Method and system for calculating stratum density based on mass attenuation coefficient function, terminal and medium

Abstract

The invention discloses a formation density calculation method and system based on a mass attenuation coefficient function, a terminal and a medium, and belongs to the technical field of logging, specifically, a formation density calculation model is constructed based on a pulsed neutron gamma density logging instrument; establishing a scale coefficient library including scale coefficients corresponding to different lithologic stratums through numerical simulation; identifying the lithology of the formation to be measured, and matching in the scale coefficient library to obtain a scale coefficient corresponding to the lithology of the formation to be measured; and measuring the stratum to be measured through the pulsed neutron gamma density logger, and calculating the stratum density according to the stratum density calculation model and the scale coefficient. According to the method, the densities of different lithologic stratums can be calculated more accurately, the requirements of an actual measurement well are met, and the absolute error is controlled within a reasonable range.

Description

Technical Field

[0001] The invention belongs to the technical field of well logging, and in particular relates to a method, system, terminal and medium for calculating formation density based on a mass attenuation coefficient function. Background Art

[0002] In the field of oil exploration, traditional radioactive chemical source density logging (Gamma Gamma Density, GGD) technology has long occupied an important position. With the increasing awareness of environmental protection, the potential environmental pollution risks and operational safety hazards of GGD technology have gradually become prominent, becoming a key factor restricting its further development. Neutron Gamma Density (NGD) technology has gradually become a new focus of research and application with its advantages of no radioactive pollution and high measurement accuracy. The core of NGD technology is to use the secondary gamma rays generated by the complex interaction between high-energy fast neutrons emitted by a pulsed neutron source and formation materials to achieve the measurement of formation density.

[0003] NGD technology involves two important aspects: neutron field distribution and inelastic gamma field distribution. Neutron field distribution is closely related to the transport process of neutrons in the formation. When the pulsed neutron source emits fast neutrons with an energy of up to 14MeV and enters the formation, these neutrons will interact with various elements in the formation in three main types: inelastic scattering, elastic scattering and capture reaction. Among them, inelastic scattering and capture reaction are particularly important because they can produce secondary gamma rays. Compared with captured gamma rays, inelastic gamma rays can more directly reflect the information of neutron-formation interaction and are less affected by neutron transport, so they are more suitable for density calculation. The generation of inelastic gamma rays is closely related to the type and content of formation elements. When gamma rays pass through the formation, they will gradually attenuate due to absorption and scattering by the formation material. This attenuation process is also restricted by many factors such as formation lithology, porosity and pore fluid properties. These complex physical processes and formation factors constitute the main challenges faced by NGD technology.

[0004] At present, when measuring formation density, people generally tend to use inelastic gamma rays, which has been widely used in the industry. However, in the correction process of neutron transport, the physical quantities selected by different methods are very different, including captured gamma, thermal neutrons, and epithermal neutrons. Most of these methods focus on the detailed analysis and expression of the neutron transport process, but ignore the complexity of the gamma ray attenuation process. For example, GGD technology mainly focuses on the single physical phenomenon of Compton scattering, but the secondary gamma rays produced by the pulsed neutron source have high energy and are discrete, which makes the gamma attenuation process more complicated, and it is difficult to accurately describe the complex mechanism of gamma rays in different lithological formations by conventional means. The current NGD technology often regards the mass attenuation coefficient as a constant. This simplified processing method ignores the important influence of gamma attenuation on the measurement results, resulting in limited measurement accuracy.

[0005] Therefore, in order to improve the accuracy of formation density measurements, future research needs to pay more attention to the complexity of the gamma attenuation process and explore more sophisticated modeling and correction methods. Summary of the invention

[0006] In view of the important influence of gamma attenuation on the measurement results is ignored in the existing formation density measurement, the present invention provides a method, system, terminal and medium for calculating formation density based on the mass attenuation coefficient function, and constructs a new formation density calculation model by utilizing the dynamic change of the mass attenuation coefficient under different lithology formations, thereby improving the accuracy of the formation density calculation results in pulse neutron logging.

[0007] In order to achieve the above purpose, the technical method adopted by the present invention is as follows:

[0008] The method for calculating formation density based on the mass attenuation coefficient function includes the following steps:

[0009] Step 1: Based on the pulsed neutron gamma density logging tool, a formation density calculation model is constructed, and the formula is:

[0010]

[0011] Where ρ is the formation density; R inγ is the ratio of the inelastic scattering gamma counts at near-source distance to the inelastic scattering gamma counts at far-source distance; R ETN is the ratio of epithermal neutron counts at near-source distance to epithermal neutron counts at far-source distance; N lith is the count within the specific energy window of the near-source inelastic gamma ray; A, a0, a1, a2, b, c and d are all scale factors; the denominator That is the mass attenuation coefficient function;

[0012] Step 2: Establish a calibration coefficient library through numerical simulation, including calibration coefficients A, a0, a1, a2, b, c and d corresponding to different lithology formations;

[0013] Step 3, identifying the lithology of the formation to be measured, and matching the calibration coefficients A, a0, a1, a2, b, c and d corresponding to the lithology of the formation to be measured in the calibration coefficient library;

[0014] Step 4: The pulsed neutron gamma density logging instrument is used to measure the near-source epithermal neutron count, far-source epithermal neutron count, near-source inelastic scattering gamma count, far-source inelastic scattering gamma count and near-source inelastic gamma count within the specific energy window of the formation to be measured, and then the density ρ of the formation to be measured is calculated according to the formation density calculation model and the calibration coefficients A, a0, a1, a2, b, c and d corresponding to the lithology of the formation to be measured.

[0015] Furthermore, the pulsed neutron gamma density logging instrument comprises a neutron source 1, a near-source neutron detector group, a near-gamma detector 4, a far-source neutron detector group and a far-gamma detector 7 arranged in sequence from bottom to top; wherein the near-source neutron detector group comprises a near-epitothermal neutron detector 2 and a near-thermal neutron detector 3 arranged side by side, and the far-source neutron detector group comprises a far-epitothermal neutron detector 5 and a far-thermal neutron detector 6 arranged side by side, the near-epitothermal neutron detector 2 is opposite to the far-epitothermal neutron detector 5, and the near-thermal neutron detector 3 is opposite to the far-thermal neutron detector 6.

[0016] Furthermore, the calibration coefficients corresponding to different lithology formations in step 2 are obtained by numerically simulating formations with different porosities under each lithology, and the simulated near-source epithermal neutron counts, far-source epithermal neutron counts, near-source inelastic scattering gamma counts, far-source inelastic scattering gamma counts and near-source inelastic gamma counts within the specific energy window are brought into the formation density calculation model and fitted.

[0017] Furthermore, the specific process of step 2 is:

[0018] Step 2.1, establishing a pulsed neutron gamma density logging tool model identical to the pulsed neutron gamma density logging tool in step 1 in a numerical simulation software;

[0019] Step 2.2, selecting formation models of different lithologies, constructing formations of different porosities under different lithologies by adding water in different proportions, and obtaining simulated formation density values ​​of formations of different porosities under different lithologies;

[0020] Step 2.3, using the pulsed neutron gamma density logging model, obtain the near-source epithermal neutron counts, far-source epithermal neutron counts, near-source inelastic scattering gamma counts, far-source inelastic scattering gamma counts and near-source inelastic gamma counts within the specific energy window of different porosity formations under each lithology, bring them into the formation density calculation model, and fit the corresponding calibration coefficients A, a0, a1, a2, b, c and d for each lithology, and then establish a calibration coefficient library.

[0021] Furthermore, each lithology in step 2.2 includes at least 7 strata with different porosities.

[0022] Furthermore, the lithology of the formation to be measured is identified based on the detector response change or element spectrum analysis.

[0023] Furthermore, the mass attenuation coefficient is a function of the macroscopic atomic number of the formation, and the near-source inelastic gamma counts within a specific energy window show a good quadratic function relationship with the macroscopic atomic number of the formation. Therefore, by measuring the near-source inelastic gamma counts within a specific energy window, the variation law of the mass attenuation coefficient of the formation can be effectively characterized.

[0024] Furthermore, the near-source distance inelastic gamma specific energy window counts N lith The specific energy window of MgO is in the range of 0.07 to 0.35 MeV.

[0025] Furthermore, the numerical simulation is specifically a Monte Carlo numerical simulation.

[0026] Further, the different lithologies include limestone, dolomite and sandstone.

[0027] The system for calculating formation density based on the mass attenuation coefficient function is used to implement the method for calculating formation density based on the mass attenuation coefficient function, and specifically includes a calibration coefficient library construction module, a formation lithology identification module, a calibration coefficient matching module, a pulsed neutron gamma density logging instrument and a data processing module; wherein:

[0028] The scale coefficient library construction module is used to determine the scale coefficients A, a0, a1, a2, b, c and d corresponding to different lithological formations through numerical simulation according to the configured formation density calculation model, establish a scale coefficient library, and input it into the scale coefficient matching module;

[0029] The formation lithology identification module is used to identify the formation lithology to be measured and input the lithology identification result into the calibration coefficient matching module;

[0030] The scale coefficient matching module is used to determine the scale coefficients A, a0, a1, a2, b, c and d corresponding to the lithology of the formation to be measured, and input them into the data processing module;

[0031] The pulsed neutron gamma density logging instrument is used to measure the near-source epithermal neutron count, far-source epithermal neutron count, near-source inelastic scattering gamma count, far-source inelastic scattering gamma count and near-source inelastic gamma specific energy window count of the formation to be measured, and input them into the data processing module;

[0032] The data processing module is used to calculate the density ρ of the formation to be measured according to the configured formation density calculation model.

[0033] A terminal comprises a memory, a processor and a computer program stored in the memory, wherein the processor executes the computer program to implement the steps of the method for calculating formation density based on the mass attenuation coefficient function.

[0034] A computer-readable storage medium stores a computer program, which, when executed by a processor, implements the steps of the method for calculating formation density based on a mass attenuation coefficient function.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] The present invention proposes a method, system, terminal and medium for calculating formation density based on a mass attenuation coefficient function. By constructing a formation density calculation model based on the mass attenuation coefficient function, the density of formations of different lithologies can be calculated more accurately, which can meet the needs of actual well logging, and the absolute error is controlled within a reasonable range; wherein, the ratio of near-source inelastic scattering gamma counts to far-source inelastic scattering gamma counts is used as the main variable, which can effectively reduce environmental impact and reduce the interference of formation hydrogen index; the ratio of near-source epithermal neutron counts to far-source epithermal neutron counts is used as the correction term, which can reduce the influence of formation hydrogen index; constructing a mass attenuation coefficient function based on near-source inelastic gamma counts within a specific energy window can more dynamically reflect the real physical characteristics of the formation, characterize the complexity of gamma attenuation in formations of different lithologies, and thus improve the accuracy of the calculation results under different lithologies;

[0037] The present invention adopts the counting rate ratio instead of the absolute counting rate, which can reduce the error caused by the unstable yield of the controllable neutron source. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0039] Figure 1 is a schematic diagram of the structure of the pulsed neutron gamma density logging tool proposed in Example 1;

[0040] Figure 2 is a graph showing the relationship between the simulated formation density value and the calculated formation density value in Example 1;

[0041] Figure 3 is a graph of absolute errors between the simulated formation density value and the calculated formation density value in Example 1;

[0042] The reference numerals are as follows:

[0043] 1- neutron source; 2- near epithermal neutron detector; 3- near thermal neutron detector; 4- near gamma detector; 5- near epithermal neutron detector; 6- far thermal neutron detector; 7- far gamma detector; 8- wellbore; 9- formation. DETAILED DESCRIPTION

[0044] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the invention.

[0045] Example 1

[0046] This embodiment proposes a method for calculating formation density based on a mass attenuation coefficient function, comprising the following steps:

[0047] Step 1: Design Figure 1 The pulsed neutron gamma density logging instrument shown is placed in a wellbore 8 of a formation 9, and specifically includes a neutron source 1, a near-source neutron detector group, a near-gamma detector 4, a far-source neutron detector group and a far-gamma detector 7 arranged in sequence from bottom to top; wherein the near-source neutron detector group includes a near-epi-thermal neutron detector 2 and a near-thermal neutron detector 3 arranged side by side, and the far-source neutron detector group includes a far-epi-thermal neutron detector 5 and a far-thermal neutron detector 6 arranged side by side, the near-epi-thermal neutron detector 2 is opposite to the far-epi-thermal neutron detector 5, and the near-thermal neutron detector 3 is opposite to the far-thermal neutron detector 6.

[0048] Step 2: Based on the pulsed neutron gamma density logging tool, a formation density calculation model is constructed, and the formula is:

[0049]

[0050] Where ρ is the formation density; R inγ is the ratio of the inelastic scattering gamma counts at near-source distance to the inelastic scattering gamma counts at far-source distance; R ETN is the ratio of epithermal neutron counts at near-source distance to epithermal neutron counts at far-source distance; N lithThe specific energy window for the near-source inelastic gamma is 0.07 to 0.35 MeV. A, a0, a1, a2, b, c and d are all scale factors. The denominator is This is the mass attenuation coefficient function.

[0051] Step 2: Through Monte Carlo numerical simulation, a calibration coefficient library is established, which contains the calibration coefficients A, a0, a1, a2, b, c and d corresponding to different lithology formations. The specific process is as follows:

[0052] Step 2.1, in order to better obtain the fitting parameters, a pulsed neutron gamma density logging instrument model identical to that of the pulsed neutron gamma density logging instrument in step 1 is established in the Monte Carlo numerical simulation software, wherein the neutron source 1 is a surface source, the emission period is 35 μs, the emission time is set at 0-10 μs, and the emission is isotropically directed to all directions;

[0053] Step 2.2, selecting formation models of different lithologies, constructing formations of different porosities under different lithologies by adding water in different proportions, and obtaining simulated formation density values ​​of formations of different porosities under different lithologies;

[0054] This example takes the formation model of three lithologies, limestone, dolomite and sandstone, as an example. By adding water in different proportions, different porosity formations under each lithology are constructed. Each lithology includes 7 different porosity formations. The simulated formation density values ​​of different porosity formations under each lithology are obtained. The selected density range is 2.040 g / cm 3 ~2.841g / cm 3 , as shown in Table 1:

[0055] Table 1

[0056]

[0057] Step 2.3, using the pulsed neutron gamma density logging tool model, obtain the near-source epithermal neutron counts, far-source epithermal neutron counts, near-source inelastic scattering gamma counts, far-source inelastic scattering gamma counts and near-source inelastic gamma specific energy window counts of formations with different porosities under various lithologies, bring them into the formation density calculation model, fit and obtain the calibration coefficients A, a0, a1, a2, b, c and d corresponding to various lithologies, and then establish a calibration coefficient library;

[0058] In the Monte Carlo numerical simulation, the inelastic gamma and captured gamma counts are obtained in the way of physical process, and the epithermal neutron counts are obtained in the way of energy cutoff and region screening.

[0059] Step 3: Identify the lithology of the formation to be measured according to the detector response change or element spectrum analysis, and match the calibration coefficients A, a0, a1, a2, b, c and d corresponding to the lithology of the formation to be measured in the calibration coefficient library;

[0060] Step 4: The pulsed neutron gamma density logging instrument of step 1 is used to measure the near-source epithermal neutron count, far-source epithermal neutron count, near-source inelastic scattering gamma count, far-source inelastic scattering gamma count and near-source inelastic gamma count within the specific energy window of the formation to be measured, and then the density ρ of the formation to be measured is calculated according to the formation density calculation model and the calibration coefficients A, a0, a1, a2, b, c and d corresponding to the lithology of the formation to be measured.

[0061] In this embodiment, based on the formation models of three lithologies, namely limestone, dolomite and sandstone, a new set of simulated formation density value data of formations with different porosities under each lithology is constructed in the Monte Carlo numerical simulation software, as shown in Table 2:

[0062] Table 2

[0063]

[0064] By using the method in step 4, the density of strata with different porosities under various lithologies is calculated and recorded as the calculated stratum density value. The following is plotted: Figure 2 The relationship between the simulated formation density value and the calculated formation density value is shown in the figure. It can be seen that the data points are evenly distributed near the straight line where the simulated formation density value is equal to the calculated formation density value, and the fitting linearity is as high as 0.9978. Figure 3 The absolute error diagram of the simulated formation density value and the calculated formation density value shows that the absolute error is less than 0.02g / cm 3 , indicating that the method for calculating formation density based on the mass attenuation coefficient function proposed in this embodiment has excellent formation density calculation accuracy under different lithologies and can meet the needs of actual well logging.

[0065] The above embodiments are intended to provide a better understanding of the present invention, but are not limited to the optimal implementation mode and do not constitute a limitation on the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the present invention with the features of other prior arts shall be within the protection scope of the present invention.

Claims

1. A method for calculating formation density based on a mass attenuation coefficient function, characterized in that: The following steps are involved: Step 1: Based on the pulsed neutron gamma density logging tool, a formation density calculation model is constructed, and the formula is: Where ρ is the formation density; R inγ is the ratio of the inelastic scattering gamma counts at near-source distance to the inelastic scattering gamma counts at far-source distance; R ETN is the ratio of epithermal neutron counts at near-source distance to epithermal neutron counts at far-source distance; N lith is the count within the specific energy window of the near-source inelastic gamma ray; A, a0, a1, a2, b, c and d are all scale factors; the denominator This is the mass attenuation coefficient function; Step 2: Establish a calibration coefficient library through numerical simulation, which contains calibration coefficients corresponding to different lithology formations; Step 3, identifying the lithology of the formation to be measured, and matching the calibration coefficient corresponding to the lithology of the formation to be measured in the calibration coefficient library; Step 4: Using a pulsed neutron gamma density logging instrument, the near-source epithermal neutron count, far-source epithermal neutron count, near-source inelastic scattering gamma count, far-source inelastic scattering gamma count and near-source inelastic gamma count within the specific energy window of the formation to be measured are measured, and then the density ρ of the formation to be measured is calculated based on the formation density calculation model and the calibration coefficient corresponding to the lithology of the formation to be measured.

2. The method for calculating formation density based on mass attenuation coefficient function according to claim 1, characterized in that: The pulsed neutron gamma density logging instrument comprises a neutron source, a near-source neutron detector group, a near-gamma detector, a far-source neutron detector group and a far-gamma detector arranged in sequence from bottom to top; wherein the near-source neutron detector group comprises a near-epi-thermal neutron detector and a near-thermal neutron detector arranged side by side, the far-source neutron detector group comprises a far-epi-thermal neutron detector and a far-thermal neutron detector arranged side by side, the near-epi-thermal neutron detector is opposite to the far-epi-thermal neutron detector, and the near-thermal neutron detector is opposite to the far-thermal neutron detector.

3. The method for calculating formation density based on mass attenuation coefficient function according to claim 2, characterized in that: The calibration coefficients corresponding to different lithology formations in step 2 are obtained by numerically simulating formations with different porosities under each lithology, and the simulated near-source epithermal neutron counts, far-source epithermal neutron counts, near-source inelastic scattering gamma counts, far-source inelastic scattering gamma counts, and near-source inelastic gamma counts within the specific energy window are brought into the formation density calculation model and fitted.

4. The method for calculating formation density based on mass attenuation coefficient function according to claim 3 is characterized in that: The specific process of step 2 is: Step 2.1, establishing a pulsed neutron gamma density logging tool model identical to the pulsed neutron gamma density logging tool in step 1 in a numerical simulation software; Step 2.2, selecting formation models of different lithologies, constructing formations of different porosities under different lithologies by adding water in different proportions, and obtaining simulated formation density values ​​of formations of different porosities under different lithologies; Step 2.3, using the pulsed neutron gamma density logging model, obtain the near-source epithermal neutron counts, far-source epithermal neutron counts, near-source inelastic scattering gamma counts, far-source inelastic scattering gamma counts and near-source inelastic gamma counts within the specific energy window of different porosity formations under each lithology, bring them into the formation density calculation model, fit the calibration coefficients corresponding to each lithology, and then establish a calibration coefficient library.

5. The method for calculating formation density based on mass attenuation coefficient function according to claim 4, characterized in that: In step 2.2, each lithology includes at least 7 strata with different porosities.

6. The method for calculating formation density based on mass attenuation coefficient function according to claim 1, characterized in that: The lithology of the formation to be tested can be identified based on the detector response changes or element spectrum analysis.

7. The method for calculating formation density based on mass attenuation coefficient function according to claim 1, characterized in that: The number of counts N in the near-source distance inelastic gamma specific energy window lith The specific energy window of MgO is in the range of 0.07 to 0.35 MeV.

8. A system for calculating formation density based on a mass attenuation coefficient function, used to implement the method for calculating formation density based on a mass attenuation coefficient function as claimed in any one of claims 1 to 7, characterized in that: It includes a calibration coefficient library building module, a formation lithology identification module, a calibration coefficient matching module, a pulsed neutron gamma density logging tool and a data processing module; among which: The scale coefficient library construction module is used to determine the scale coefficients corresponding to different lithological formations through numerical simulation according to the configured formation density calculation model, establish a scale coefficient library, and input it into the scale coefficient matching module; The formation lithology identification module is used to identify the formation lithology to be measured and input the lithology identification result into the calibration coefficient matching module; The scale factor matching module is used to determine the scale factor corresponding to the lithology of the formation to be measured and input it into the data processing module; The pulsed neutron gamma density logging instrument is used to measure the near-source epithermal neutron count, far-source epithermal neutron count, near-source inelastic scattering gamma count, far-source inelastic scattering gamma count and near-source inelastic gamma specific energy window count of the formation to be measured, and input them into the data processing module; The data processing module is used to calculate the density ρ of the formation to be measured according to the configured formation density calculation model.

9. A terminal comprising a memory, a processor and a computer program stored in the memory, characterized in that: The processor executes the computer program to implement the steps of the method for calculating formation density based on the mass attenuation coefficient function as described in any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method for calculating formation density based on the mass attenuation coefficient function described in any one of claims 1 to 7 are implemented.

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