CN111123379A - Pure non-elastic gamma energy spectrum acquisition method based on dual-spectrum combination - Google Patents
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 5
- 229910052799 carbon Inorganic materials 0.000 description 4
- 229910052760 oxygen Inorganic materials 0.000 description 4
- 238000004088 simulation Methods 0.000 description 4
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 3
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 3
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- 239000001301 oxygen Substances 0.000 description 3
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 description 2
- 239000004568 cement Substances 0.000 description 2
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- G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
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- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V5/00—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity
- G01V5/04—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging
- G01V5/08—Prospecting or detecting by the use of ionising radiation, e.g. of natural or induced radioactivity specially adapted for well-logging using primary nuclear radiation sources or X-rays
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Abstract
The invention discloses a pure non-elastic gamma energy spectrum acquisition method based on double-ordinary combination, and particularly relates to the technical field of formation physical logging. The method comprises the following steps: acquiring a gamma total spectrum, a capture gamma energy spectrum and a time spectrum of a pulse zone according to logging data; calculating the gamma attenuation slope by using the time spectrum and deducting the background influence of the gamma total spectrum of the pulse area and the captured gamma energy spectrum; calculating the capture gamma count of a certain energy channel at the pulse ending time and the capture gamma increasing time, determining a capture gamma increasing function, integrating the capture gamma increasing function of the pulse area, and making a ratio with the capture part to obtain a deduction coefficient of each energy channel; and calculating the deduction amount of the corresponding energy channel according to the product of the deduction coefficient of each energy channel and the capture gamma energy spectrum, and subtracting the deduction amount of the corresponding energy channel by using the pulse area gamma total spectrum to obtain a net inelastic scattering gamma energy spectrum.
Description
Technical Field
The invention relates to the technical field of formation physical logging, in particular to a pure non-elastic gamma-ray energy spectrum acquisition method based on double-ordinary combination.
Background
The carbon-oxygen ratio logging is to utilize a pulse neutron source to emit 14MeV high-energy fast neutrons to a stratum, generate inelastic scattering effect with C, O elements in stratum pores and release gamma rays of 4.43MeV and 6.14MeV, and because petroleum contains a large amount of C elements and almost no O elements, while water contains a large amount of O elements and no C elements, the oil and water identification and the residual oil saturation monitoring can be realized by measuring the counting of C windows and O windows and calculating the C/O ratio.
The pulse neutron source emits 14MeV fast neutrons to the stratum with a certain pulse width and a certain repetition period, after the neutrons enter the stratum, the neutrons firstly generate inelastic scattering with atomic nuclei of certain elements in the stratum and emit 'inelastic scattering' gamma rays, and 10 after the fast neutrons are emitted-8-10-6In the s time interval, inelastic scattering is the dominant way neutrons lose energy, so inelastic scattering occurs for the duration of the pulse emission, and this process also stops immediately when the neutron emission stops. In the following 10-6-10-3During s time, the main processes of action are elastic scattering, neutron thermalization and capture of radiation. In the pulse emission cycle time, due to multiple cycles, the gamma energy spectrum measured in the pulse region is a mixed spectrum of the pulse region gamma total spectrum and the partial capture gamma energy spectrum, so that when measuring C/O, the pure pulse region gamma total spectrum is obtained by deducting the partial capture gamma energy spectrum, and further the residual oil saturation is accurately obtained by utilizing the C/O.
At present, the method for the gamma total spectrum of the net pulse region mainly comprises a fixed coefficient method, a hydrogen reduction peak method and a sectional deduction method, and because the formation environment is changeable in the actual measurement process, the error of the fixed coefficient method is large, and the sectional deduction method cannot determine the deduction coefficient of a specific formation. Meanwhile, due to radioactive statistical errors, when the speed measurement is fast, the measured energy spectrum fluctuation is large, the fluctuation of the net spectral coefficient of the hydrogen reduction peak method is large, the gamma total spectrum of a net pulse region of the stratum cannot be accurately acquired, the correlation with the theoretical net non-elastic gamma scattering energy spectrum is low, and the accuracy of calculating the C/O ratio to obtain the saturation of the residual oil is low.
Disclosure of Invention
The invention aims to overcome the defects, and provides a pure non-elastic gamma energy spectrum acquisition method based on double-ordinary combination, which can still accurately acquire a pure non-elastic gamma scattering energy spectrum when the statistical fluctuation is large.
The invention specifically adopts the following technical scheme:
a pure non-elastic gamma energy spectrum acquisition method based on bispectrum combination comprises the following specific steps:
the method comprises the following steps: in the C/O mode well logging process, simultaneously recording a non-elastic scattering gamma energy spectrum, a capture gamma energy spectrum and a time spectrum;
step two: the time spectrum meets the logarithmic attenuation characteristic of the gamma ray, the specific gravity of the pulse section and the capture section in the next cycle of the previous cycle is calculated according to the specific gravity, the proportion of the previous pulse in the next pulse calculated by the time spectrum is used as the 'background proportion', and the background in the non-bullet scattering gamma energy spectrum and the capture gamma energy spectrum is correspondingly deducted;
step three: recording the energy channel count of the capture gamma, wherein the energy channel count accords with a logarithmic linear attenuation rule in time, and calculating the total capture gamma count of the energy at the pulse emission ending moment by reverse extrapolation;
step four: in the pulse section, the generation of the capture gamma meets the exponential growth, an exponential growth function is determined by using the starting time and the ending time of pulse emission, and the function is subjected to time domain integration to obtain the total capture gamma count of each energy channel generated in the pulse emission time; dividing capture gamma counts generated by each energy channel in the pulse time period by corresponding energy channel counts in the capture time period to obtain a deduction coefficient of the corresponding energy channel;
step five: and calculating the deduction amount of the corresponding energy channel according to the product of the deduction coefficient of each energy channel and the capture gamma energy spectrum, and subtracting the deduction amount of the corresponding energy channel by using the pulse area gamma total spectrum to obtain a net inelastic scattering gamma energy spectrum.
Preferably, in the step one, the non-elastic scattering gamma energy spectrum includes the influence of the capture gamma energy spectrum, and the time spectrum covers the whole pulse time period and part of the capture time period, including the pulse emission time and 100 μ s after the emission is finished.
Preferably, the logarithmic attenuation characteristic of the gamma ray in the second step has a slope comprising:
the count rate at the end of the previous cycle continues to decay for a cycle time with the same logarithmic decay slope that depends on the borehole and formation conditions and is obtained using a least squares method.
Preferably, in the third step, the capture gamma count of each energy channel with the background deducted is used for calculating the total capture gamma count of the energy at the pulse emission end time according to logarithmic time linear attenuation reverse-push.
Preferably, the pulse region capture gamma in step four increases with an exponential function, which is equation (1),
Ni＝Nαi(1-e-tΣ) (1)
wherein N isiCapture gamma count rate for the ith energy track in the burst region; n is a radical ofαiThe capture gamma counting rate of the ith energy channel at the pulse emission cut-off moment is obtained by logarithmic linear attenuation reverse-deducing in the fourth step; t is t1-t0，t1At the end of the pulse, t0The moment when the pulse count starts to increase; Σ is the neutron capture cross section, affected by the borehole medium.
Preferably, the subtraction coefficient corresponding to each energy channel in the fourth step satisfies formula (2),
wherein, ηiIs the ith energyCoefficient of subtraction of traces, NCapiCount the ith trace in the capture gamma spectrum.
Preferably, the pure inelastic scattering gamma obtained in step five can be represented by formula (3):
wherein N isIne_pIs a pure inelastic scattering gamma spectrum, NIneη for recorded non-elastic scattering gamma spectraiIs the subtraction factor of the ith energy track, NCapiCount the ith trace in the capture gamma spectrum.
The invention has the following beneficial effects:
starting from the time spectrum attenuation characteristic, the method adopts an exponential growth function to approximate the capture gamma change rule in the pulse emission time period, further calculates the capture gamma count of each energy channel in the pulse area and the capture gamma count ratio of the capture area to obtain a deduction coefficient, calculates the deduction amount of the corresponding energy channel according to the product of the deduction coefficient of each energy channel and the capture gamma energy spectrum, and subtracts the deduction amount of the corresponding energy channel by using the pulse area gamma total spectrum to obtain a net inelastic scattering gamma energy spectrum.
Drawings
FIG. 1 is a flow chart of a pure non-elastic gamma energy spectrum acquisition method based on bipode combination;
FIG. 2 is a schematic diagram of time spectrum counting and measurement periods of the burst section gamma total spectrum and the capture gamma energy spectrum within the corresponding time window;
FIG. 3 is a schematic diagram of a C/O tool logging MCNP model;
FIG. 4 is a time-averaged spectrum of 20% porosity sandstone simulated using MCNP, a net non-ballistic time spectrum, and a capture time spectrum subtracted therefrom;
FIG. 5 is a schematic diagram showing the capture gamma time spectrum under the condition of 20% porosity sandstone formation, the capture gamma time spectrum fitted by using a growth function, and the capture gamma time spectrum obtained by MCNP simulation calculation;
FIG. 6 is a schematic diagram of the deduction coefficients of each energy channel of a 20% water-containing sandstone scale well stratum;
figure 7 is a graphical representation of the carbon window and oxygen window count coincidence for two sandstone oil and water containing graduated wells.
Wherein, 1 is the stratum, 2 is near gamma detector, 3 is well gamma detector, 4 is far gamma detector, 5 is cement sheath, 6 is the sleeve pipe, 7 is gamma shielding body, 8 is D-T neutron source, 9 is oil pipe, 10 is 4 spaces.
Detailed Description
The following description of the embodiments of the present invention will be made with reference to the accompanying drawings:
the capture gamma is information to be recorded by the detector, and comprises counting information on time and energy, and the capture gamma energy spectrum is only one of the information.
With reference to fig. 1, a pure non-elastic gamma energy spectrum acquisition method based on bispectrum combination includes the following specific steps:
the method comprises the following steps: in the C/O mode well logging process, simultaneously recording a non-elastic scattering gamma energy spectrum, a capture gamma energy spectrum and a time spectrum; the non-elastic scattering gamma energy spectrum comprises the influence of a capture gamma energy spectrum, and the time spectrum covers the whole pulse time period and part of the capture time period, including the pulse emission time and 100 mu s after the emission is finished.
Step two: the time spectrum meets the logarithmic attenuation characteristic of the gamma ray, the specific gravity of the pulse section and the capture section in the next cycle of the previous cycle is calculated according to the specific gravity, the proportion of the previous pulse in the next pulse calculated by the time spectrum is used as the 'background proportion', and the background in the non-bullet scattering gamma energy spectrum and the capture gamma energy spectrum is correspondingly deducted;
the slope of the method comprises the following steps: the count rate at the end of the previous cycle continues to decay for a cycle time with the same logarithmic decay slope that depends on the borehole and formation conditions and is obtained using a least squares method.
Step three: recording the energy channel count of the capture gamma, wherein the energy channel count accords with a logarithmic linear attenuation rule in time, and calculating the total capture gamma count of the energy at the pulse emission ending moment by reverse extrapolation; and (4) obtaining the total capture gamma count of the energy at the pulse emission ending moment by utilizing the capture gamma counts of each energy channel after background deduction according to logarithmic time linear attenuation reverse-deducing calculation.
Step four: in the pulse section, the generation of the capture gamma meets the exponential growth, an exponential growth function is determined by using the starting time and the ending time of pulse emission, and the function is subjected to time domain integration to obtain the total capture gamma count of each energy channel generated in the pulse emission time; dividing capture gamma counts generated by each energy channel in the pulse time period by corresponding energy channel counts in the capture time period to obtain a deduction coefficient of the corresponding energy channel;
in the fourth step, the capture gamma of the pulse region satisfies the growth of exponential function, the exponential function is the formula (1),
Ni＝Nαi(1-e-tΣ) (1)
wherein N isiCapture gamma count rate for the ith energy track in the burst region; n is a radical ofαiThe capture gamma counting rate of the ith energy channel at the pulse emission cut-off moment is obtained by logarithmic linear attenuation reverse-deducing in the fourth step; t is t1-t0，t1At the end of the pulse, t0The moment when the pulse count starts to increase; Σ is a neutron capture cross section, and is affected by the borehole medium, as shown in fig. 5, under the condition of the simulated 20% porosity sandstone formation, the capture gamma time spectrum obtained by MCNP simulation calculation coincides with the capture gamma time spectrum obtained by using the exponential growth function.
At t0To t1In the time period, the capture gamma count of each energy channel of the gamma energy spectrum in the pulse zone can be obtained by integrating the formula (4) in time, and the corresponding deduction coefficient of each energy channel satisfies the formula (2):
wherein, ηiIs the subtraction factor of the ith energy track, NCapiCount the ith trace in the capture gamma spectrum.
Step five: and calculating the deduction amount of the corresponding energy channel according to the product of the deduction coefficient of each energy channel and the capture gamma energy spectrum, and subtracting the deduction amount of the corresponding energy channel by using the pulse area gamma total spectrum to obtain a net inelastic scattering gamma energy spectrum.
The pure inelastic scattering gamma obtained in the fifth step can be expressed by formula (3):
wherein N isIne_pIs a pure inelastic scattering gamma spectrum, NIneη for recorded non-elastic scattering gamma spectraiIs the subtraction factor of the ith energy track, NCapiFor the capture of the ith count in the gamma spectrum, figure 6 is the 20% pore water sandstone subtraction factor.
Fig. 2 is a schematic diagram of an MCNP calculation model, which is installed in a formation 1, and includes a casing 6, a cement sheath 5 is sleeved on an outer wall of the casing 6, a near gamma detector 2 is arranged in the middle of the casing 6, a middle gamma detector 3 is arranged on the upper portion of the near gamma detector 2, a far gamma detector 4 is arranged above the middle gamma detector 3, a D-T neutron source 8 is arranged under the near gamma detector 2, gamma shields 7 are arranged between the near gamma detector 2 and the middle gamma detector 3, between the middle gamma detector 3 and the far gamma detector 4, and between the near gamma detector 3 and the D-T neutron source 8, an oil pipe 9 is arranged between the casing 6 and each gamma detector, and a space 10 of a part 4 is reserved between the oil pipe 9 and the casing 6. The model can be used for obtaining a pulse zone gamma total spectrum, a capture gamma energy spectrum and a time spectrum under different formation conditions in a computer simulation mode, and obtaining a net pulse zone gamma total spectrum under corresponding formation conditions in a mode of MCNP cutting 1MeV neutrons, a theoretical capture gamma time spectrum can be obtained by making a difference between a non-bomb gamma time spectrum and a net non-bomb gamma time spectrum, and a capture time spectrogram obtained by subtracting the time total spectrum and the net non-bomb time spectrum of 20% porosity sandstone obtained through simulation is shown in figure 4.
As shown in fig. 3, the pulse emission time period, the capture time period, the gamma total spectrum of the pulse region measured in the pulse emission time period, and the capture gamma energy spectrum measured in the capture gamma time period are set, and the non-bomb gamma count and capture gamma count acquisition of the corresponding channel can be realized by using the time period setting.
The model can be used for obtaining a pulse zone gamma total spectrum, a capture gamma energy spectrum and a time spectrum under different formation conditions in a computer simulation mode, and obtaining a net pulse zone gamma total spectrum under corresponding formation conditions in a mode of MCNP cutting 1MeV neutrons, a theoretical capture gamma time spectrum can be obtained by making a difference between a non-bomb gamma time spectrum and a net non-bomb gamma time spectrum, and a capture time spectrogram obtained by subtracting the time total spectrum and the net non-bomb time spectrum of 20% porosity sandstone obtained through simulation is shown in figure 4.
In order to verify the accuracy of the method in obtaining the net spectral coefficient, an MCNP numerical calculation model shown in fig. 2 is adopted, and 2 types of calibration well stratums are set, specifically: (1) an oil-bearing sandstone formation with the porosity of 33.8%, 28.7%, 25%, 21.4% and 15.8% respectively; (2) an aqueous sandstone formation having a porosity of 33.8%, 28.7%, 25%, 21.4%, 16.8%, respectively. The method of truncation and the clean spectrum method are respectively used for obtaining a stratum pure non-elastic gamma energy spectrum, and the counting coincidence rate of a carbon window and an oxygen window is calculated, and the result is shown in fig. 7. The counting coincidence rate of the carbon window and the oxygen window is about 97 percent.
It is to be understood that the above description is not intended to limit the present invention, and the present invention is not limited to the above examples, and those skilled in the art may make modifications, alterations, additions or substitutions within the spirit and scope of the present invention.
Claims (7)
1. A pure non-elastic gamma energy spectrum acquisition method based on bispectrum combination is characterized by comprising the following specific steps:
the method comprises the following steps: in the C/O mode well logging process, simultaneously recording a non-elastic scattering gamma energy spectrum, a capture gamma energy spectrum and a time spectrum;
step two: the time spectrum meets the logarithmic attenuation characteristic of the gamma ray, the specific gravity of the pulse section and the capture section in the next cycle of the previous cycle is calculated according to the specific gravity, the proportion of the previous pulse in the next pulse calculated by the time spectrum is used as the 'background proportion', and the background in the non-bullet scattering gamma energy spectrum and the capture gamma energy spectrum is correspondingly deducted;
step three: recording the energy channel count of the capture gamma, wherein the energy channel count accords with a logarithmic linear attenuation rule in time, and calculating the total capture gamma count of the energy at the pulse emission ending moment by reverse extrapolation;
step four: in the pulse section, the generation of the capture gamma meets the exponential growth, an exponential growth function is determined by using the starting time and the ending time of pulse emission, and the function is subjected to time domain integration to obtain the total capture gamma count of each energy channel generated in the pulse emission time; dividing capture gamma counts generated by each energy channel in the pulse time period by corresponding energy channel counts in the capture time period to obtain a deduction coefficient of the corresponding energy channel;
step five: and calculating the deduction amount of the corresponding energy channel according to the product of the deduction coefficient of each energy channel and the capture gamma energy spectrum, and subtracting the deduction amount of the corresponding energy channel by using the pulse area gamma total spectrum to obtain a net inelastic scattering gamma energy spectrum.
2. The pure non-bomb gamma energy spectrum acquisition method based on bispectrum combination as claimed in claim 1, wherein in step one, the non-bomb scattering gamma energy spectrum includes the capture gamma energy spectrum influence, and the time spectrum covers the whole pulse time period and part of the capture time period, including the pulse emission time and 100 μ s after the emission is finished.
3. The method for obtaining a pure non-elastic gamma energy spectrum based on bispectrum combination as claimed in claim 1, wherein the slope of the logarithmic attenuation characteristic of the gamma ray in the second step comprises:
the count rate at the end of the previous cycle continues to decay for a cycle time with the same logarithmic decay slope that depends on the borehole and formation conditions and is obtained using a least squares method.
4. The pure non-elastic gamma energy spectrum acquisition method based on bispectrum combination as claimed in claim 1, wherein in the third step, the total capture gamma count of the energy at the pulse emission ending moment is obtained by utilizing the capture gamma counts of each energy channel with background deduction and performing logarithmic time linear attenuation inverse calculation.
5. The method for acquiring pure non-elastic gamma energy spectrum based on bispectrum combination as claimed in claim 1, wherein the capture gamma of the pulse region in the four steps satisfies the growth of exponential function, the exponential function is formula (1),
Ni＝Nαi(1-e-tΣ) (1)
wherein N isiCapture gamma count rate, N, for the ith energy track in the pulse zoneαiThe capture gamma counting rate of the ith energy channel at the pulse emission cut-off moment is obtained by carrying out inverse extrapolation on logarithmic linear attenuation in the fourth step, wherein t is t1-t0，t1At the end of the pulse, t0At the moment the pulse count starts to increase, Σ is the neutron capture cross section, affected by the borehole medium.
6. The method for obtaining pure non-elastic gamma energy spectrum based on bispectrum combination as claimed in claim 1, wherein the subtraction coefficients corresponding to each energy channel in step four satisfy equation (2),
wherein, ηiIs the subtraction factor of the ith energy track, NCapiCount the ith trace in the capture gamma spectrum.
7. The pure non-elastic gamma energy spectrum acquisition method based on bispectrum combination as claimed in claim 1, wherein the pure inelastic scattering gamma obtained in the fifth step can be represented by formula (3):
wherein N isIne_pIs a pure inelastic scattering gamma spectrum, NIneη for recorded non-elastic scattering gamma spectraiIs the subtraction factor of the ith energy track, NCapiCount the ith trace in the capture gamma spectrum.
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