtain a linear relationship between the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity. Using the linear relationship, a deuterium-deuterium neutron porosity correction method is established.

[0021] Preferably, the linear relationship between the capture cross section and the deuterium-deuterium neutron porosity and the compensation neutron porosity is:

[0022] Porc=PorDD+k*sigma+b

[0023] Among them, PorDD is the deuterium-deuterium neutron porosity, pu; Porc is the corrected deuterium-deuterium neutron porosity, pu; sigma is the formation capture cross section calculated based on the pulse time spectrum, cu; k and b are correction coefficients, which are determined based on the relationship between the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity.

[0024] Compared with the prior art, the present invention has the following beneficial technical effects:

[0025] The present invention provides a deuterium-deuterium neutron porosity correction method based on capture cross section, which improves the porosity sensitivity of controlled source neutron porosity logging by optimizing the acquisition time sequence, solves the problem that tritium-deuterium neutron porosity logging is insensitive to high-porosity formations, and improves the measurement range of tritium-deuterium neutron porosity logging. Based on the neutron time spectrum calculation of the pulse mode, the capture cross section is used to correct the deuterium-deuterium neutron porosity, so that the corrected deuterium-deuterium neutron porosity is consistent with the traditional chemical source compensation neutron porosity, and the deuterium-deuterium neutron porosity logging instrument is developed and applied. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Schematic diagram of a device for deuterium-deuterium neutron porosity and compensating neutron porosity provided in an embodiment of the present invention.

[0027] Figure 2 A schematic diagram of a flow chart of a method for determining a deuterium-deuterium neutron porosity logging response correction method provided in an embodiment of the present invention.

[0028] Figure 3 The relationship between the capture cross section, deuterium-deuterium neutron porosity and the compensating neutron porosity difference provided in the embodiments of the present invention. DETAILED DESCRIPTION

[0029] The present invention is further described in detail below in conjunction with specific embodiments, which are intended to explain the present invention rather than to limit it.

[0030] The present invention provides a deuterium-deuterium neutron porosity correction method based on capture cross section, which uses a pulse mode deuterium-deuterium neutron porosity and a compensated neutron porosity with a response difference, especially the later the acquisition sequence is, the greater the response difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity. Neutron porosity mainly reflects the deceleration effect and capture effect of the formation, the deceleration effect is mainly related to the formation porosity, and the capture effect is mainly related to the formation capture cross section. The present invention calculates the capture cross section based on the neutron time spectrum in the pulse mode, uses the capture cross section to correct the deuterium-deuterium neutron porosity, and makes the corrected deuterium-deuterium neutron porosity consistent with the traditional chemical source compensated neutron porosity, thereby realizing the development and application of a deuterium-deuterium neutron porosity logging instrument.

[0031] The present invention provides a deuterium-deuterium neutron porosity correction method based on capture cross section, studies the difference in logging response between deuterium-deuterium neutron porosity and compensated neutron porosity under well conditions with different calibrations; realizes capture cross section calculation based on sliding time window method; studies the relationship between capture cross section, deuterium-deuterium neutron porosity and compensated neutron porosity, and establishes a correction method. The method that the corrected deuterium-deuterium neutron porosity is consistent with the traditional chemical source compensated neutron porosity realizes the development and application of deuterium-deuterium neutron porosity logging instrument.

[0032] To achieve the above object, the present invention provides a deuterium-deuterium neutron porosity correction method based on capture cross section, comprising the following steps:

[0033] The difference in logging response between deuterium-deuterium neutron porosity and compensated neutron porosity under well conditions of different scales is studied;

[0034] The capture cross section calculation is realized based on the sliding time window method;

[0035] Study the relationship between capture cross section, deuterium-deuterium neutron porosity and compensated neutron porosity, and establish a correction method;

[0036] Specifically, the steps of studying the difference characteristics of the logging responses of deuterium-deuterium neutron porosity and compensated neutron porosity under different scale well conditions include:

[0037] In wells with different lithology, porosity, and shale content, the deuterium-deuterium neutron porosity and compensated neutron porosity logging responses of the americium-beryllium neutron source are obtained. The compensated neutron porosity logging of the americium-beryllium neutron source measures the near and far neutron counts. The deuterium-deuterium neutron porosity logging based on the pulse mode can obtain the near and far detector neutron time spectrum. The near and far detector neutron ratio is obtained by setting the neutron time spectrum acquisition time window. Generally, the later the acquisition time window, the higher the sensitivity of the neutron ratio to porosity, but the greater the difference in the logging response between the deuterium-deuterium neutron porosity and the compensated neutron porosity.

[0038] Specifically, the steps of calculating the capture cross section based on the sliding window method include:

[0039] When calculating the capture cross section based on the neutron time spectrum in pulse mode, the time spectrum attenuation of formations with different capture cross sections is different. Generally, the larger the formation capture cross section, the greater the time spectrum attenuation. When using the single exponential fitting solution method, if a fixed time window is used, the formation capture cross section cannot be accurately obtained. Therefore, the sliding time window method is used to calculate the capture cross section. First, the initial time window is set to obtain the initial sigma value. According to the size of the initial sigma value, the time window used to calculate the capture cross section is determined, and the sigma value is calculated based on the single exponential fitting.

[0040] Specifically, the steps of studying the relationship between the capture cross section, the deuterium-deuterium neutron porosity and the compensation neutron porosity include:

[0041] Based on the above steps, the deuterium-deuterium neutron porosity and formation capture cross section of different acquisition time windows are obtained, and the relationship between the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity is compared and analyzed. It is found that the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity are in a linear relationship. Using the linear relationship, a deuterium-deuterium neutron porosity correction method is established. The corrected deuterium-deuterium neutron porosity is consistent with the traditional chemical source compensated neutron porosity method, realizing the development and application of the deuterium-deuterium neutron porosity logging tool.

[0042] By adopting the above method, the deuterium-deuterium neutron porosity and capture cross section are calculated based on the neutron time spectrum in pulse mode, and the deuterium-deuterium neutron porosity is corrected by using the capture cross section. The corrected deuterium-deuterium neutron porosity is made consistent with the traditional chemical source compensated neutron porosity, thereby realizing the development and application of the deuterium-deuterium neutron porosity logging tool.

[0043] The present invention improves the porosity sensitivity of controlled source neutron porosity logging by optimizing the acquisition timing, solves the problem that tritium-tritium neutron porosity logging is insensitive to high-porosity formations, and increases the measurement range of tritium-tritium neutron porosity logging.

[0044] Embodiment 1

[0045] The deuterium-deuterium neutron porosity logging device used in this embodiment can be referenced to the schematic diagram of its simulation model. Figure 1 The instrument is mainly composed of a deuterium-deuterium neutron generator, a shield, and two near and far detectors. This embodiment provides a deuterium-deuterium neutron porosity correction method based on capture cross section, such as Figure 2 As shown, the specific steps include:

[0046] S1. Study the difference in logging response between deuterium-deuterium neutron porosity and compensated neutron porosity under different well conditions;

[0047] S2, capture cross section calculation based on sliding time window method;

[0048] S3. Study the relationship between capture cross section, deuterium-deuterium neutron porosity and compensated neutron porosity, and establish a correction method;

[0049] In step S1, the research on the difference in logging response between deuterium-deuterium neutron porosity and compensated neutron porosity under well conditions of different scales is specifically carried out as follows:

[0050] In wells with different lithology, porosity, and shale content, the deuterium-deuterium neutron porosity and compensated neutron porosity logging responses of the americium-beryllium neutron source are obtained. The compensated neutron porosity logging of the americium-beryllium neutron source measures the near and far neutron counts. The deuterium-deuterium neutron porosity logging based on the pulse mode can obtain the near and far detector neutron time spectrum. The near and far detector neutron ratio is obtained by setting the neutron time spectrum acquisition time window. Generally, the later the acquisition time window, the higher the sensitivity of the neutron ratio to porosity, but the greater the difference in the logging response between the deuterium-deuterium neutron porosity and the compensated neutron porosity, as shown in Table 1.

[0051] Table 1 Logging responses of deuterium-deuterium neutron porosity and compensated neutron porosity

[0052]

[0053] In step S2, the capture cross section calculation is implemented based on the sliding window method; specifically:

[0054] When calculating the capture cross section based on the neutron time spectrum in pulse mode, the time spectrum attenuation of formation measurements with different capture cross sections is different. Generally, the larger the formation capture cross section, the greater the time spectrum attenuation. When using the single exponential fitting solution method, if a fixed time window is used, the formation capture cross section cannot be accurately obtained. Therefore, the sliding time window method is used to calculate the capture cross section. First, the initial time window t = 611-683 is set to obtain the initial sigma value. According to the size of the initial sigma value, if sigma < 38, the calculation time window is t = 816-1258, if sigma > 45, the calculation time window is t = 611-683, and the rest are time windows t = 611-683. The time window used to calculate the capture cross section is determined, and the sigma value is calculated based on the single exponential fitting, as shown in Formula 1 and Formula 2.

[0055]

[0056]

[0057] Among them, C i is the gamma count rate of the ith channel, in Cps; t i is the time of the ith channel, in μs; t0 is the initial time for calculating the formation capture cross section; τ F is the gamma decay time constant related to the formation capture cross section, in μs; Σ is the formation capture cross section, in cu. A is the amplitude, and the initial value can be determined by the counting rate corresponding to time t0.

[0058] In step S3, the relationship between the capture cross section, the deuterium-deuterium neutron porosity and the compensation neutron porosity is studied, specifically:

[0059] Based on the above steps, the deuterium-deuterium neutron porosity and formation capture cross section of different acquisition time windows are obtained, and the relationship between the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity is compared and analyzed. It is found that the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity are in a linear relationship. Using the linear relationship, a deuterium-deuterium neutron porosity correction method is established, as shown in Formula 3. The corrected deuterium-deuterium neutron porosity is consistent with the traditional chemical source compensated neutron porosity method, realizing the development and application of the deuterium-deuterium neutron porosity logging tool.

[0060] Porc=PorDD+k*sigma+b (3)

[0061] Among them, PorDD is the deuterium-deuterium neutron porosity, pu; Porc is the corrected deuterium-deuterium neutron porosity, pu; sigma is the formation capture cross section calculated based on the pulse time spectrum, cu; k and b are correction coefficients, which can be determined based on the relationship between the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity.

[0062] The porosity sensitivity of controlled source neutron porosity logging is improved by optimizing the acquisition timing. However, the later the acquisition time, the greater the difference in the logging response between deuterium-deuterium neutron porosity and compensated neutron porosity. The deuterium-deuterium neutron porosity and capture cross section are calculated based on the neutron time spectrum in pulse mode. The deuterium-deuterium neutron porosity is corrected using the capture cross section. The corrected deuterium-deuterium neutron porosity is made consistent with the traditional chemical source compensated neutron porosity, thus realizing the development and application of deuterium-deuterium neutron porosity logging instrument.

Claims

1. A deuterium-deuterium neutron porosity correction method based on capture cross section, characterized in that: Including the following process, According to the difference in logging response between deuterium-deuterium neutron porosity and compensated neutron porosity under different calibration well conditions, the neutron time spectrum of near and far detectors is obtained; The capture cross section is calculated based on the neutron time spectrum by the sliding time window method; The relationship among the capture cross section, deuterium-deuterium neutron porosity and compensated neutron porosity is obtained, and a linear relationship between the capture cross section, deuterium-deuterium neutron porosity and compensated neutron porosity is established for correction.

2. A deuterium-deuterium neutron porosity correction method based on capture cross section according to claim 1, characterized in that: The specific process of acquiring the neutron time spectrum of near and far detectors is as follows: in wells with different lithology, porosity, and mud content, the deuterium-deuterium neutron porosity and compensated neutron porosity logging responses of the americium-beryllium neutron source are obtained; the compensated neutron porosity logging of the americium-beryllium neutron source measures the near and far neutron counts, and the deuterium-deuterium neutron porosity logging in pulse mode is used to obtain the neutron time spectrum of near and far detectors.

3. A deuterium-deuterium neutron porosity correction method based on capture cross section according to claim 2, characterized in that: By setting the time window for neutron time spectrum acquisition, the neutron ratio of near and far detectors is obtained, and the neutron porosity is calculated based on the neutron ratio of near and far detectors.

4. The deuterium-deuterium neutron porosity correction method based on capture cross section according to claim 1, characterized in that: The capture cross section is calculated by a sliding time window method, including: setting an initial time window, obtaining an initial sigma value, determining a time window for calculating the capture cross section according to the size of the initial sigma value, and calculating the sigma value based on a single exponential fitting.

5. A deuterium-deuterium neutron porosity correction method based on capture cross section according to claim 4, characterized in that: If the initial sigma is less than 38, the calculation time window is t=816-1258 channels. If the initial sigma is greater than 45, the calculation time window is t=611-683 channels. For other initial sigmas, the calculation time window is t=611-683 channels.

6. The deuterium-deuterium neutron porosity correction method based on capture cross section according to claim 1, characterized in that: The capture cross section is calculated based on the count rate. The count rate formula is: Among them, C i is the gamma count rate of the ith channel, in Cps; t i is the time of the ith channel, in μs; t0 is the initial time for calculating the formation capture cross section; τ F is the gamma decay time constant related to the formation capture cross section, in μs; Σ is the formation capture cross section, in cu; A is the amplitude, and the initial value can be determined by the counting rate corresponding to time t0.

7. The deuterium-deuterium neutron porosity correction method based on capture cross section according to claim 1, characterized in that: The capture cross section calculation formula is: Where, Σ is the formation capture cross section, in units of cu; τ F is the gamma decay time constant related to the formation capture cross section, in μs.

8. The deuterium-deuterium neutron porosity correction method based on capture cross section according to claim 1, characterized in that: The deuterium-deuterium neutron porosity and formation capture cross section in different acquisition time windows are obtained, and the relationship between the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity is compared and analyzed. It is found that the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity are in a linear relationship. Using the linear relationship, a deuterium-deuterium neutron porosity correction method is established.

9. The deuterium-deuterium neutron porosity correction method based on capture cross section according to claim 1, characterized in that: The linear relationship between the capture cross section and the deuterium-deuterium neutron porosity and the compensation neutron porosity is: Porc=PorDD+k*sigma+b Among them, PorDD is the deuterium-deuterium neutron porosity, pu; Porc is the corrected deuterium-deuterium neutron porosity, pu; sigma is the formation capture cross section calculated based on the pulse time spectrum, cu; k and b are correction coefficients, which are determined based on the relationship between the capture cross section and the difference between the deuterium-deuterium neutron porosity and the compensated neutron porosity.

Images