A high-sensitivity neutron porosity measurement method based on DT pulsed neutron source

Abstract

The present invention discloses a high-sensitivity neutron porosity measurement method based on a D-T pulsed neutron source. The present invention uses a four-detector pulsed neutron logging instrument provided with a D-T pulsed neutron source, two neutron detectors and two gamma detectors for measurement, obtains the inelastic gamma count ratio of the near and far gamma detectors and the thermal neutron count ratio of the near and far neutron detectors, calculates the neutron porosity evaluation parameters of the formation, and establishes the relationship between the neutron porosity evaluation parameters and the porosity through a Monte Carlo numerical calculation method, so as to accurately determine the porosity of the formation. The present invention combines the thermal neutron count ratio of the dual neutron detectors with the inelastic gamma count ratio of the dual gamma detectors, uses the neutron porosity evaluation parameters to characterize the formation porosity, eliminates the influence of the formation density on the measurement of the hydrogen index, accurately obtains the formation porosity, and effectively improves the porosity sensitivity of the neutron porosity logging instrument in high-porosity formations.

Description

Technical Field

[0001] The invention belongs to the technical field of mine geophysical logging, and in particular relates to a high-sensitivity neutron porosity measurement method based on a DT pulse neutron source. Background Art

[0002] Formation porosity is one of the most important geological parameters in the process of oil exploration and development. It is of great significance in calculating the oil production capacity of the formation and identifying gas layers. Compensated neutron porosity logging is the most commonly used formation porosity measurement method. The Am-Be neutron source it uses has a series of problems in terms of safety, health and environment. In order to meet the needs of new logging methods, with the development of neutron sources, the Am-Be neutron source has gradually been replaced by the DT pulsed neutron source. The average energy of neutrons emitted by the DT pulsed neutron source is 14MeV, which is significantly higher than the average energy of neutrons generated by the Am-Be source. However, due to the significant influence of formation density on the measurement of hydrogen index, the porosity sensitivity of the neutron porosity logging instrument is reduced when the DT pulsed neutron source is used to measure formation porosity.

[0003] In previous studies, using the thermal neutron count ratio of the detector for density correction can improve the sensitivity of formation porosity measurement. By establishing the relationship between the thermal neutron count ratio, formation density and hydrogen index, the influence of formation density on the sensitivity of porosity measurement can be eliminated. However, this method is extremely dependent on the density information of the formation and the formation density curve must be used as an input parameter, so this method cannot be applied to wells lacking open hole data. Therefore, it is urgent to propose a high-sensitivity neutron porosity measurement method using a DT pulsed neutron source to eliminate the influence of formation density on hydrogen index measurement and improve the porosity sensitivity of neutron porosity logging instruments. Summary of the invention

[0004] In view of the above-mentioned shortcomings, the present invention proposes a high-sensitivity neutron porosity measurement method based on a DT pulsed neutron source, a measurement system based on a DT pulsed neutron source, a dual neutron detector and a dual gamma detector, and utilizes the thermal neutron count ratio of near and far neutron detectors and the inelastic gamma count ratio of near and far gamma detectors to characterize the formation porosity, thereby effectively eliminating the influence of formation density on the measurement of formation hydrogen index, achieving accurate acquisition of formation porosity, and improving the porosity sensitivity of the neutron porosity logging instrument in high-porosity formations.

[0005] In order to achieve the above object, the present invention adopts the following technical solution:

[0006] A high-sensitivity neutron porosity measurement method based on a DT pulsed neutron source is used for measurement using a four-detector pulsed neutron logging instrument. The four-detector pulsed neutron logging instrument is provided with a measurement system consisting of a DT pulsed neutron source, two neutron detectors and two gamma detectors. The method specifically includes the following steps:

[0007] Step 1: Place a four-detector pulsed neutron logging instrument in the well for measurement close to the well wall. The DT pulsed neutron source emits fast neutrons in the form of pulses, and records the inelastic gamma counts of the near gamma detector, the inelastic gamma counts of the far gamma detector, the thermal neutron counts of the near neutron detector, and the thermal neutron counts of the far neutron detector.

[0008] Step 2: Determine the neutron porosity evaluation parameter R of the formation based on the thermal neutron count ratio of the near neutron detector and the far neutron detector, combined with the inelastic gamma count ratio of the near gamma detector and the far gamma detector. ti for:

[0009]

[0010] In the formula, R th is the thermal neutron count ratio of the near neutron detector to the far neutron detector, R in is the ratio of inelastic gamma counts of the near gamma detector to the far gamma detector;

[0011] Step 3: Based on the Monte Carlo numerical calculation method, an MCNP numerical calculation model is established according to the four-detector pulsed neutron logging instrument. The formation porosity of the MCNP numerical calculation model is changed, and the inelastic gamma counts of the near gamma detector, the inelastic gamma counts of the far gamma detector, the thermal neutron counts of the near neutron detector and the thermal neutron counts of the far neutron detector under different formation porosity conditions are simulated. The neutron porosity evaluation parameters corresponding to different formation porosities are calculated, and the relationship between the neutron porosity evaluation parameters and the formation porosity is established as follows:

[0012] φ=AR ti 2 +BR ti +C (2)

[0013] Where φ is the formation porosity, R ti is the neutron porosity evaluation parameter, A, B, and C are calibration coefficients;

[0014] Step 4: Based on the relationship between the neutron porosity evaluation parameter and the formation porosity, the neutron porosity evaluation parameter of the formation is used to obtain accurate formation porosity.

[0015] Preferably, the instrument diameter of the four-detector pulsed neutron logging instrument is set to 89 mm, the source distances of the near gamma detector and the far gamma detector are set to 31.25 cm and 70 cm respectively, and the source distances of the near neutron detector and the far neutron detector are set to 31.25 cm and 57.5 cm respectively.

[0016] Preferably, a tungsten-nickel-iron shield is provided between the DT pulse neutron source and the near-neutron detector.

[0017] Preferably, in step three, the well diameter in the MCNP numerical calculation model is set to 200 mm, the formation lithology is set to limestone, and the formation porosity in the MCNP numerical calculation model varies in the range of 0 to 100%.

[0018] Beneficial technical effects brought by the present invention:

[0019] The present invention is based on a four-detector pulsed neutron logging instrument, and adopts a measurement system consisting of a DT pulsed neutron source, two neutron detectors and two gamma detectors to simultaneously obtain thermal neutron counts of near and far neutron detectors and inelastic gamma counts of near and far gamma detectors, and calculates the neutron porosity evaluation parameters of the formation by using the thermal neutron count ratio of the near and far neutron detectors and the inelastic gamma count ratio of the near and far gamma detectors to determine the formation porosity, thereby achieving accurate measurement of the formation porosity.

[0020] The present invention determines the formation porosity by combining the thermal neutron count ratio and the inelastic gamma count ratio of a four-detector pulsed neutron logging instrument. The influence of the formation density on the measurement result of the formation hydrogen index can be eliminated without relying on the density information of the formation. The porosity sensitivity of the neutron porosity logging instrument in high-porosity formations is effectively improved, laying a foundation for the exploration and development of oil and gas resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The schematic diagram of the structure of the four-detector pulsed neutron logging instrument is shown in the figure. In the figure, 1, far gamma detector, 2, far neutron detector, 3, near neutron detector, 4, near gamma detector, 5, tungsten nickel iron shield, 6, DT pulsed neutron source, 7, wellbore, 8, formation.

[0022] Figure 2 The response law of thermal neutron count ratio of four-detector pulsed neutron logging instrument under different formation conditions. In the figure, the formation condition corresponding to the change of hydrogen index curve with fixed formation density is 2.19g / cm 3 , change the hydrogen index of the formation, the range of the hydrogen index is 0-1; the formation conditions corresponding to the formation density curve with a fixed hydrogen index of 0.3, change the formation density, the range of the formation density is 1-2.708g / cm 3; At the same time, the formation conditions corresponding to the formation density and hydrogen index curve are changed to change the formation density range to 1-2.708g / cm 3 , corresponding to the change in the hydrogen index of the formation, the range of the hydrogen index is 0 to 1.

[0023] Figure 3 The response law of the inelastic gamma count ratio of the four-detector pulsed neutron logging tool under different formation conditions. In the figure, the formation condition corresponding to the change of hydrogen index curve with fixed formation density is 2.19g / cm 3 , change the hydrogen index of the formation, the range of the hydrogen index is 0-1; the formation conditions corresponding to the formation density curve with a fixed hydrogen index of 0.3, change the formation density, the range of the formation density is 1-2.708g / cm 3 ; At the same time, the formation conditions corresponding to the formation density and hydrogen index curve are changed to change the formation density range to 1-2.708g / cm 3 , corresponding to the change in the hydrogen index of the formation, the range of the hydrogen index is 0 to 1.

[0024] Figure 4 The response law of thermal neutron counting ratio and neutron porosity evaluation parameters under different formation porosity conditions.

[0025] Figure 5 This is a comparison chart of the relative sensitivity of using thermal neutron counting ratio and neutron porosity evaluation parameters to characterize porosity under different formation porosity conditions. DETAILED DESCRIPTION

[0026] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0027] After the fast neutrons emitted by the DT pulsed neutron source enter the formation, they react with the nuclei of the formation material through inelastic scattering, elastic scattering and radiation capture, and the neutron energy is reduced to become epithermal neutrons and thermal neutrons. The distribution of thermal neutron flux in the formation depends on the distance r that the neutrons move and the deceleration length L of the formation medium. S, the influence of formation density on the distribution of thermal neutrons cannot be ignored. Therefore, the distribution of thermal neutrons in the formation is jointly affected by the hydrogen index and formation density. When the formation porosity is small, the distribution range of thermal neutrons in the formation is very wide, but with the increase of formation porosity, the formation's hydrogen index increases, and the deceleration ability of fast neutrons increases, causing the distribution range of thermal neutrons in the formation to gradually decrease. The thermal neutron count rate recorded by the detector of the neutron porosity logging instrument decreases, among which the thermal neutron count rate of the far detector decreases faster than that of the near detector. Therefore, it is manifested that the thermal neutron count ratio of the near and far detectors increases with the increase of formation porosity. At the same time, the formation density decreases with the increase of porosity, and the deceleration and absorption effect of the formation skeleton on neutrons gradually decreases, which slows down the speed of the detector count rate decrease, making the sensitivity of the thermal neutron count ratio to porosity decrease. In order to improve the porosity sensitivity of neutron porosity logging tools, it is necessary to consider the role of formation density in neutron transport and perform density compensation on thermal neutron distribution to eliminate the influence of formation density on porosity characterization.

[0028] According to the double-group diffusion theory, in an infinite homogeneous medium, the thermal neutron counts recorded by two neutron detectors with source distances r1 and r2 are equal to R th It can be roughly described as:

[0029]

[0030] In the formula, is the thermal neutron count rate near the neutron detector, is the thermal neutron count rate of the far neutron detector, r1 is the source distance of the near neutron detector, r2 is the source distance of the far neutron detector, L S is the deceleration length of the formation medium.

[0031] For Am-Be neutron source, the hydrogen index of the formation is the main reason that determines the distribution of thermal neutrons in the formation. Therefore, when using Am-Be neutron source for measurement, the relationship between the detector thermal neutron count ratio and the formation hydrogen index can be directly used. However, when using DT pulsed neutron source for measurement, the use of the detector thermal neutron count ratio to characterize the hydrogen index is affected by the formation density. Therefore, it is inappropriate to use only the thermal neutron count ratio to characterize the hydrogen index of the formation. Therefore, when neutron porosity logging is measured using DT pulsed neutron source, it is necessary to use a combination of thermal neutron count ratio and formation density to determine the hydrogen index of the formation.

[0032] According to the neutron-gamma coupling field theory, the inelastic scattered gamma-ray flux Φ(r) at the neutron moving distance r is:

[0033]

[0034] In the formula, Σ inis the macroscopic inelastic scattering cross section of the formation, S0 is the neutron source intensity, λ s is the fast neutron scattering free path, ρ is the formation density, μ m is the mass attenuation coefficient, and i is the number of gamma photons emitted by the inelastic scattering of a single fast neutron with formation elements.

[0035] According to the Lagrange mean value theorem, formula (4) is transformed to obtain:

[0036]

[0037] Among them, ξ is the m ,1 / λ s ) as shown in formula (6):

[0038] ξ=(1-α)(1 / λ s )+αρμ m α∈(0,1) (6)

[0039] Then the inelastic gamma count ratio R of the near gamma detector (source distance is r3) and the far gamma detector (source distance is r4) is in It is expressed as:

[0040]

[0041] Therefore, when the source distance is fixed, the ratio of the inelastic gamma counts of the near and far gamma detectors is R in It is only related to the formation density, and the inelastic gamma count ratio can be directly used to characterize the formation density. Therefore, in order to eliminate the influence of formation density on formation porosity measurement and improve the sensitivity of the instrument to porosity, the thermal neutron count ratio and the inelastic gamma count ratio are combined to obtain a new porosity characterization parameter, the neutron porosity evaluation parameter R ti .

[0042] Based on the above principle, the present invention proposes a high-sensitivity neutron porosity measurement method based on a DT pulsed neutron source, which uses a four-detector pulsed neutron logging instrument for measurement. The four-detector pulsed neutron logging instrument is provided with a measurement system consisting of a DT pulsed neutron source, two neutron detectors and two gamma detectors, and specifically includes the following steps:

[0043] Step 1: Place a four-detector pulsed neutron logging instrument in the wellbore 7, and measure the wellbore wall by the instrument. Figure 1As shown, the diameter of the four-detector pulse neutron logging instrument is set to 89 mm, and a measurement system consisting of a DT pulse neutron source 6, two neutron detectors and two gamma detectors is arranged inside the instrument, wherein the neutron detector and the gamma detector are both cylindrical structures, the diameter of the near neutron detector 3 is 4 cm and the length is 8 cm, the diameter of the far neutron detector 2 is 6 cm and the length is 9 cm, the diameter of the near gamma detector 4 is 4 cm and the length is 5 cm, the diameter of the far gamma detector 1 is 6 cm and the length is 9 cm, the source distances between the near gamma detector 4 and the near neutron detector 3 are both set to 31.25 cm, the source distance of the far gamma detector 1 is set to 70 cm, and the source distance of the far neutron detector 2 is set to 57.5 cm; a tungsten-nickel-iron shield 5 with a cylindrical structure is arranged between the DT pulse neutron source 6 and the near neutron detector 3, and the diameter of the tungsten-nickel-iron shield 5 is 89 mm and the length is 10 cm.

[0044] The DT pulsed neutron source of the four-detector pulsed neutron logging instrument is used to emit fast neutrons into the formation 8 in the form of pulses, and the inelastic gamma counts of the near gamma detector, the inelastic gamma counts of the far gamma detector, the thermal neutron counts of the near neutron detector and the thermal neutron counts of the far neutron detector are recorded.

[0045] Step 2: Determine the thermal neutron count ratio based on the thermal neutron counts of the near neutron detector and the thermal neutron counts of the far neutron detector, and then determine the inelastic gamma count ratio using the inelastic gamma counts of the near gamma detector and the inelastic gamma counts of the far gamma detector. Combine the thermal neutron count ratio with the inelastic gamma count ratio to determine the neutron porosity evaluation parameter R of the formation. ti for:

[0046]

[0047] In the formula, R th is the thermal neutron count ratio of the near neutron detector to the far neutron detector, R in is the ratio of the inelastic gamma counts of the near gamma detector to the far gamma detector.

[0048] Step three, based on the Monte Carlo numerical calculation method, an MCNP numerical calculation model is established according to the instrument structure of the four-detector pulsed neutron logging instrument, the formation porosity of the MCNP numerical calculation model is changed, the formation porosity change range is set to 0-100%, and the formation porosity value is increased by 10% each time for simulation, and the inelastic gamma counts of the near gamma detector, the inelastic gamma counts of the far gamma detector, the thermal neutron counts of the near neutron detector and the thermal neutron counts of the far neutron detector under different formation porosity conditions are obtained, and the neutron porosity evaluation parameters corresponding to different formation porosities are calculated.

[0049] According to different formation porosities and their corresponding neutron porosity evaluation parameters, the relationship between the neutron porosity evaluation parameters and the formation porosity is established by fitting the formation porosity and the neutron porosity evaluation parameters:

[0050] φ=-0.01385R ti 2 +0.62636R ti -0.20035 (8)

[0051] Where φ is the formation porosity, R ti is the neutron porosity evaluation parameter.

[0052] Step 4: Based on the relationship between the neutron porosity evaluation parameter and the formation porosity, the neutron porosity evaluation parameter of the formation is used to obtain accurate formation porosity.

[0053] As an example, the improvement of the porosity sensitivity of the DT pulsed neutron source neutron porosity logging instrument by the method of the present invention is better verified. The response laws of the thermal neutron count ratio and the inelastic gamma count ratio of the four-detector pulsed neutron logging instrument under different formation conditions are simulated by the MCNP numerical calculation model. Figure 2 and Figure 3 As shown, through analysis Figure 2 and Figure 3 The response law in the results shows that both the thermal neutron count ratio and the inelastic gamma count ratio are affected by the formation hydrogen index and formation density. As the formation porosity increases, the change in the hydrogen index makes the neutron porosity evaluation parameter R ti (i.e. the ratio of thermal neutron count ratio to inelastic gamma count ratio) increases, and the change in formation density makes the neutron porosity evaluation parameter R ti However, when the formation porosity is large, the thermal neutron count ratio is more affected by the formation density, which reduces the sensitivity of the thermal neutron count ratio to the formation porosity. The non-elastic gamma count ratio mainly reflects the change of formation density and can be used to eliminate the influence of formation density on the thermal neutron count ratio of the four-detector pulsed neutron logging tool.

[0054] The response rules of thermal neutron count ratio and neutron porosity evaluation parameter (ratio of thermal neutron count ratio to inelastic gamma count ratio) under different formation porosity conditions are compared, such as Figure 4 As shown, the relative sensitivity of the neutron porosity logging tool under different formation porosity conditions when using thermal neutron counting ratio and neutron porosity evaluation parameters to characterize porosity is calculated, as shown in Figure 5 As shown, the relative sensitivity calculation formula is shown in formula (9):

[0055]

[0056] Where S is the relative sensitivity of the logging tool to porosity.

[0057] By analyzing Figure 4 and Figure 5 It can be obtained that the thermal neutron count ratio and neutron porosity evaluation parameters of the four-detector pulsed neutron logging instrument both decrease with the increase of formation porosity. When the formation porosity is lower than 12%, the sensitivity of the thermal neutron count ratio to porosity is higher than the sensitivity of the neutron porosity evaluation parameter to porosity, and when the formation porosity is higher than 12%, the porosity sensitivity of the neutron porosity evaluation parameter is significantly higher than the porosity sensitivity of the thermal neutron count ratio. When the formation porosity is 35%, the porosity sensitivity of the neutron porosity evaluation parameter is 1.98 times the porosity sensitivity of the thermal neutron count ratio, and the greater the formation porosity, the more significant the improvement of the porosity sensitivity of the neutron porosity evaluation parameter, which proves that the method of the present invention can effectively improve the sensitivity of the neutron porosity logging instrument in high-porosity formations.

[0058] 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 high-sensitivity neutron porosity measurement method based on a DT pulsed neutron source, characterized in that: The measurement is performed using a four-detector pulsed neutron logging instrument, which is equipped with a measurement system consisting of a DT pulsed neutron source, two neutron detectors and two gamma detectors, and specifically includes the following steps: Step 1: Place a four-detector pulsed neutron logging instrument in the well for measurement close to the well wall. The DT pulsed neutron source emits fast neutrons in the form of pulses, and records the inelastic gamma counts of the near gamma detector, the inelastic gamma counts of the far gamma detector, the thermal neutron counts of the near neutron detector, and the thermal neutron counts of the far neutron detector. Step 2: Determine the neutron porosity evaluation parameter R of the formation based on the thermal neutron count ratio of the near neutron detector and the far neutron detector, combined with the inelastic gamma count ratio of the near gamma detector and the far gamma detector. ti for: In the formula, R th is the thermal neutron count ratio of the near neutron detector to the far neutron detector, R in is the ratio of inelastic gamma counts of the near gamma detector to the far gamma detector; Step 3: Based on the Monte Carlo numerical calculation method, an MCNP numerical calculation model is established according to the four-detector pulsed neutron logging instrument. The formation porosity of the MCNP numerical calculation model is changed, and the inelastic gamma counts of the near gamma detector, the inelastic gamma counts of the far gamma detector, the thermal neutron counts of the near neutron detector and the thermal neutron counts of the far neutron detector under different formation porosity conditions are simulated. The neutron porosity evaluation parameters corresponding to different formation porosities are calculated, and the relationship between the neutron porosity evaluation parameters and the formation porosity is established as follows: φ=AR ti 2 +BR ti +C (2) Where φ is the formation porosity, R ti is the neutron porosity evaluation parameter, A, B, and C are calibration coefficients; Step 4: Based on the relationship between the neutron porosity evaluation parameter and the formation porosity, the neutron porosity evaluation parameter of the formation is used to obtain accurate formation porosity.

2. The high-sensitivity neutron porosity measurement method based on a DT pulsed neutron source according to claim 1, characterized in that: The instrument diameter of the four-detector pulsed neutron logging instrument is set to 89 mm, the source distances of the near gamma detector and the far gamma detector are set to 31.25 cm and 70 cm respectively, and the source distances of the near neutron detector and the far neutron detector are set to 31.25 cm and 57.5 cm respectively.

3. The high-sensitivity neutron porosity measurement method based on a DT pulsed neutron source according to claim 2, characterized in that: A tungsten-nickel-iron shield is arranged between the DT pulse neutron source and the near-neutron detector.

4. The high-sensitivity neutron porosity measurement method based on a DT pulsed neutron source according to claim 1, characterized in that: In the step 3, the well diameter in the MCNP numerical calculation model is set to 200 mm, the formation lithology is set to limestone, and the formation porosity in the MCNP numerical calculation model varies in the range of 0 to 100%.

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