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

Abstract

The present invention discloses a method, system, terminal and medium for calculating formation density based on a mass attenuation coefficient function, belonging to the field of logging technology. Specifically: Based on a pulsed neutron gamma density logging tool, a formation density calculation model is constructed; through numerical simulation, a calibration coefficient library is established, including calibration coefficients corresponding to different lithology formations; the lithology of the formation to be measured is identified, and the calibration coefficient corresponding to the lithology of the formation to be measured is matched in the calibration coefficient library; the formation to be measured is measured by a pulsed neutron gamma density logging tool, and then the formation density is calculated according to the formation density calculation model and the calibration coefficient. The present invention can calculate the density of formations with different lithologies more accurately, meet the requirements of actual measurement wells, and control the absolute error within a reasonable range.

Description

Technical Field

[0001] The present invention belongs to the technical field of logging, and particularly 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, the 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 operation safety hazards of the GGD technology have gradually emerged, becoming the key factors restricting its further development. The neutron gamma density logging (Neutron Gamma Density, NGD) technology, with its advantages of no radioactive pollution and high measurement accuracy, has gradually become a new focus of research and application. The core of the NGD technology lies in using the secondary gamma rays generated by the complex interactions between the high-energy fast neutrons emitted by the pulsed neutron source and the formation materials to measure the formation density.

[0003] The NGD technology involves two important aspects: neutron field distribution and inelastic gamma field distribution. The 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 14 MeV into the formation, these neutrons will interact with various elements in the formation in three main types: inelastic scattering, elastic scattering, and capture reactions. Among them, inelastic scattering and capture reactions are particularly important because they can generate secondary gamma rays. Compared with capture gamma rays, inelastic gamma rays can more directly reflect the information of the interaction between neutrons and the formation and are less affected by neutron transport. Therefore, they are more suitable for density calculation. The generation of inelastic gamma rays is closely related to the types and contents of formation elements. When gamma rays pass through the formation, they will gradually attenuate due to the absorption and scattering effects of the formation materials, and this attenuation process is also restricted by various factors such as formation lithology, porosity, and pore fluid properties. These complex physical processes and formation factors constitute the main challenges faced by the NGD technology.

[0004] When conducting formation density measurements, there is a general tendency to use inelastic gamma rays, which has been widely applied in the industry. However, during the correction process of neutron transport, the physical quantities selected by different methods vary significantly, including various methods such as capture gamma rays, thermal neutrons, and epithermal neutrons. Most of these methods focus on the detailed analysis and expression of the neutron transport process, but neglect the complexity of the gamma ray attenuation process. For example, the GGD technology mainly focuses on a single physical phenomenon, Compton scattering. However, the secondary gamma rays generated by pulsed neutron sources have high energy and are discrete, making the gamma attenuation process more complex and difficult to accurately describe the complex action mechanism of gamma rays in different lithologic formations through conventional means. The current NGD technology often treats the mass attenuation coefficient as a constant. This simplified treatment neglects the important influence of gamma attenuation on the measurement results, thus limiting the 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 refined modeling and correction methods. Summary of the Invention

[0006] Aiming at the neglect of the important influence of gamma attenuation on the measurement results during 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. By utilizing the dynamic changes of the mass attenuation coefficient under different lithologic formations, a new formation density calculation model is constructed, thereby improving the accuracy of the formation density calculation results in pulsed neutron logging.

[0007] In order to achieve the above object, the technical methods adopted by the present invention are 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 a pulsed neutron gamma density logging tool, construct a formation density calculation model, and the formula is:

[0010]

[0011] In the formula, ρ is the formation density; R inγ is the ratio of the inelastic scattering gamma count at a near source distance to the inelastic scattering gamma count at a far source distance; R ETN is the ratio of the epithermal neutron count at a near source distance to the epithermal neutron count at a far source distance; N lith is the count within a specific energy window of inelastic gamma rays at a near source distance; A, a0, a1, a2, b, c, and d are all calibration coefficients; the denominator 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 lithologic formations.

[0013] Step 3: Identify the lithology of the formation to be measured, 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.

[0014] Step 4: 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 a specific energy window of the formation to be measured by a pulsed neutron gamma density logging tool. Then, calculate the density ρ of the formation to be measured 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] Further, the pulsed neutron gamma density logging tool 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. Among them, the near-source neutron detector group includes a near epithermal neutron detector 2 and a near thermal neutron detector 3 arranged side by side, the far-source neutron detector group includes a far epithermal neutron detector 5 and a far thermal neutron detector 6 arranged side by side, the near epithermal neutron detector 2 is opposite to the far epithermal neutron detector 5, and the near thermal neutron detector 3 is opposite to the far thermal neutron detector 6.

[0016] Further, the calibration coefficients corresponding to different lithologic formations in Step 2 are obtained by numerically simulating different porosity formations under each lithology, and substituting the simulated 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 a specific energy window into the formation density calculation model and then fitting.

[0017] Further, the specific process of Step 2 is as follows:

[0018] Step 2.1: Establish a pulsed neutron gamma density logging tool model in the numerical simulation software that is the same as the pulsed neutron gamma density logging tool in Step 1.

[0019] Step 2.2: Select formation models of different lithologies, and construct different porosity formations under each lithology by adding different proportions of water to obtain the simulated formation density values of different porosity formations under each lithology.

[0020] Step 2.3: Using the pulsed neutron gamma density logging tool model, obtain 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 a specific energy window for different porosity formations of each lithology. Substitute these into the formation density calculation model and fit to obtain the calibration coefficients A, a0, a1, a2, b, c, and d corresponding to each lithology, and then establish a calibration coefficient library.

[0021] Further, each lithology in Step 2.2 includes at least 7 different porosity formations.

[0022] Further, identify the lithology of the formation to be measured based on the detector response change or elemental spectrum analysis.

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

[0024] Further, the count N within the near-source inelastic gamma specific energy window lith has a specific energy window range of 0.07 to 0.35 MeV.

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

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

[0027] A formation density calculation system based on the mass attenuation coefficient function, used to implement the formation density calculation method based on the mass attenuation coefficient function, specifically includes a calibration coefficient library construction module, a formation lithology identification module, a calibration coefficient matching module, a pulsed neutron gamma density logging tool, and a data processing module; where:

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

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

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

[0031] The pulsed neutron gamma density logging tool 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 a specific energy window 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 includes a memory, a processor, and a computer program stored on the memory. 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, and when the computer program is executed by a processor, it implements the steps of the method for calculating formation density based on the mass attenuation coefficient function.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] The present invention proposes a method, system, terminal, and medium for calculating formation density based on the mass attenuation coefficient function. By constructing a formation density calculation model based on the mass attenuation coefficient function, the density of different lithology formations can be calculated more accurately, meeting the requirements of actual measurement wells, and the absolute error is controlled within a reasonable range; among them, taking the ratio of the near-source inelastic scattering gamma count to the far-source inelastic scattering gamma count as the main variable can effectively reduce the environmental impact and reduce the interference of the formation hydrogen index; taking the ratio of the near-source epithermal neutron count to the far-source epithermal neutron count as a correction term can reduce the influence of the formation hydrogen index; constructing a mass attenuation coefficient function based on the near-source inelastic gamma count within a specific energy window can more dynamically reflect the true physical properties of the formation, depict the complexity of gamma attenuation in different lithology formations, and thus improve the accuracy of the calculation results under different lithologies;

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

[0038] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

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

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

[0041] Figure 3 It is an absolute error diagram between the simulated formation density value and the calculated formation density value in Example 1;

[0042] The reference signs 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 - Borehole; 9 - Formation. Detailed implementation manners

[0044] To further understand the present invention, the preferred implementation manners of the present invention will be described below in conjunction with embodiments. However, it should be understood that these descriptions are only for further explaining the features and advantages of the present invention, rather than limiting the claims of the invention.

[0045] Example 1

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

[0047] Step 1, design a pulsed neutron gamma density logging tool as shown in Figure 1 and place it in the borehole 8 of the formation 9. Specifically, it 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 epithermal neutron detector 2 and a near thermal neutron detector 3 arranged side by side, the far-source neutron detector group includes a far epithermal neutron detector 5 and a far thermal neutron detector 6 arranged side by side, the near epithermal neutron detector 2 is opposite to the far epithermal 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, construct a formation density calculation model, and the formula is:

[0049]

[0050] In the formula, ρ is the formation density; R inγ is the ratio of the inelastic scattering gamma count at the near source distance to the inelastic scattering gamma count at the far source distance; R ETN is the ratio of the epithermal neutron count at the near source distance to the epithermal neutron count at the far source distance; N lithCounts within a specific energy window of inelastic gamma rays at a short source-detector spacing. The specific energy window ranges from 0.07 to 0.35 MeV; A, a0, a1, a2, b, c, and d are all calibration coefficients; the denominator is the mass attenuation coefficient function.

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

[0052] Step 2.1: In order to better obtain the fitting parameters, establish a pulsed neutron gamma density logging tool model in the Monte Carlo numerical simulation software that is the same as the pulsed neutron gamma density logging tool in Step 1. The neutron source 1 is a plane source with an emission period of 35 μs, and the emission time is set from 0 to 10 μs, and it emits isotropically in all directions.

[0053] Step 2.2: Select formation models of different lithologies, and construct formations with different porosities under each lithology by adding different proportions of water to obtain the simulated formation density values of formations with different porosities under each lithology.

[0054] In this embodiment, formation models of three lithologies, namely limestone, dolomite, and sandstone, are taken as examples. By adding different proportions of water, formations with different porosities under each lithology are constructed. Each lithology includes 7 formations with different porosities, and the simulated formation density values of formations with different porosities under each lithology are obtained. The selected density range is 2.040 g / cm 3 ~2.841 g / 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 short source-detector spacing epithermal neutron counts, long source-detector spacing epithermal neutron counts, short source-detector spacing inelastic scattering gamma counts, long source-detector spacing inelastic scattering gamma counts, and counts within the specific energy window of short source-detector spacing inelastic gamma rays for formations with different porosities under each lithology, and substitute them into the formation density calculation model to fit and obtain the calibration coefficients A, a0, a1, a2, b, c, and d corresponding to each lithology, and then establish a calibration coefficient library.

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

[0059] Step 3: Identify the lithology of the formation to be measured based on the change in detector response or elemental 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: Using the pulsed neutron gamma density logging tool in Step 1, 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 a specific energy window of the formation to be measured. Then, based on 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, calculate the density ρ of the formation to be measured.

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

[0062] Table 2

[0063]

[0064] Using the method in Step 4, calculate the density of the formation with different porosities under each lithology, denoted as the calculated formation density value, and draw a relationship diagram between the simulated formation density value and the calculated formation density value as shown in Figure 2 . 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. And according to the Figure 3 absolute error diagram between the simulated formation density value and the calculated formation density value, it can be known that the absolute error is less than 0.02 g / cm 3 , indicating that the method for calculating the 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 requirements of actual measurement wells.

[0065] The above embodiments are for better further understanding of the present invention, and are not limited to the best implementation manner, and do not constitute a limitation to the content and protection scope of the present invention. Any product that is the same as or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other existing technologies is 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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