Gamma ray spectrum unfolding method for elemental capture spectroscopy logging and device therefor

Abstract

A gamma ray spectrum unfolding method for elemental capture spectroscopy logging and a device therefor including the steps of first preprocessing the data obtained from an elemental capture spectrometry instrument; constructing a primary element group and an auxiliary element group according to the degree of interactions among the elements via theoretical analysis and numerical calculation of spectrum profiles, characteristic peak channels, and backgrounds of different elements; unfolding by using the least square method based on the construction of the primary element group and the auxiliary element group; and finally reconstructing the spectrum based on theory according to the yield of each element obtained by unfolding with the least square method, and comparing the measured gamma ray spectrum with the reconstructed gamma ray spectrum for error control, thereby improving the spectrum unfolding precision.

Claims

1. A gamma ray spectrum unfolding method for elemental capture spectroscopy logging, comprising the following steps: Step 1: acquiring and analyzing research area data at least including elemental capture gamma ray spectroscopy logging data and whole rock oxides analysis data, and determining the primary types of elements in the area; Step 2: preprocessing the elemental capture gamma ray spectroscopy logging data, including selection of energy window, energy spectrum smoothing and filtering, normalization, and inelastic scattering information deduction, to obtain a preprocessed elemental capture gamma ray spectrum; Step 3: constructing a primary element group and an auxiliary element group composed of different elements, to determine an order of spectrum unfolding for different elements; Step 4: according to the primary element group and the auxiliary element group obtained in step 3, first unfolding the primary elements in the elemental capture gamma ray spectrum preprocessed in Step 2 by using the least square method, deducting the contribution of all the primary elements from the elemental capture gamma ray spectrum preprocessed in Step 2, and then unfolding the auxiliary elements by using the least square method, to obtain the relative yield of each element; Step 5: according to the relative yield of each element obtained in Step 4, in combination with the normalized elemental capture standard gamma ray spectrum of single element, reconstructing the elemental capture gamma ray spectrum and comparing the reconstructed gamma ray spectrum with the measured gamma ray spectrum, to determine whether the unfolding results are reliable.

2. The unfolding method according to claim 1, wherein, in Step 1, the acquired and analyzed research area data further include one or a combination of more of conventional logging data, logging data, and geological data; and the types of the primary elements in the research area are determined by: analyzing the whole rock oxides analysis data and optionally, one or a combination of more of conventional logging data, logging data, and geological data from the research area, determining the weight percentages of different kinds of oxides in the rock, and determining the primary types of elements in the area based on the weight percentage of each oxide.

3. The unfolding method according to claim 1, wherein, in Step 2, the energy window selected ranges in channels 30-210.

4. The unfolding method according to claim 1, wherein, in Step 2, the energy spectrum smoothing and filtering is carried out using a Savitzky-Golay filter and the Savitzky-Golay five-point filtering method by filtering the elemental capture gamma ray spectroscopy logging data in the range of the selected energy window, and the equation for the Savitzky-Golay five-point filtering method is shown in Equation 1 as below: $\begin{array}{cc}\stackrel{\_}{y}=\frac{1}{35}\left(-3y_{i-2}+12y_{i-1}+17y_i+12y_{i+1}-3y_{i+2}\right)& \left(\mathrm{Equation}1\right)\end{array}$ wherein, y represents a count at an address after filtering, y.sub.i represents a count at said address, y.sub.i−1 represents a count at the first address before said address, y.sub.i−2 represents a count at the second address before said address, y.sub.i+1 represents a count at the first address after said address, and y.sub.i+2 represents a count at the second address after said address.

5. The unfolding method according to claim 1, wherein in Step 2, the normalization is carried out in a process where, with respect to the elemental capture gamma ray spectroscopy logging data having been subjected to energy window range selection and energy spectrum smoothing and filtering, the sum of the energy spectrum data of 181 channels is taken as 10 to provide a normalized elemental capture gamma ray spectrum by using the equation as shown in Equation 2 below: $\begin{array}{cc}{N}_{\mathrm{Gkj}}=\frac{N_{\mathrm{kj}}}{\underset{k=30}{\overset{210}{.Math.}}{N}_{\mathrm{kj}}}k=30,31,\mathrm{.Math.},210& \left(\mathrm{Equation}2\right)\end{array}$ wherein, N.sub.Gkj is a count of the normalized elemental capture gamma ray spectrum in the k.sup.th channel corresponding to the j depth point, N.sub.kj is a count of the elemental capture gamma ray spectrum upon energy window range selection and energy spectrum smoothing and filtering in the k.sup.th channel corresponding to the j depth point, k is the channel address, and j is the depth point below the formation.

6. The unfolding method according to claim 1, wherein, in Step 2, the inelastic scattering information deduction is carried out in a process where the counts of three segments of the channel addresses of channels 30-54, 55-75, and 76-210 selected from the normalized elemental capture gamma ray spectrum are subjected to inelastic information deduction using a deduction coefficient of 0.9, 0.7, and 0.8, respectively.

7. The unfolding method according to claim 1, wherein, in Step 3, the primary element group constructed comprises one or more elements of Si, Ca, S, H, Cl, Ti, Fe, Na, Ba, and Gd; and the auxiliary element group constructed comprises one or more elements of Mg, K, Cr, Ni, I, Tb, and Al.

8. The unfolding method according to claim 1, wherein, in Step 4, the unfolding by using the least square method is performed according to a calculation as shown in the following Equation 3 to obtain the relative yield of each element: $\begin{array}{cc}{c}_i=\underset{j=1}{\overset{m}{.Math.}}{a}_{\mathrm{ij}}{y}_j+{\mathrm{.Math.}}_i& \left(\mathrm{Equation}3\right)\end{array}$ wherein, y.sub.j is a relative yield of the j.sup.th element, a.sub.ij is a count of the normalized elemental capture standard gamma ray spectrum of single element of the j.sup.th element in the i.sup.th channel, c.sub.i is a count of the preprocessed elemental capture gamma ray spectrum in the i.sup.th channel, and ε.sub.i is a correction factor, where ε.sub.i≤0.1.

9. The unfolding method according to claim 8, wherein, in Step 5, the normalized elemental capture standard gamma ray spectrum of single element is obtained in a process where, with respect to the elemental capture standard gamma ray spectrum of single element, the sum of the energy spectrum data of 256 channels is taken as 10 by using the equation as shown in Equation 4 below: $\begin{array}{cc}{N}_{\mathrm{Gkj}}=\frac{N_{\mathrm{kj}}}{\underset{k=0}{\overset{255}{.Math.}}{N}_{\mathrm{kj}}}k=0,1,\mathrm{.Math.},255& \left(\mathrm{Equation}4\right)\end{array}$ wherein, N.sub.Gkj is a count of the normalized elemental capture standard gamma ray spectrum of single element in the k.sup.th channel corresponding to the j depth point, N.sub.kj is a count of the elemental capture standard gamma ray spectrum of single element before normalization in the k.sup.th channel corresponding to the j depth point, k is the channel address, and j is the depth point below the formation.

10. The unfolding method according to claim 1, wherein, in Step 5, reconstructing the elemental capture gamma ray spectrum comprises calculating the count of the reconstructed gamma ray spectrum in the i.sup.th channel according to the following Equation 5 to obtain the count in each channel, and plotting the reconstructed gamma ray spectrum: $\begin{array}{cc}{X}_i=\underset{j=1}{\overset{m}{.Math.}}{a}_{\mathrm{ij}}{y}_j+{\mathrm{.Math.}}_i& \left(\mathrm{Equation}5\right)\end{array}$ wherein, X.sub.i is a count of the reconstructed gamma ray spectrum in the i.sup.th channel, y.sub.j is a relative yield of the j.sup.th element, and a.sub.ij is a count of the normalized elemental capture standard gamma ray spectrum of single element of the j.sup.th element in the i.sup.th channel, and ε.sub.i is a correction factor, where ε.sub.i≤0.1.

11. The unfolding method according to claim 1, wherein, in Step 5, comparing the reconstructed gamma ray spectrum with the measured gamma ray spectrum is carried out according to Equation 6:
|c.sub.i−X.sub.i|ε  (Equation 6) c.sub.i is a count of the measured gamma ray spectrum in the i.sup.th channel, X.sub.i is a count of the reconstructed gamma ray spectrum in the i.sup.th channel, and ε is the relative error; and when the relative error is less than or equal to 5%, the unfolding results are regarded as reliable; when the relative error is greater than 5%, the unfolding results are regarded as unreliable.

12. A gamma ray spectrum unfolding device for elemental capture spectroscopy logging, comprising: a module for acquiring data and determining the primary element types, configured to acquire and analyze research area data at least including elemental capture gamma ray spectroscopy logging data and whole rock oxide analysis data, in order to determine the primary types of elements in the area; a module for preprocessing the elemental capture gamma ray spectroscopy logging data, configured to preprocess the elemental capture gamma ray spectroscopy logging data, including selection of energy window, energy spectrum smoothing and filtering, normalization, and inelastic scattering information deduction, in order to obtain a preprocessed elemental capture gamma ray spectrum; a module for constructing a primary element group and an auxiliary element group, configured to construct a primary element group and an auxiliary element group composed of different elements, in order to determine an order of spectrum unfolding for different elements; a module for unfolding with the least square method on the basis of element groups, configured to, according to the primary element group and the auxiliary element group, firstly unfold the primary elements in the preprocessed elemental capture gamma ray spectrum by using the least square method, deduct the contribution of all the primary elements from the preprocessed elemental capture gamma ray spectrum, and then unfold the auxiliary elements by using the least square method, thereby obtaining the relative yield of each element; and a module for reconstructing gamma ray spectrum and error control, configured to reconstruct the elemental capture gamma ray spectrum according to the relative yield of each element in combination with the normalized elemental capture standard gamma ray spectrum of single element, and compare the reconstructed gamma ray spectrum with the measured gamma ray spectrum to determine whether the unfolding results are reliable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic diagram of the selection of the energy window for unfolding.

(2) FIG. 2 shows a comparison diagram between the elemental capture standard gamma ray spectra of single element: the element Na and the element Mg.

(3) FIG. 3 shows the unfolding effects for each element in a DAS3 well at 2403.1956 m.

(4) FIG. 4 shows a comparison diagram of the reconstructed gamma ray spectrum and the measured gamma ray spectrum of the DAS3 well at 2403.1956 m.

(5) FIG. 5 shows a comparison diagram of the preprocessed elemental capture gamma ray spectrum and the measured gamma ray spectrum of the DAS3 well at 2403.1956 m.

(6) FIG. 6 shows a comparison diagram of the reconstructed gamma ray spectrum of the primary element group and the measured gamma ray spectrum of the DAS3 well at 2403.1956 m.

(7) FIG. 7 shows a schematic diagram of a gamma ray spectrum unfolding device for elemental capture spectroscopy logging provided in Example 2.

DETAILED DESCRIPTION

(8) In order to provide a clearer understanding of the technical features, the objects and the advantages of the present disclosure, detailed description of the technical solutions of the present disclosure will be made hereinafter in conjunction with the practical field application examples in oilfields, which is not to be construed as limitation to the scope of the present disclosure.

Example 1

(9) This example provides a gamma ray spectrum unfolding method for elemental capture spectroscopy logging, which may comprise the following steps:

(10) 1. Acquirement of the measured spectrums of total elemental gamma ray spectroscopy and determination of the distribution of primary elements in the research area

(11) A DAS3 well in the Daqing Oilfield in the reservoir section 3230-3270 m was logged by using an elemental capture energy spectrum logging instrument, and the total gamma ray spectrum of the stratigraphic elements at each depth point was acquired, that is, the elemental capture gamma ray spectroscopy logging data, with each depth point corresponding to one spectrum. The whole rock oxide analysis experiment results and logging data of the DAS3 well in the Daqing Oilfield were acquired and analyzed, and it was found that the reservoir section 3230 m-3270 m primarily comprised the elements Si, Ca, Al, Fe, S, K, Na, Mg, Ti, Gd, H, and Ba, with the oxides of these elements accounting for 98.9% by weight of the content of rock.

(12) 2. Preprocessing of the elemental capture gamma ray spectroscopy data

(13) The measured spectra at various depth points in the DAS3 well reservoir section 3230-3270 m were respectively preprocessed, and the preprocessed elemental capture gamma ray spectrum corresponding to each depth point was obtained respectively, wherein the preprocessing included selection of energy window, energy spectrum smoothing and filtering, normalization, and inelastic scattering information deduction.

(14) Here, the selected energy window range was channels 30-210, as shown in FIG. 1.

(15) The energy spectrum smoothing and filtering was carried out using a Savitzky-Golay filter and the Savitzky-Golay five-point filtering method by filtering the measured spectrum in the range of the selected energy window. The equation for the Savitzky-Golay five-point filtering method is shown in Equation 1 as below:

(16) $\begin{array}{cc}\stackrel{\_}{y}=\frac{1}{35}\left(-3y_{i-2}+12y_{i-1}+17y_i+12y_{i+1}-3y_{i+2}\right)& \left(\mathrm{Equation}1\right)\end{array}$

(17) wherein, y represents a count at an address after filtering, y.sub.i represents a count at said address, y.sub.i−1 represents a count at the first address before said address, y.sub.i−2 represents a count at the second address before said address, y.sub.i+1 represents a count at the first address after said address, and y.sub.i+2 represents a count at the second address after said address.

(18) The normalization was carried out in a process where, with respect to the measured spectrum having been subjected to energy window range selection and energy spectrum smoothing and filtering, the sum of the energy spectrum data of 181 channels (channel by channel) was taken as 10 to provide a normalized measured spectrum by using the equation as shown in Equation 2 below:

(19) $\begin{array}{cc}{N}_{\mathrm{Gkj}}=\frac{N_{\mathrm{kj}}}{\underset{k=30}{\overset{210}{.Math.}}{N}_{\mathrm{kj}}}k=30,31,\mathrm{.Math.},210& \left(\mathrm{Equation}2\right)\end{array}$

(20) wherein, N.sub.Gkj was a count of the normalized measured spectrum in the k.sup.th channel corresponding to the j depth point, N.sub.kj was a count of the measured spectrum upon energy window range selection and energy spectrum smoothing and filtering in the k.sup.th channel corresponding to the j depth point, k was the channel address, and j was the depth point below the formation.

(21) The inelastic scattering information deduction was carried out in a process where the counts of three segments of the channel addresses of channels 30-54, 55-75, and 76-210 selected from the normalized measured spectrum were subjected to inelastic information deduction using a deduction coefficient of 0.9, 0.7, and 0.8, respectively.

(22) 3. Construction of the primary element group and the auxiliary element group

(23) Based on Step 1, it was acknowledged that the primary element types in the DAS3 well reservoir section 3230 m-3270 m were Si, Ca, Al, Fe, S, K, Na, Mg, Ti, Gd, H, and Ba. A primary element group was constructed on the basis thereof, comprising: Si, Ca, S, Ti, Fe, Na, and Gd. In the construction of the auxiliary element group, the influence of the instrument background was taken into account, and the element Tb was included so that the auxiliary element group was constructed as: Mg, K, Al, and Tb.

(24) 4. Spectrum unfolding with the least square method based on element groups

(25) Based on the primary element group and the auxiliary element group determined in Step 3, in the elemental capture gamma ray spectrum preprocessed in Step 2, the primary elements (i.e., Si, Ca, S, Ti, Fe, Na, H, Ba, and Gd) were first unfolded by using the least square method, and then the contribution of all the primary elements was deducted from the preprocessed total element gamma ray spectrum (i.e., the elemental capture gamma ray spectrum preprocessed in Step 2); the auxiliary elements (i.e., Mg, K, Al, and Tb) were unfolded by using the least square method, so that the relative yield of each element was determined.

(26) The unfolding with the least square method was performed according to a calculation shown in the following Equation 3 so as to obtain the relative yield of each element:

(27) $\begin{array}{cc}{c}_i=\underset{j=1}{\overset{m}{.Math.}}{a}_{\mathrm{ij}}{y}_j+{\mathrm{.Math.}}_i& \left(\mathrm{Equation}3\right)\end{array}$

(28) wherein, y.sub.j is a relative yield of the j.sup.th element (i.e., the contribution of the element to the total count), a.sub.ij is a count of the normalized elemental capture standard gamma ray spectrum of single element of the j.sup.th element in the i.sup.th channel, c.sub.i is a count of the preprocessed elemental capture gamma ray spectrum in the i.sup.th channel, and ε.sub.i is a correction factor, where ε.sub.i≤0.1.

(29) Taking the elemental capture gamma ray spectrum obtained at the depth point of 2403.1956 m in the DAS3 well as an example, the relative yield of each element obtained by unfolding was shown in FIG. 3. Since Td was background yield with no reference value, it was not shown in FIG. 3.

(30) 5. Reconstruction of the gamma ray spectrum and error control

(31) According to the relative yield of each element obtained in Step 4 in combination with the normalized elemental capture standard gamma ray spectrum of single element, the elemental capture gamma ray spectrum was reconstructed, and the reconstructed gamma ray spectrum was compared with the measured gamma ray spectrum to determine whether the unfolding results were reliable.

(32) Here, the normalized elemental capture standard gamma ray spectrum of single element was obtained in a process where, with respect to the elemental capture standard gamma ray spectrum of single element, the sum of the energy spectrum data of 256 channels (channel by channel) was taken as 10 by using the equation as shown in Equation 4 below:

(33) $\begin{array}{cc}{N}_{\mathrm{Gkj}}=\frac{N_{\mathrm{kj}}}{\underset{k=0}{\overset{255}{.Math.}}{N}_{\mathrm{kj}}}k=0,1,\mathrm{.Math.},255& \left(\mathrm{Equation}4\right)\end{array}$

(34) wherein, N.sub.Gkj was a count of the normalized elemental capture standard gamma ray spectrum of single element in the k.sup.th channel corresponding to the j depth point, N.sub.kj was a count of the elemental capture standard gamma ray spectrum of single element before normalization in the k.sup.th channel corresponding to the j depth point, k was the channel address, and j was the depth point below the formation.

(35) Because the count of the first two channel addresses was considered practically meaningless, the elemental capture standard gamma ray spectrum of single element could also be normalized by using the following Equation 4-1:

(36) $\begin{array}{cc}{N}_{\mathrm{Gkj}}=\frac{N_{\mathrm{kj}}}{\underset{k=2}{\overset{255}{.Math.}}{N}_{\mathrm{kj}}}k=2,3,\mathrm{.Math.},255.& \left(\mathrm{Equation}4\text{-}1\right)\end{array}$

(37) The reconstruction of the elemental capture gamma ray spectrum was carried out by calculating the count of the reconstructed gamma ray spectrum in the i.sup.th channel (i.e., the count of all unfolded elements in the i.sup.th channel) according to the following Equation 5 to obtain the count in each channel, and then plotting the reconstructed gamma ray spectrum:

(38) $\begin{array}{cc}{X}_i=\underset{j=1}{\overset{m}{.Math.}}{a}_{\mathrm{ij}}{y}_j+{\mathrm{.Math.}}_i& \left(\mathrm{Equation}5\right)\end{array}$

(39) wherein, X.sub.i was a count of the reconstructed gamma ray spectrum in the i.sup.th channel, y.sub.j was a relative yield of the j.sup.th element (calculated from Equation 3), and a.sub.ij was a count of the normalized elemental capture standard gamma ray spectrum of single element of the j.sup.th element in the i.sup.th channel (calculated from Equation 4), and ε.sub.i was a correction factor, where ε.sub.i≤0.1.

(40) The comparison between the reconstructed gamma ray spectrum with the measured gamma ray spectrum was performed according to Equation 6:
|c.sub.i−X.sub.i∥C.sub.i−X.sub.i|ε  (Equation 6)

(41) c.sub.i was a count of the measured gamma ray spectrum in the i.sup.th channel, X.sub.i was a count of the reconstructed gamma ray spectrum in the i.sup.th channel, and ε was the relative error; and

(42) when the relative error was less than or equal to 5%, the unfolding results were regarded as reliable; when the relative error was greater than 5%, the unfolding results were regarded as unreliable.

(43) Taking the unfolding results of the elements at the depth point of 2403.1956 m in the DAS3 well as an example, the reconstructed gamma ray spectrum and the measured gamma ray spectrum (FIG. 4), the preprocessed elemental capture gamma ray spectrum and the measured gamma ray spectrum (FIG. 5), the reconstructed gamma ray spectrum of the primary element group (that is, the gamma ray spectrum reconstructed after the count of each channel was determined by calculating the count of all the primary elements in the i.sup.th channel according to Equation 5) and the measured gamma ray spectrum (FIG. 6) were compared. As can be seen from FIGS. 4-6, the unfolding results of this example were reliable, and the unfolding method was precise and effective.

Example 2

(44) This example provided a gamma ray spectrum unfolding device for elemental capture spectroscopy logging, as shown in FIG. 7, including a module 101 for acquiring data and determining the primary element types, configured to acquire and analyze research area data at least including elemental capture gamma ray spectroscopy logging data and whole rock oxide analysis data, in order to determine the primary types of elements in the area; a module 102 for preprocessing the elemental capture gamma ray spectroscopy logging data, configured to preprocess the elemental capture gamma ray spectroscopy logging data, including selection of energy window, energy spectrum smoothing and filtering, normalization, and inelastic scattering information deduction, in order to obtain a preprocessed elemental capture gamma ray spectrum; a module 103 for constructing a primary element group and an auxiliary element group, configured to construct a primary element group and an auxiliary element group composed of different elements, in order to determine an order of spectrum unfolding for different elements; a module 104 for unfolding with the least square method on the basis of element groups, configured to, according to the primary element group and the auxiliary element group, firstly unfold the primary elements in the preprocessed elemental capture gamma ray spectrum by using the least square method, deduct the contribution of all the primary elements from the preprocessed total elemental capture gamma ray spectrum (i.e., the preprocessed elemental capture gamma ray spectrum), and then unfold the auxiliary elements by using the least square method, thereby obtaining the relative yield of each element; and a module 105 for reconstructing gamma ray spectrum and error control, configured to reconstruct the elemental capture gamma ray spectrum according to the relative yield of each element in combination with the normalized elemental capture standard gamma ray spectrum of single element, and compare the reconstructed gamma ray spectrum with the measured gamma ray spectrum to determine whether the unfolding results were reliable.