RU2422857C1 - Method of calibrating downhole spectrometers - Google Patents

Abstract

FIELD: physics. ^ SUBSTANCE: natural or induced gamma radiation is detected. Spectral flux density is determined. The null reading of the spectrometer is corrected. The first channel is that channel in which non-zero spectrum intensity was detected. The current spectrum is compared with spectrum of Compton scattering of the gamma radiation and the observed spectrum is transformed through normalisation thereof to an analytically approximated spectrum of Compton radiation using a given formula. The peak of the maximum amplitude ia and the peak with the highest number in are then selected from significant peaks. One of these peaks is then matched with the reference line of known energy E0 present in the detected radiation, and energy E of the detected radiation is then calculated using the formula E = ki, where k=E0/in, (or k=E0/ia), E is radiation energy, MeV, i is the number of channels. ^ EFFECT: high measurement accuracy, easy calibration of downhole equipment and less time spent on implementing the method. ^ 2 dwg

Description

The invention relates to the field of nuclear geophysics and is used for automatic energy calibration of downhole spectrometers with a steel casing, recording natural gamma radiation or neutron gamma radiation, having a non-linearity of not more than ± 2% and intended for the study of cased and uncased oil and gas, ore and engineering wells, using stationary or pulsed neutron sources.

A number of methods for energy calibration of downhole spectrometers are known.

Guidelines governing the conduct of LPG (spectrometric neutron gamma ray logging) in ore deposits, the spectrometer’s energy scale is prescribed to be installed on the surface before logging using the position of the pair peak of iron (6.6 MeV) on the instrument spectrum of the field calibration device (PCU), followed by exposure windows ; during the logging process, the initial energy calibration is not adjusted [1].

The disadvantage of this method is the low accuracy and the inability to use for oil and gas wells.

Known downhole tool for gamma-ray logging equipment (SGK - spectrometric gamma-ray logging), which provides a system of automatic gain control (AGC), tuned to peak Cs-137 (0.66 MeV) [2].

This method requires the use of a radioactive source, which is undesirable; in addition, there is a danger of overloading the spectrometric path.

A known method of calibrating detectors when recording natural or induced Y-radiation (gamma radiation) by detectors, in which the intensities of the spectral flows are determined in the given energy intervals of each spectrum, the boundaries of which are determined by the position of the reference Y-line in the spectrum of the first detector. The first detector is surrounded by a screen that absorbs Y-radiation and displays X-ray radiation (RI). The Y-spectra from each of the detectors are recorded simultaneously in two energy ranges, for which threshold selection and amplification of signals with coefficients K ₁ and K _{2 are} carried out, then the channel numbers of Y-radiation peaks of naturally radioactive nuclides are found in hollow Y-spectra by position the channel number, the peak of the RI peak n ₁ and the value of its energy E _{1 are} calculated in a first approximation, the energy scales of the Y-spectrum of the first detector according to certain formulas. In the main high-energy spectrum, a peak is found with a minimum measurement error and, using the energy scale value, approximate energy values are calculated using a certain formula. Identify the peak energy with a minimum measurement error E _i ^min using certain ratios. Using the found energy and the channel number of the peak E _i ^min and n _i ^min and the position of the maximum RI (E ₁ , n ₁ ), we calculate the second approximation for the energy scale and the calibration equation by certain expressions, identify the energy value for all other peaks in the spectrum of the hard component of the first detector, according to the found E _i and n _i for all peaks of the spectrum of the rigid components of the first detector, the true scale is calculated, for which the calibration equation is found by the least squares method (n.c.). The energy scales for the remaining detectors are determined to a first approximation by the position of the peak E _i ^min and n _i ^min in them, after which calibration equations are found for each detector, approximate energy values for the remaining peaks of each spectrum are calculated, and they are also identified by the n method. to. establish the final form of the calibration equations and the scales of the detectors, then adjust the amplification factors a certain number of times. The technical result of the invention is to increase the accuracy of measurements [3].

This method makes high demands on the spectrometer (the identity of the energy calibrations of several detectors) and is very difficult to implement.

In another known method, the energy calibration of the INGKS (pulsed neutron gamma ray spectrometric) equipment is refined, which is carried out at the processing stage in an interactive mode by comparing the current spectrum with the reference one [4].

This method makes high demands on the skills of the contractor and significantly increases the processing time of logging materials.

The objective of the proposed method is to increase the accuracy of measurements, simplifying the technology of calibration of downhole equipment and reducing the time spent on its implementation.

The problem is solved in that in a method for calibrating downhole spectrometers that provide a measurement of the spectrum in at least 128 channels (discrimination levels) and are characterized by a nonlinearity of the energy scale of not more than 2%, which includes recording natural or induced gamma radiation, determining the intensities of spectral flows, the instrument zero of the spectrometer is corrected, and the channel in which nonzero spectrum intensities are first recorded is taken as the first channel; comparing the current spectrum with the spectrum of Compton scattering of gamma radiation and transforming the observed spectrum by normalizing it to an analytically approximated spectrum of Compton radiation according to the formula:

Where

SpTr (i) - transformed spectrum, relative unit (p.u.)

Sp (i) - initial spectrum after zero correction, pulse / s

i - channel numbers after correction of the "zero" of the spectrometer,

a _j (j = 0, 1, 2, 3), are the coefficients determined by the least squares method (n.c.),

Further, in the transformed spectrum, the channel numbers of i significant peaks are determined by the criterion:

Where

SpTr (i) - transformed spectrum,

i - channel numbers,

then, among the significant peaks, a peak of maximum amplitude i _a and a peak with the highest number i _n are distinguished, one of these peaks is identified with a reference line of known energy E ₀ present in the detected radiation, and the energy E of the detected radiation is calculated by the formula:

Where

k = E ₀ / i _n (or k = E ₀ / i _a )

E is the radiation energy, MeV,

i - channel numbers.

Figure 1 shows an example of calibration of the equipment SGK,

where 1-A is the initial spectrum, 1-B is the transformable spectrum, 1-C is the spectrum after calibration.

Figure 2 shows an example of calibration of the equipment LPG,

where 2-A is the initial spectrum, 2-B is the transformable spectrum, 2-C is the spectrum after calibration.

The method is as follows.

When researching wells with spectrometers that record natural gamma activity (GCS), or neutron gamma radiation caused by ampoule or pulsed neutron sources (CGCs), the intensities of the recorded spectral fluxes are determined, to improve the measurement accuracy, the instrument zero of the spectrometer is corrected, while the first the channel receives a channel in which nonzero spectrum intensities are first recorded; an operation is performed to compare the current spectrum with the spectrum of Compton scattered radiation and transform the observed spectrum by normalizing it to an analytically approximated spectrum of Compton radiation according to formula (1).

Due to this transformation, the contrast of the spectrum and its information content in the high-energy region significantly increase.

After this operation, significant peaks of energies are isolated in the transformed spectrum, using which, based on the presence of known peaks in the registered spectrum of reference energies, the spectrometer is calibrated with energy. The automatic energy calibration algorithm for the equipment is embedded in the control computer program.

Examples of practical application.

Figure 1 shows an example of automatic energy calibration of the equipment SGK.

When exploring wells with spectrometers, natural gamma activity (SGC) is recorded.

First, the hardware zero of the spectrometer (1-A) is corrected.

Next, the measured spectrum is transformed according to formula (1). In the transformed spectrum, peaks i _n and i _a (1-B) are distinguished. The peak i _n corresponds to an energy of 2.62 MeV (T1-208) (Reference data p.470 [5]). Hence, the initial value of the coefficient k ₀ is equal to k ₀ = 2.62 / i _n . Peak i _a can have one of three energies: 0.93 (E ₁ - Ac-228), 1.46 (E ₂ - K-40), 1.76 (E ₃ - Bi-214).

We calculate E ₄ = k ₀ · i _a . Abs (E ₄ -E ₁ ) <0.1; therefore, we take E ₄ = E ₁ , k ₁ = E ₁ / i _a , E = k ₁ · i. The resulting energy calibration of the SGC spectrum is shown in 1-C (Fig. 1).

Figure 2 shows an example of automatic calibration of LPG equipment.

When researching wells with spectrometers, neutron gamma radiation caused by ampoule or pulsed neutron sources (CIS) is recorded.

Correction 0 in this case was not required.

After spectrum transformation, the peak i _n (2-B) is isolated. This peak corresponds to an energy of 7.6 MeV - gamma radiation of radiation capture (GIRZ) of iron (Tabular data. Appendix 4, p.520 [5]). The initial value of the coefficient k ₀ is 7.6 / i _n . Based on the obtained value of k ₀ , we find the spectral range (i _n1 , i _n2 ) (2.2.4) MeV, in which the peak of the GIRD of hydrogen (2.2 MeV) is located. In this interval, we find the peak i _a . The adjusted value of the coefficient k ₁ will be equal to 2.2 / i _a . The energy calibration of the CIS spectrum obtained as a result of the described algorithm is shown in 2-C (Fig. 2).

The proposed method is intended for automatic energy calibration at the stage of processing logging materials of multichannel (with the number of channels not less than 128) spectrometers designed for well research by any of the methods - SGK, SNGK, the nonlinearity of the energy scale of which does not exceed ± 2%; it is assumed that there were no “noises” of the spectrometer during the logging process.

Information sources

1. Guidelines for the use of spectrometric gamma-ray neutron logging in solid mineral deposits. - M., 1986.

2. Urmanov E.G. Spectrometric gamma-ray logging of oil and gas wells. - M.: VNIIOENG, 1994.

3. RF patent No. 2159451, G01V 5/04, “Gamma-ray logging method”, publ. 11/20/2000.

4. Instructions for conducting pulsed spectrometric neutron gamma-ray logging and processing of measurement results in assessing the current oil saturation of rocks (terrigenous deposits). - Tver, 2008.

5. Filippov EM Nuclear exploration of minerals. Directory. - K .: "Science Dumka", 1978.

Claims (1)

A method for calibrating downhole spectrometers, comprising recording natural or induced gamma radiation, determining spectral flux intensities, characterized in that the spectrometer is instrumented for zero, while the first channel is taken to be a channel in which nonzero spectrum intensities are first recorded, and the current spectrum is compared with the spectrum of Compton scattering of gamma radiation and transform the observed spectrum by normalizing it to analysis a practically approximated spectrum of Compton radiation according to the formula:

where SpTr (i) is the transformed spectrum, the relative unit (pu);
Sp (i) is the initial spectrum, momentum / s;
i - channel numbers after correction of the “zero” of the spectrometer;
a _j (j = 0, 1, 2, 3) are the coefficients determined by the least squares method (n.c.), dimensionless further in the transformed spectrum, determine the channel numbers of I significant peaks by the criterion:

where SpTr (i) is the transformed spectrum;
i - channel numbers,
then, among the significant peaks, a peak of maximum amplitude i _a and a peak with the highest number i _n are distinguished, one of these peaks is identified with a reference line of known energy E ₀ present in the detected radiation, and the energy E of the detected radiation is calculated by the formula:

where k = E ₀ / i _n , (or k = E ₀ / i _a );
E - radiation energy, MeV;
i - channel numbers.

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