A method for density correction of a gamma-gamma density log

Abstract

This application relates to the field of gamma-gamma density logging technology, specifically providing a density correction method for gamma-gamma density logging, including: obtaining the count rate ratio R of a long-source detector (GGFR) and a natural gamma detector (NGR) using a model well. GGFR / NGR And the count rate ratio R of the short-source detector GGNR to the natural gamma detector NGR GGNR / NGR ; Obtain the density calibration coefficient of the density logging tool; Perform logging using the density logging tool and obtain the count rate N of the natural gamma detector NGR, long-source detector GGFR, and short-source detector GGNR. NGR N GGFR and N GGNR Based on the obtained count rate N NGR N GGFR N GGNR The ratio of the count rate to the count rate R GGFR / NGR R GGNR / NGR The net count rate of the long-source detector (GGFR) and the short-source detector (GGNR) is calculated to correct the count rate used for density parameter calculation; the formation density is then calculated based on the density calibration coefficient and the net count rate. This application can effectively correct the density and meet the accuracy requirements of density logging.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of gamma-gamma density logging, and particularly relates to a density correction method for gamma-gamma density logging. BACKGROUND

[0002] Density is one of the parameters that must be measured in conventional geophysical logging, and gamma-gamma density logging is a logging method for measuring the density of a formation according to the Compton effect generated when gamma rays interact with matter. Density logging based on the principle of gamma-gamma scattering is irreplaceable in identifying the lithology of a formation, obtaining the wave impedance of a formation and the porosity of a formation according to the density and acoustic velocity of ore.

[0003] In the exploration of sandstone-type uranium deposits, the density logging curve has a relatively obvious difference between mudstone and sandstone formations. Generally, as the argillaceous content of sandstone increases, the density value decreases, and the permeability decreases. Therefore, in order to determine the lithology and permeability of the ore-bearing and water-bearing layer of an in-situ leaching sandstone-type uranium deposit and provide basic physical parameter information for in-situ leaching mining, density logging cannot be avoided.

[0004] However, the gamma rays of radioactive formations are an interference to density logging, which can cause distortion of the density curve, affect the correct identification of lithology, and even divide the permeable lithology into impermeable formations, which seriously affects the selection of subsequent in-situ leaching mining processes. SUMMARY

[0005] In order to solve the above problems, the embodiments of the present application provide a density correction method for gamma-gamma density logging, which can effectively correct the density and meet the accuracy requirements of density logging.

[0006] To this end, the embodiments of the present application adopt the following technical solutions:

[0007] A density correction method for gamma-gamma density logging is applied to a density logging instrument, the density logging instrument comprising a natural gamma detector NGR, a long source detector GGFR and a short source detector GGNR; the density correction method comprising: obtaining the count rate ratio R GGFR / NGR of the long source detector GGFR and the natural gamma detector NGR and the count rate ratio R GGNR / NGRThe model well was obtained through testing of rock samples with known density and nominal content. The response differences of the long-source detector (GGFR) and short-source detector (GGNR) to natural gamma rays in the model well were obtained relative to the natural gamma ray detector (NGR) using count rate ratios. The density calibration coefficient of the density logging tool was obtained; this coefficient was determined based on calibration data of the density logging tool using standard density samples. The density logging tool was used for logging, and the count rates N of the natural gamma ray detector (NGR), long-source detector (GGFR), and short-source detector (GGNR) were obtained. NGR N GGFR and N GGNR ; Take the count rate N of the natural gamma detector NGR NGR Multiply by the count rate ratio R respectively GGFR / NGR and R GGNR / NGR The count rate N of natural gamma rays from the radioactive formation in the well was obtained for the long-source detector GGFR and the short-source detector GGNR. GGFR N GGNR The interference value is then calculated using the count rate N of the long-source detector GGFR. GGFR The count rate N of the short-source detector GGNR GGNR Subtract the corresponding interference values ​​from each, and calculate the net count rate N of the long-source detector GGFR and the short-source detector GGNR. GGFR净 and N GGNR净 The count rate is corrected to complete the density parameter calculation; the formation density is calculated based on the density scale factor and the net count rate.

[0008] As one possible implementation, the model well is selected from either a saturation logging model or a field verification model.

[0009] As one feasible implementation, the ratio R of the count rate of the long-source detector GGFR to the natural gamma detector NGR is obtained using a model well. GGFR / NGR And the count rate ratio R of the short-source detector GGNR to the natural gamma detector NGR GGNR / NGR This includes: obtaining the nominal content of N model wells, where N is greater than or equal to 2; and obtaining the count rates NGR, GGFR, and GGNR of the natural gamma detector, GGFR, and GGNR in the N model wells. NGR '、N GGFR 'and N GGNR Based on the nominal content of N model wells and N count rates N NGR '、N GGFR 'and N GGNR 'Obtain the linear fitting curve of count rate versus nominal content in the model well; obtain the corresponding count rate ratio R based on the ratio of the slopes of the linear fitting curves of the natural gamma detector (NGR), long-source detector (GGFR), and short-source detector (GGNR).GGFR / NGR and R GGNR / NGR .

[0010] As an implementable embodiment, the nominal content of the model well is 0.2-5167x10 -6 g / g.

[0011] As an implementable embodiment, the net count rates N GGFR净 and N GGNR净 of the long source detector GGFR and the short source detector GGNR are obtained by using the following formula, and the correction of the count rate for the density parameter solution is completed:

[0012] N GGFR净 =N GGFR -N NGR x R GGFR / NGR

[0013] N GGNR净 =N GGNR -N NGR x R GGNR / NGR .

[0014] As an implementable embodiment, the density calibration coefficient of the density logging tool is obtained by including: obtaining an aluminum module and a plexiglass module with known density as calibration modules; using a cesium source as a radioactive source of the density logging tool; sequentially installing the aluminum module and the plexiglass module on a probe of the density logging tool, and starting the density logging tool so that the cesium source emits rays to the calibration modules; recording count rate readings of the logging tool and the known density values of the calibration modules, and drawing a relationship curve between the count rate readings and the density according to the recorded count rate readings and the module density; and obtaining the density calibration coefficient A and B of the density logging tool by fitting and analyzing the relationship curve, wherein ln N=Aρ b +B, and the density calibration coefficient A and B of the density logging tool are obtained, wherein ρ b is the density, the unit is g / cm 3 ; N is the detector count rate, the unit is s -1 ; A is the sensitivity coefficient, the unit is s -1 / (g / cm 3 ); and B is the intercept, which is a constant.

[0015] As an implementable embodiment, the formation density is calculated according to the density calibration coefficient and the net count rate, including:

[0016] The formation density is calculated by using the following formula:

[0017] ρ b =(1 / A)ln N-B / A

[0018] In the formula, ρ b is the formation density, the unit is g / cm3 N—Detector count rate, in seconds. -1 A—Sensitivity coefficient, unit is s. -1 / (g / cm 3 B—intercept, which is a constant.

[0019] As one feasible implementation, the cesium source is spaced 200 mm from the short-source detector GGNR; the cesium source is spaced 350 mm from the long-source detector GGFR; and the cesium source is spaced 2500 mm from the natural gamma detector NGR.

[0020] As one feasible implementation, the natural gamma detector NGR, the long-source detector GGFR, and the short-source detector GGNR all use sodium iodide crystals with diameters and lengths of 30mm and 80mm, 23mm and 40mm, and 13mm and 10mm, respectively, and an energy threshold of 130keV.

[0021] As one possible implementation, the short-source detector GGNR incorporates a tantalum-silver sheet.

[0022] This application embodiment utilizes the ratio of the natural gamma detector count rate to the count rates of long- and short-source detectors (R0). GGFR / NGR R GGNR / NGR The density correction method is used to correct the count rates of long and short source detectors by subtracting the influence of natural gamma rays from radioactive formations on density measurements using the count rate ratio. Then, the density value is calculated using a density calibration coefficient to reduce density curve distortion. Density calculation and correction were performed using both field verification models and borehole measurement data. The results show that the density correction method provided in this application is an effective density correction method that meets the accuracy requirements of density logging. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0024] The various regions, shapes, and their relative sizes and positional relationships shown in the figure are merely illustrative and may deviate from reality due to manufacturing tolerances or technical limitations. Furthermore, those skilled in the art can design regions / layers with different shapes, sizes, and relative positions as needed.

[0025] In the various drawings, like elements are designated with like reference numerals throughout. Various portions of the drawings have been exaggerated or omitted for clarity, and certain features can be shown in somewhat generalized or schematic form and not to scale to more clearly demonstrate and discuss the principles of the application.

[0026] Figure 1 A structural schematic diagram of a density logging instrument provided in an embodiment of the present application;

[0027] Figure 2 A flow schematic diagram of a density correction method provided in an embodiment of the present application;

[0028] Figure 3 is a linear fitting curve of the count rate and the model nominal content obtained by the natural gamma-ray detector NGR, the long source spacing detector GGFR and the short source spacing detector GGNR of the MD604-2005 density tri-lateral logging instrument on four saturated logging models;

[0029] Figure 4 A structural schematic diagram of an on-site verification model provided in an embodiment of the present application;

[0030] Figure 5 is a density correction schematic diagram of borehole 1-2-16Z well completion logging;

[0031] Figure 6 is a density curve correction schematic diagram of borehole SYG-1 open hole logging. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be described below with reference to the drawings in the embodiments of the present application.

[0033] In the description of the present specification, the description of the terms "one embodiment", "some embodiments", "exemplary", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any appropriate manner in any one or more embodiments or examples. In addition, the person skilled in the art can combine and combine the different embodiments or examples described in the present specification and the features of the different embodiments or examples without contradiction.

[0034] Stratum density is a very useful and characteristic parameter for stratum evaluation, and is an important basis for identifying stratum sequence and dividing ore bed in a region, and plays an important role in stratum division. Gamma-gamma density logging is very important in the exploration of sandstone type uranium deposit, and is an indispensable logging method. The main purpose of gamma-gamma density logging is to identify stratum lithology, and to obtain stratum wave impedance and porosity according to ore density and acoustic logging.

[0035] However, in the sandstone type uranium mineralized area, the density measurement data will be greatly disturbed by the radioactivity in the ore bed, thereby affecting stratum division and lithology identification, and greatly affecting the judgment of stratum lithology. When the ore bed contains calcareous layer, mudstone and other non-permeable bottom strata, the non-permeable layer should be removed for calculation in logging interpretation and resource evaluation. If the interlayer cannot be accurately divided, it will affect the logging interpretation results and resource estimation. Therefore, it is necessary to correct the interference of the density logging value caused by the radioactivity in the ore bed, so as to obtain more accurate density data and more accurate stratum division and estimation of sandstone type uranium resource.

[0036] In order to eliminate the interference of natural gamma rays on density measurement, workers of traditional density logging have proposed various methods, such as using density three lateral logging instrument to measure twice, i.e. hanging source logging and non-hanging source logging. In density calculation, the influence of natural radiation count on density measurement is deducted by using the difference of detector count rate, which is an effective method, but the working efficiency is relatively low. Some workers also proposed a method of using empirical coefficient fitting to deduct the influence of natural gamma on long source detector count rate, but the universality of this method has certain limitations.

[0037] In order to solve the problem of interference of strong radioactive ore in stratum on density logging data, the density correction method of gamma-gamma density logging is applied in the process of measuring stratum density by the density logging instrument in the embodiments of the present application. The method uses the ratio (R GGFR / NGR , R GGNR / NGR ) of natural gamma detector count rate to long and short source detector count rate to correct the long and short source detector count rate, and then uses the density calibration coefficient to calculate the density value.

[0038] Before introducing the density correction method in the embodiments of the present application in detail, the structure of the density logging instrument provided by the embodiments of the present application needs to be introduced. Referring to Figure 1 , the density logging instrument is a density three lateral logging instrument (i.e. compensated density logging instrument), which includes a pusher 1, a probe 2 (also called a slide plate), a circuit (not shown in the figure) and a gamma ray detector. The gamma ray detector includes a short source detector GGNR, a long source detector GGFR and a natural gamma detector NGR.

[0039] The radioactive source, short-source detector GGNR, and long-source detector GGFR are sequentially mounted on probe 2. The natural gamma detector NGR is mounted at the front end of the density logging tool. Furthermore, one end of the pusher 1 is mounted on the sidewall of the density logging tool, while the other end can abut against the wellbore during logging, allowing the density logging tool to also abut against the wellbore. The distance between the end of the pusher 1 abutting against the wellbore and the radioactive source is 369 mm.

[0040] The interaction between gamma rays and matter takes three forms: 1. photoelectric absorption; 2. Compton scattering; 3. electron-electron pair effect. For example, the radioactive source selection for density logging tools... 137 Cs source. Because in this implementation scheme, the density logging tool selected is... 137 The Cs source emits gamma rays with an energy of 0.662 MeV, thus eliminating the possibility of electron-electron pair formation. By appropriately selecting the detector threshold, the influence of the photoelectric absorption effect can be minimized, ensuring that the detector records only gamma rays that have undergone one or more Compton scattering events with the formation. For example, in this embodiment, the natural gamma detector NGR, the long-source detector GGFR, and the short-source detector GGNR all employ sodium iodide crystals with diameters and lengths of 30 mm and 80 mm, 23 mm and 40 mm, and 13 mm and 10 mm, respectively, and an energy threshold of 130 keV. To reduce the influence of the photoelectric effect caused by low-energy gamma rays, the short-source detector GGNR incorporates a tantalum-silver sheet to filter out low-energy scattered gamma rays.

[0041] For the selected 137 Under the conditions of a Cs radioactive source and a measurement energy threshold, for a typical stratum composed of atoms of medium atomic number, the Compton effect dominates the interaction between photons and the stratum. In this case, the stratum attenuation coefficient is proportional to the electron density, which in turn is proportional to the volume density. For most minerals and most strata, the mass attenuation coefficient μ... m It is essentially a constant, which is the basis of density logging.

[0042] During well logging, the probe is first lowered into the well and, with the help of the pusher, is brought close to the well wall. The radiation source emits gamma photons into the formation through the emission window. After being scattered and absorbed by the formation, a portion of the gamma photons that undergo one or more Compton scatterings are received by the crystals of two gamma-ray detectors (short-source detector GGNR and long-source detector GGFR) at different distances from the radiation source. These detectors then undergo a photoelectric effect, generating an electrical signal, which is amplified and processed by a circuit before being recorded. A shielding material separates the radiation source and the gamma-ray detectors, ensuring that the photons received by the detectors are those scattered by the formation. Different formation densities result in different scattering and absorption capacities for gamma photons, leading to different readings recorded by the detectors. Through calibration using the density correction method in this embodiment, the formation density value can be determined based on the detector readings.

[0043] It is worth mentioning that the formation detection sensitivity A of the density logging tool satisfies the following formula:

[0044] A = -u m ·d

[0045] In the formula, d—source distance; μ m —The mass absorption coefficient of the formation, which is theoretically the sum of the absorption coefficients of the photoelectric effect, Compton effect, and pair production effect. Due to the use of... 137 The Cs source gamma rays have an energy of 662 keV, and their interaction with the formation material is almost entirely the Compton effect, therefore μ m It is approximately equal to the Compton absorption coefficient. From the above formula, it can be seen that the larger the source distance d, the higher the sensitivity A of the density logging tool, and the greater the detection depth. However, if the source distance d is too large, it will reduce the count rate, thereby increasing the statistical error.

[0046] It should be noted that the distance between the gamma-ray detector and the radiation source is called the source distance. In this embodiment, the source distance between the short-source detector GGNR and the cesium source is 200 mm. The source distance between the long-source detector GGFR and the cesium source is 350 m. The source distance between the natural gamma-ray detector NGR and the cesium source is 2500 mm.

[0047] In addition, in actual logging, due to irregular well wall, pushing and other factors, mud cake is inevitably sandwiched between the probe and the formation, at this time, the density value measured by the instrument (called apparent density) is not only related to the formation density, but also related to the thickness, density and average atomic number of the mud cake. Therefore, the long source probe GGFR and the short source probe GGNR arranged in the probe can realize the use of the dual source distance compensation method to obtain the formation density. Specifically, when there is mud cake, the mud cake has different effects on the long source probe and the short source probe. By comparing the difference between the gamma radiation intensity measured by the long source probe and the short source probe, and according to the known relationship between the mud cake properties and the absorption of gamma radiation, the thickness, density and average atomic number of the mud cake can be estimated, and the long source probe GGFR measurement value is compensated by using the calculated mud cake parameters, so that more accurate formation density value is obtained.

[0048] In the absence of mud cake, the count rate of two detectors with different source distances is measured, and the relationship between the count rate and the formation density is as follows:

[0049] ln N=Aρ b +B

[0050] That is

[0051] ρ b =(1 / A)ln N-B / A

[0052] Where: ρ b —formation density, unit g / cm 3 ; N—probe count rate, unit s -1 ; A—sensitivity coefficient, unit s -1 / (g / cm 3 ); B—intercept, constant.

[0053] For long source probe GGFR: ρ GGFR =(1 / A GGFR )lnN GGFR -B GGFR / A GGFR ;

[0054] For short source probe GGNR: ρ GGNR =(1 / A GGNR )lnN GGNR -B GGNR / A GGNR .

[0055] Assuming ρ GGFR =ρ GGNR , then:

[0056] lnN GGFR =(A GGFR / A GGNR )lnNGGNR -(A GGFR / A GGNR )B GGNR +B GGFR .

[0057] That is, lnN GGFR and lnN GGNR is a linear relationship, the slope of which is A GGFR / A GGNR , and we call this line the ridge line, and the angle α = arctg(A GGFR / A GGNR ) between the ridge line and the horizontal axis is called the ridge angle. Since A GGFR / A GGNR only relates to the source distance, the ridge angle does not change when the instrument geometry parameters are set, and each point on the ridge line corresponds to a density value.

[0058] As described above, the natural radioactivity gamma rays of the formation are received by the long source detector GGFR, causing the measured density value to deviate from the true value. Therefore, in the present embodiment, a natural gamma detector NGR is installed on the density tri-lateral logging tool to measure the natural radioactivity of the formation, which is used for depth alignment and formation correlation. The natural gamma detector NGR is located at the top of the logging tool, while the long source detector GGFR and the short source detector GGNR for measuring the density parameters are located at the bottom of the density tri-lateral logging tool. Since the source distance between the natural gamma detector NGR and the cesium source is 2500 mm, it can be considered that the natural gamma detector NGR is basically not affected by the active radioactive source (cesium source). Therefore, during actual borehole logging, the count rate ratio (R GGFR / NGR ,R GGNR / NGR ) of the natural gamma detector and the long and short source detectors can be used to proportionally deduct the count rates of the long and short source detectors for density calculation, in order to reduce the influence of natural gamma rays.

[0059] The density correction method for gamma-gamma density logging provided by the embodiment of the present application will be described in detail below with reference to the accompanying drawings.

[0060] Referring to Figure 2 , the density correction method for gamma-gamma density logging disclosed by the embodiment of the present application includes the following steps:

[0061] S201, obtaining the count rate ratio R GGFR / NGR of the long source detector GGFR and the natural gamma detector NGR and the count rate ratio R GGNR / NGR of the short source detector GGNR and the natural gamma detector NGR by using a model well.

[0062] It is understood that the count rate refers to the number of rays or particles received by the detector per unit time, which is usually measured by the number of rays or particles received per second.

[0063] The model well is a rock sample with known density and nominal content obtained by testing to obtain the difference in response of the long source detector GGFR and the short source detector GGNR to the natural gamma rays of the model well relative to the natural gamma detector NGR in terms of count rate ratio. For example, the model well is selected from any one of the saturated logging model or the field verification model. It is assumed here that the count rate of each detector is proportional to the radioactivity intensity of the formation. That is, the proportional coefficient of the count rate ratio can be obtained on a standard model well such as the saturated logging model, or can be obtained using a non-standard model well such as the field verification model.

[0064] In an implementable embodiment, the count rate ratio R GGFR / NGR of the long source detector GGFR to the natural gamma detector NGR and the count rate ratio R GGNR / NGR of the short source detector GGNR to the natural gamma detector NGR are obtained using the model well, including the following steps: obtaining the nominal content of N model wells, N being greater than or equal to 2; obtaining the count rates N NGR ', N GGFR ' and N GGNR ' of the natural gamma detector NGR, the long source detector GGFR and the short source detector GGNR in the N model wells; obtaining a linear fitting curve of the count rate and the nominal content of the model well (energy scale linearization processing) according to the nominal content of the N model wells and the N count rates N NGR ', N GGFR ' and N GGNR '; obtaining the corresponding count rate ratios R GGFR / NGR and R GGNR / NGR according to the slope ratio of the linear fitting curves of the natural gamma detector NGR, the long source detector GGFR and the short source detector GGNR.

[0065] For example, the count rate proportional coefficient is obtained using the saturated logging model. Specifically, in order to more accurately obtain the count rate ratios of the long source detector GGFR, the short source detector GGNR and the natural gamma detector NGR, data acquisition was performed on the saturated logging model (UF-0.03-I, UF-0.1-I, UF-0.2-I, UF-0.5-I) of the National Defense Science and Technology Industry Radioactivity Measurement Station (1313) using the MD604 density tri-lateral logging instrument, dead time correction and energy scale linearization processing (Table 1).

[0066] Table 1 Dead time and count rate ratio of NGR, GGFR and GGNR of MD604 density tri-lateral logging tool

[0067]

[0068] It can be understood that the above saturation logging models (UF-0.03-I, UF-0.1-I, UF-0.2-I, UF-0.5-I) only differ in the nominal content of uranium content. Specifically, refer to Figure 3 , Figure 3 is the linear fitting curve of the count rate of NGR, GGFR and GGNR of MD604-2005 density tri-lateral logging tool on the above four saturation logging models and the nominal content of the model, and the corresponding R GGFR / NGR and R GGNR / NGR ratio is obtained according to the slope ratio.

[0069] For example, the count rate ratio of the detector is obtained by using the field verification model. Specifically, as long as the number of model wells is sufficient, such as N is more than 3, the R GGFR / NGR and R GGNR / NGR ratio can also be obtained by using the field verification model developed by the Measurement Station of Nuclear Industry Aerial Survey and Remote Sensing Center. Figure 4 is a schematic diagram of the structure of the field verification model. Refer to Figure 4 , the field verification model includes a handle and a bottom foot, which facilitates the staff to carry and place the field verification model in the logging site; refer to Figure 4 the top view on the right side, the middle part of the field verification model is a cylindrical model well.

[0070] For example, the field verification model provided in the present embodiment has six. Table 2 is a list of the content of main elements and impurity elements of the six verification models.

[0071] Table 2 Element content table of field verification model

[0072]

[0073] Note: a The fixed value result is for reference only (provided by the Nuclear Industry Aerial Survey and Remote Sensing Center).

[0074] As can be seen from the above, whether it is a field verification model or a saturation logging model, the nominal content of the model well in the present embodiment is 0.2-5167x10 -6 g / g, so that the density correction method in the present embodiment has the ability to perform density correction in a wider range of formation radioactivity intensity.

[0075] S202, obtaining the density calibration coefficient of the density logging tool.

[0076] The density calibration coefficient is the calibration data of the density logging instrument determined according to the standard density sample. Specifically, aluminum modules and organic glass modules with known densities are obtained as calibration modules;

[0077] The cesium source (US14CS001184 and US14CS001224) is used as the radioactive source of the density logging instrument;

[0078] The aluminum modules and organic glass modules are sequentially installed on the probe of the density logging instrument, and the density logging instrument is started to make the cesium source emit rays to the calibration modules;

[0079] The count rate reading of the logging instrument and the known density value of the calibration module are recorded, and a relationship curve between the count rate reading and the density is drawn according to the recorded count rate reading and the module density;

[0080] Through fitting and analysis of the relationship curve, ln N=Aρ b +B is obtained, and the density calibration coefficient A and B of the density logging instrument are obtained, wherein ρ b —density, unit: g / cm 3 ; N—detector count rate, unit: s -1 ; A—sensitivity coefficient, unit: s -1 / (g / cm 3 ); B—intercept, constant.

[0081] That is, in the present embodiment, the MD604 logging instrument is calibrated for density parameters using the cesium source (US14CS001184 and US14CS001224) with aluminum modules and organic glass modules, and the sensitivity and ridge angle are obtained, and the results are shown in Table 3.

[0082] Table 3 MD604 density calibration results

[0083]

[0084] S203, using the density logging instrument to log and obtaining the count rates N NGR , N GGFR and N GGNR of the natural gamma detector NGR, the long source detector GGFR and the short source detector GGNR.

[0085] S204, according to the obtained count rates N NGR , N GGFR , N GGNR and the count rate ratio R GGFR / NGR , R GGNR / NGR, the net count rates of the long source detector GGFR and the short source detector GGNR are calculated, and the correction of the count rates for the density parameter calculation is completed.

[0086] The count rate N NGR of the natural gamma detector NGR is taken as an example. GGFR / NGR and R GGNR / NGR , the interference values of the natural gamma ray count rates N GGFR and N GGNR of the measured downhole radioactive formation to the long source detector GGFR and the short source detector GGNR are obtained, and then the count rate N GGFR of the long source detector GGFR and the count rate N GGNR of the short source detector GGNR are respectively subtracted by the interference values corresponding to themselves, so as to calculate the net count rates N GGFR净 and N GGNR净 of the long source detector GGFR and the short source detector GGNR, and complete the correction of the count rates for the density parameter calculation.

[0087] Specifically, the following formula is used to complete the correction of the count rates for the density parameter calculation:

[0088] N GGFR净 = N GGFR -N NGR × R GGFR / NGR

[0089] N GGNR净 = N GGNR -N NGR × R GGNR / NGR .

[0090] S205, calculate the formation density according to the density calibration coefficient and the net count rate.

[0091] The following formula is used to calculate the formation density:

[0092] ρ b = (1 / A) ln N-B / A

[0093] In the formula, ρ b is the formation density, with the unit of g / cm 3 ; N is the detector count rate, with the unit of s -1 ; A is the sensitivity coefficient, with the unit of s -1 / (g / cm 3 ); and B is the intercept, which is a constant.

[0094] It should be noted that the short source detector GGNR in the compensated density logging tool is mainly used to correct the influence of mud cake because its detection depth is shallower than that of the long source detector GGFR. In the following logging test process, whether using the field verification model or open hole logging, the density calculation and correction process is based on the long source detector count.

[0095] For example, next, using the field verification model, the count rate direct deduction method in the two downhole measurements in the conventional density correction method is compared with the density correction method in the present application. For the convenience of description, the density correction method provided in the present application is referred to as the count rate ratio method.

[0096] The nominal uranium content of the verification model UH-0.03-II is 396×10 -6 g / g. Four MD604 logging tools are used to correct the influence of the change in long and short source counts caused by the gamma rays of the ore layer, resulting in a lower density calculation result, by using: 1) the count rate direct deduction method (i.e. the long and short source counts with the cesium source are subtracted from the long and short source counts without the cesium source, which is equivalent to two downhole measurements, one with the source and one without the source); and 2) the detector count rate ratio method (R GGFR / NGR ), and the results are shown in Table 4.

[0097] Table 4 Density correction results of the verification model (UH-0.03-II)

[0098]

[0099] As can be seen from Table 4, the measured density before correction is obviously low, with an average of 1.84 g / cm 3 . The count rate deduction method simulating two measurements obtains a density average of 2.01 g / cm 3 . The count rate ratio deduction method provided in the present application (which is equivalent to one measurement) obtains a density average of 2.00 g / cm 3 . For the UH-0.03-II model, the density correction results are ideal, especially the count rate direct deduction method and the count rate ratio deduction method are equivalent. Since there is no nominal value of the field verification model, the error is within ±0.03 g / cm 3 when the measured average is used as the reference value.

[0100] For the UH-0.5-II verification model (with a nominal uranium content of 5 167×10 -6 g / g), the long source detector count rate is used to calculate the density value of the field verification model by using the count rate deduction method and the count rate ratio method (R GGFR / NGR ), respectively. The count rate ratio deduction method is closer to the actual value (1.12 g / cm 3, the count rate deduction method is 2.54 g / cm 3 , and the count rate ratio method is 2.05 g / cm 3 , which indicates that the count rate ratio method provided in the embodiments has the ability to correct the density in a wider range of formation radioactivity intensity.

[0101] Next, the open hole logging and cased hole logging test comparison is carried out by using the density logging instrument and the density correction method provided in the embodiments. It should be noted that the open hole logging refers to a logging method in which the density logging instrument is lowered into the well hole to collect and record data before the well completion operation is performed during the drilling process. The cased hole logging refers to a method of logging through the casing after the well completion operation is performed.

[0102] Figure 5 and Figure 6 In the above, GR represents the quantitative gamma logging, expressed in the irradiation rate (nC / (kg·h)), unless otherwise specified; N NGR , N GGFR , N GGNR respectively represent the natural gamma detector NGR, the long source detector GGFR, and the short source detector GGNR count rate in the MD604 logging instrument, and Den-correction before and Den-correction after respectively represent the density values before and after the density correction by using the density correction method in the embodiments.

[0103] Figure 5 is a schematic diagram of the cased hole logging density correction of the borehole 1-2-16Z. Referring to Figure 5 , the cased hole 1-2-16Z: the gamma-gamma density logging is used to determine the filter position and the thickness of fine gravel and coarse gravel in the cased hole, and according to the cased hole process requirements, the filter position is coarse gravel; the fine gravel is noted above and below the filter position, and the coarse gravel has a relatively low density compared with the fine gravel. From Figure 5 , it can be seen from the logging curves in the above that the short source detector GGNR count rate is basically not affected by the formation radioactive gamma rays, and the density calculation and correction are based on the long source detector count rate. Figure 5 The two curves on the far right in the above are the density logging curves before and after the density correction, and the logging curve after the Den-correction is basically a reflection of the mine layer process structure compared with the logging curve before the Den-correction, because the detection depth of the density logging is limited, and the corrected density value basically reflects the coarse particles (the depth interval is 390.0-396.15 m, and the average density is 1.767 g / cm 3), fine particle density (depth interval 386.60-389.75 m, and 396.15-399.40 m, the average density is about 2.017 g / cm 3 ). Quantitative gamma logging, the exposure rate at the ore section location can be as high as 700 nC / (kg·h). Well density logging, using long source detector count rate GGFR for density correction is more ideal.

[0104] Figure 6 is a schematic diagram of the borehole SYG-1 bare hole logging density curve correction. See Figure 6 , SYG-1 borehole, bare hole logging, in the depth interval 295.0-310.0 m, the count rate of long and short source detectors changes, which truly reflects the change of the density of the formation around the wellbore. In the depth interval 405.36-406.35 m and 423.80-426.34 m, which are exactly in the ore layer location, the count rate of the long source detector is obviously disturbed. The corresponding density correction before and after is obvious, the density measurement value before density correction is low, which directly affects the inference of the formation lithology. After density correction, the average density of the depth interval 405.36-406.35 m is 2.289 g / cm 3 , which is close to the average density of the upper formation 2.279 g / cm 3 ; and the average density of the depth interval 423.80-426.34 m after correction is 2.363 g / cm 3 , which is also close to the average density of the upper formation 2.331 g / cm 3 , which belongs to the same lithology (coarse sandstone).

[0105] The drilling example verification result shows that in the density correction of the radioactive formation of 1 500 nC / (kg·h) Figure 6 , using the count rate ratio method to correct the density using the count rate of the long source detector GGFR can achieve satisfactory results. For hard rock radioactive formation, there is no influence of mud cake, and it can be inferred that the density correction method proposed in the embodiment is also applicable.

[0106] In summary, the density correction method of the gamma-gamma density logging disclosed in the embodiment is an effective correction method for the problem of low formation bulk density obtained by the gamma-gamma density logging method caused by the natural gamma ray of the formation. Compared with the direct subtraction method of obtaining the count rates of the respective detectors twice, the density correction is effective in a larger natural gamma radiation intensity range of the formation, and only one trip is needed, reducing the drilling time. It should be noted that the density correction method provided in the embodiment is not limited to the use of MD604 logging instrument, as long as the density logging instrument has a natural gamma detector, the detector count rate ratio subtraction method proposed in the application can also be used to subtract the influence of natural gamma radiation on density measurement.

[0107] Finally, it should be pointed out that the above embodiments are only used to illustrate the technical solutions of the present application. Those skilled in the art should understand that although the present application has been described in detail with reference to the foregoing embodiments, modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent replacements can be made to some of the technical features. These modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions in the embodiments of the present application.

Claims

1. A density correction method of a gamma-gamma density logging, applied to a density logging instrument, the density logging instrument comprising a natural gamma detector NGR, a long source detector GGFR and a short source detector GGNR; characterized in that The density correction method comprises: Obtaining the count rate ratio R of a long source probe GGFR and a natural gamma probe NGR GGFR / NGR and the count rate ratio R of a short source probe GGNR and a natural gamma probe NGR GGNR / NGR ; the model well is a rock sample with known density and nominal content obtained by testing, to obtain the count rate ratio of the response difference of the long source probe GGFR and the short source probe GGNR relative to the natural gamma probe NGR to the natural gamma ray of the model well; obtaining a density calibration coefficient of the density logging instrument, the density calibration coefficient being calibration data of the density logging instrument determined according to a standard density sample; The density logging instrument is used for logging, and count rates N of a natural gamma detector NGR, a long source detector GGFR, and a short source detector GGNR are obtained NGR , N GGFR , and N GGNR ; Take the count rate N of the natural gamma detector NGR NGR Multiply by the count rate ratio R respectively GGFR / NGR and R GGNR / NGR The count rate N of natural gamma rays from the radioactive formation in the well was obtained for the long-source detector GGFR and the short-source detector GGNR. GGFR N GGNR The interference value is then calculated using the count rate N of the long-source detector GGFR. GGFR The count rate N of the short-source detector GGNR GGNR Subtract the corresponding interference values ​​from each, and calculate the net count rate N of the long-source detector GGFR and the short-source detector GGNR. GGFR净 and N GGNR净 The count rate is corrected to complete the density parameter calculation; calculating a formation density according to the density calibration coefficient and a net count rate.

2. The density correction method of claim 1, wherein The model well is selected from any one of a saturation logging model or a field verification model.

3. The density correction method of claim 1, wherein The count rate ratio R of the long source probe GGFR and the natural gamma probe NGR is obtained by using the model well GGFK / NGR And the count rate ratio R of the short source probe GGNR and the natural gamma probe NGR GGNR / NGR , comprising: A nominal content of N model wells is obtained, N being greater than or equal to 2; Acquiring count rates N of a natural gamma ray detector NGR, a long source detector GGFR and a short source detector GGNR in N model wells NGR ', N GGFR ', N GGNR '; According to the nominal content of N model wells and N counting rates N NGR ’, N GGFR ’ and N GGNR ’, the linear fitting curve of the counting rate and the nominal content of the model well is obtained; The corresponding count rate ratio R is obtained from the ratio of the slopes of the linear fit curves of the natural gamma detector NGR, the long source detector GGFR and the short source detector GGNR GGFR / NGR and R GGNR / NGR .

4. The density correction method of claim 3, wherein The model well has a nominal content of 0.2 to 5167 x 10 -6 g / g.

5. The density correction method according to any one of claims 1 to 4, characterized in that, The net count rates NG of the long source detector GGFR and the short source detector GGNR are obtained using the following equation GFR净 and N GGNR净 , the correction of the count rate for the density parameter solution is completed. N GGFR净 = N GGF R-N NGR x R GGFR / NGR N GGNR净 = N GGNR - N NGR x R GGNR / NGR .

6. The density correction method according to any one of claims 1 to 4, characterized in that, The method for obtaining the density calibration coefficient of the density logging instrument comprises: obtaining an aluminum module and a plexiglass module with known densities as calibration modules; using a cesium source as a radioactive source of the density logging instrument; sequentially mounting the aluminum module and the plexiglass module to a probe of the density logging instrument, and starting the density logging instrument, so that the cesium source emits rays to the calibration modules; recording a count rate reading of the logging instrument and a known density value of the calibration modules, and drawing a relationship curve between the count rate reading and the density according to the recorded count rate reading and the module density; By fitting and analyzing the relationship curve, we obtain lnN=Aρ b +B, and obtain the density calibration coefficients A and B of the density logging tool, where ρ b - Density, in g / cm³ 3 N - Detector count rate, in seconds. -1 A - Sensitivity coefficient, unit is s. -1 / (g / cm 3 B-intercept, which is a constant.

7. The density correction method of claim 1, wherein The method for calculating the formation density according to the density calibration coefficient and the net count rate comprises: calculating the formation density by using the following formula: p b = (1 / A) ln N - B / A where: p b - formation density in g / cm 3 ; N - probe count rate in s -1 ; A - sensitivity factor in s -1 / (g / cm 3 ); B - intercept, a constant.

8. The density correction method of claim 6, wherein, The distance between the cesium source and the short source detector GGNR is 200 mm, the distance between the cesium source and the long source detector GGFR is 350 mm, and the distance between the cesium source and the natural gamma detector NGR is 2500 mm.

9. The density correction method according to any one of claims 1 to 4, characterized in that, The natural gamma detector NGR, the long source detector GGFR and the short source detector GGNR all use sodium iodide crystals, the diameters and lengths of which are 30 mm and 80 mm, 23 mm and 40 mm, and 13 mm and 10 mm respectively, and the energy threshold thereof is 130 kev.

10. The density correction method of claim 9, wherein, The short source detector GGNR is internally provided with a tantalum-silver sheet.

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