CN112593923A - Method and device for predicting gas saturation based on pulsed neutrons - Google Patents
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Abstract
The embodiment of the application discloses a method and a device for predicting gas saturation based on pulse neutrons, which are applied to an instrument comprising a pulse neutron generator and a gamma probe, wherein the pulse neutron generator is used for emitting neutrons according to a pulse period; the method comprises the following steps: collecting gamma energy spectrum data through the gamma probe; calculating a pure inelastic gamma count within an acquisition time period from the acquired gamma energy spectrum data; calculating fast neutron interface FNXS data by using the pure inelastic gamma counts; and calculating the gas saturation of the corresponding depth of the gamma probe by adopting a preset algorithm according to the FNXS data. With the scheme of the present disclosure, pure inelastic gamma counts can be utilized to predict gas saturation at a target depth.
Description
Technical Field
The embodiment of the application relates to but is not limited to the field of well logging, in particular to a method and a device for predicting gas saturation based on pulsed neutrons.
Background
The natural gas is used as a clean energy source, the exploration and development scale of the natural gas is in a trend of increasing year by year along with the increase of the detected reserves, but the evaluation difficulty of a gas layer is gradually increased along with the mass exploitation of unconventional gases such as coal bed gas, shale gas, compact sandstone gas and the like. With the wide use of gas injection production increasing technology, the demand for evaluating gas injection effect is gradually increased. In the traditional well logging field, technologies such as nuclear well logging, sound wave and nuclear magnetism are mainly adopted to carry out qualitative identification on a gas layer, and quantitative identification is carried out to a certain extent by combining logging data, but due to the lack of quantitative means, the quantitative evaluation effect is not ideal. In recent years, the field of well logging generally uses a pulsed neutron technology to evaluate a gas layer, but due to low porosity, complex formation and the like, the conventional pulsed neutron-based gas layer evaluation technology faces greater challenges in terms of accuracy and reliability.
Disclosure of Invention
The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.
The invention provides a method and a device for predicting gas saturation based on pulsed neutrons, which can obtain a more reliable quantitative evaluation result of the gas saturation through atomic number density calculation.
In one aspect, the present disclosure provides a method for predicting gas saturation based on pulsed neutrons, applied in an instrument comprising a pulsed neutron generator and a gamma probe, wherein the pulsed neutron generator is configured to emit neutrons according to a pulse period; the method comprises the following steps:
collecting gamma energy spectrum data through the gamma probe;
calculating a pure inelastic gamma count within an acquisition time period from the acquired gamma energy spectrum data;
calculating fast neutron interface FNXS data by using the pure inelastic gamma counts;
and calculating the gas saturation of the corresponding depth of the gamma probe by adopting a preset algorithm according to the FNXS data.
In an exemplary embodiment, the calculating fast neutron interface FNXS data using the pure inelastic gamma counts includes:
carrying out logarithm processing on the pure inelastic gamma counts to obtain GRAT values corresponding to the pure inelastic gamma counts;
based on lithology profile information and well structure information, correcting the GRAT value according to a standard GRAT value;
and calculating the FNXS data of the fast neutron interface by using the corrected GRAT value.
In an exemplary embodiment, the correcting the GRAT value according to the standard GRAT value includes:
establishing a corresponding relation between the GRAT value and the standard GRAT value;
adjusting the slope and the offset value in the linear relation between the GRAT value and the standard GRAT value according to the one-to-one corresponding relation between the GRAT value and the standard GRAT value;
and obtaining a corrected GRAT value after adjustment.
In an exemplary embodiment, the calculating fast neutron cross-section FNXS data using the corrected GRAT value includes:
calculating fast neutron interface FNXS data according to the corrected GRAT value by using the corresponding relation between the GRAT value and the FNXS value; and when the formation has different porosities and different gas saturations, the corresponding relation between the GRAT value and the FNXS value is realized.
In an exemplary embodiment, the calculating a pure inelastic gamma count over an acquisition period from the acquired gamma spectrum data comprises:
obtaining a pure non-elastic spectrum S according to the following formula for a gamma energy spectrum S1 at a neutron explosion stage in the collected gamma energy spectrumpure：
Spure＝S1–α*S2
In the above formula, α is a preset coefficient, and the acquired gamma energy spectrum includes S1 and S2; s1 is a gamma energy spectrum of a neutron explosion stage in an acquisition period, S2 is a gamma energy spectrum of a neutron explosion pause stage in the acquisition period, SpureIs a pure nonelastomeric spectrum; wherein one of the pulse periods comprises a neutron explosion phase and a neutron explosion off-time phase;
and adding the pure inelastic spectra corresponding to the N energy channels to obtain a pure inelastic gamma count, wherein the gamma probe acquires the N energy channels.
In an exemplary embodiment, the calculating the gas saturation of the gamma probe at the corresponding depth according to the FNXS data by using a preset algorithm includes:
and calculating the gas saturation of the corresponding depth of the gamma probe by utilizing a preset volume model according to the FNXS data.
In one exemplary embodiment of the present invention,
the preset volume model is as follows:
FNXSi＝FNXSm(1-φ-Vsh)+FNXSshVsh+FNXSgφSg+FNXSwφ(1-Sg)
in the above formula: FNXSiFNXS calculated by GRAT for a certain depth point; FNXSmIs the FNXS value of the formation framework; FNXSshIs muddyFNXS value; FNXSgIs the FNXS value of the gas measured; FNXSwFNXS value for water; phi is porosity; vshThe volume of the mud is occupied; sgIs the gas saturation.
In one exemplary embodiment of the present invention,
calculating the gas saturation of the corresponding depth of the gamma probe by utilizing a preset volume model according to the FNXS data, wherein the calculation comprises the following steps:
determining the gas saturation S in the volume modelgSet to 0 and 100%, respectively;
correspondingly calculating to obtain FNXS values of pure water and pure gas;
calculating the gas saturation of the corresponding depth according to the following formula:
Sig＝(FNXSi-FNXSiwater)/(FNXSigas-FNXSiwater)
wherein S isigFor gas saturation at corresponding depth, FNXSiwaterAs the gas saturation SgSet to 0 calculated FNXS value, FNXS of pure waterigasAs the gas saturation SgThe FNXS value of the resulting pure gas was calculated set to 100%.
In another aspect, the present disclosure also provides an apparatus for predicting gas saturation based on pulsed neutrons, comprising a processor and a memory; the memory is used for storing a program for predicting gas saturation based on pulse neutrons, and the processor is used for reading and executing the program for predicting gas saturation based on pulse neutrons and executing the method in any one of the above embodiments.
In another aspect, the present disclosure also provides a storage medium having stored therein a program for predicting gas saturation based on pulsed neutrons, the program for predicting gas saturation based on pulsed neutrons being arranged to perform the method of any one of the above embodiments when executed.
The embodiment of the application discloses a method and a device for predicting gas saturation based on pulse neutrons, which are applied to an instrument comprising a pulse neutron generator and a gamma probe, wherein the pulse neutron generator is used for emitting neutrons according to a pulse period; the method comprises the following steps: collecting gamma energy spectrum data through the gamma probe; calculating a pure inelastic gamma count within an acquisition time period from the acquired gamma energy spectrum data; calculating fast neutron interface FNXS data by using the pure inelastic gamma counts; and calculating the gas saturation of the corresponding depth of the gamma probe by adopting a preset algorithm according to the FNXS data. With the scheme of the present disclosure, the gas saturation can be predicted by using pulsed neutrons.
Other aspects will be apparent upon reading and understanding the attached drawings and detailed description.
Drawings
FIG. 1 is a flow chart of a method for predicting gas saturation based on pulsed neutrons in accordance with an embodiment of the present disclosure;
FIG. 2 is a schematic diagram of a pulsed neutron reservoir evaluator in some exemplary embodiments;
FIG. 3 is a schematic diagram of pulse timing design in some exemplary embodiments;
FIG. 4 is a schematic diagram of an apparatus for predicting gas saturation based on pulsed neutrons in accordance with an embodiment of the present disclosure;
FIG. 5 is a graphical representation of typical material elastic scattering interface size in some exemplary embodiments;
FIG. 6 is a schematic of the relationship between lnN and FNXS in some exemplary embodiments;
FIG. 7 is a graphical representation of GRAT and FNXS response relationships of a multi-lithologic skeletal formation in some example embodiments;
fig. 8 is a diagram illustrating a relationship between a net non-shot total count log GRAT and a standard empirical GRAT in some exemplary embodiments.
Detailed Description
Hereinafter, embodiments of the present application will be described in detail with reference to the accompanying drawings. It should be noted that the features of the embodiments and examples of the present application may be arbitrarily combined with each other without conflict.
The steps illustrated in the flow charts of the figures may be performed in a computer system such as a set of computer-executable instructions. Also, while a logical order is shown in the flow diagrams, in some cases, the steps shown or described may be performed in an order different than here.
In some technologies, in the field of well logging technologies based on pulsed neutron technology, a natural gas/injected gas measurement and evaluation method mainly adopts two methods at present: one method is based on the fact that the capture section of gas has larger difference with the stratum and water, and the gas layer evaluation is carried out by measuring the capture section value in the stratum and combining a volume model; another method is to perform air layer evaluation based on 2-3 probe count ratios (total non-bullet count ratio or capture count ratio) in combination with a simulation model or an interpretation plate. The first evaluation method is that the mineralization degree of stratum minerals with large mineralization degree and capture cross section or the stratum minerals are unknown and variable in most cases, so that the gas layer saturation degree is difficult to accurately evaluate; the second method, because of the need to incorporate a simulation model or calibration chart, the interpretation accuracy for each well depends on how accurate the well conditions and formation conditions are. The existing evaluation method adopts a pulse neutron generator to acquire time spectrum data, and the applicant finds that the gamma energy spectrum data acquired by the pulse neutron generator can obtain a more reliable evaluation result of the gas saturation.
Fig. 1 is a flowchart of a method for predicting gas saturation based on pulsed neutrons according to an embodiment of the present disclosure, as shown in fig. 1, including steps 100 and 103:
100. collecting gamma energy spectrum data through the gamma probe;
101. calculating a pure inelastic gamma count within an acquisition time period from the acquired gamma energy spectrum data;
102. calculating fast neutron interface FNXS data by using the pure inelastic gamma counts;
103. and calculating the gas saturation of the corresponding depth of the gamma probe by adopting a preset algorithm according to the FNXS data.
In step 100, an instrument comprising a pulsed neutron generator and a gamma probe is applied, wherein the pulsed neutron generator is configured to emit neutrons in a pulsed period; the structure of the pulse neutron reservoir evaluation instrument is shown in FIG. 2, and a pulse neutron generator comprises three parts; 1 is a neutron generator based on a deuterium-tritium neutron source, and can emit fast neutrons of 14 Mev; 2 is a yield monitoring probe which can be removed if the yield of the neutron generator is stable; and 3, a gamma probe used for collecting gamma rays generated by the reaction of fast neutrons and elements in the stratum.
In the present embodiment, the instrument shown in fig. 2 is used to collect gamma spectrum data; the pulse time sequence is set to be 100-200 us one period, as shown in fig. 3, the neutron explosion stage generally lasts for dozens of microseconds, the gamma energy spectrum S1 is acquired at the stage, and the gamma energy spectrum S2 is acquired after the neutron explosion pause stage is started.
In step 101, according to the acquired gamma spectrum data S1 and S2, in combination with the gamma counting time law of the whole acquisition cycle, the spectrum S1 is stripped to obtain a pure non-elastic spectrum.
In one exemplary embodiment, calculating a pure inelastic gamma count over an acquisition period from acquired gamma spectrum data comprises: obtaining a pure non-elastic spectrum S according to the following formula for a gamma energy spectrum S1 at a neutron explosion stage in the collected gamma energy spectrumpure：
Spure＝S1–α*S2
In the above formula, α is a preset coefficient, and the acquired gamma energy spectrum includes S1 and S2; s1 is a gamma energy spectrum of a neutron explosion stage in an acquisition period, S2 is a gamma energy spectrum of a neutron explosion pause stage in the acquisition period, SpureIs a pure nonelastomeric spectrum; wherein one of the pulse periods comprises a neutron explosion phase and a neutron explosion off-time phase; and adding the pure inelastic spectra corresponding to the N energy channels to obtain a pure inelastic gamma count, wherein the gamma probe acquires the N energy channels. In this embodiment, the energy trace may refer to a certain trace in the energy spectrum. The gamma probe acquires gamma energy spectrum data, and one gamma probe can acquire 256 energy channels, such as: the energy range is 0-9Mev, and the energy per energy channel is 9/256 Mev.
In step 102, fast neutron interface FNXS data is calculated using the pure inelastic gamma counts.
The fast neutron interface FNXS theoretical calculation method is characterized in that a pure inelastic scattering gamma count N is mapped to a fast neutron elastic scattering cross section in 14Mev neutron flux measurement, namely, the fast neutron elastic scattering cross section is defined as a physical property in a stratum, and a symbol is set as FNXS, wherein the FNXS calculation formula is as follows:
the expression of this formula means the sum of the elastic scattering interfaces at all atomic numbers of 14 MeV.
In an exemplary embodiment, the calculating fast neutron interface FNXS data using the pure inelastic gamma counts includes: carrying out logarithm processing on the pure inelastic gamma counts to obtain GRAT values corresponding to the pure inelastic gamma counts; based on lithology profile information and well structure information, correcting the GRAT value according to a standard GRAT value; and calculating the FNXS data of the fast neutron interface by using the corrected GRAT value. Wherein the standard GRAT is obtained by performing measurements and monte carlo simulations in a standard well.
In an exemplary embodiment, said correcting said GRAT value according to a standard GRAT value comprises: establishing a corresponding relation between the GRAT value and the standard GRAT value; adjusting the slope and the offset value in the linear relation between the GRAT value and the standard GRAT value according to the one-to-one corresponding relation between the GRAT value and the standard GRAT value; and obtaining a corrected GRAT value after adjustment.
In an exemplary embodiment, the calculating fast neutron interface FNXS data fast neutron cross-section FNXS data using the corrected GRAT value includes: calculating fast neutron interface FNXS data according to the corrected GRAT value by using the corresponding relation between the GRAT value and the FNXS value; and when the formation has different porosities and different gas saturations, the corresponding relation between the GRAT value and the FNXS value is realized.
In step 103, calculating the gas saturation of the corresponding depth of the gamma probe by adopting a preset algorithm according to the FNXS data.
In an exemplary embodiment, the calculating the gas saturation of the gamma probe at the corresponding depth according to the FNXS data by using a preset algorithm includes: and calculating the gas saturation of the corresponding depth of the gamma probe by utilizing a preset volume model according to the FNXS data.
In an exemplary embodiment, the preset volume model is:
FNXSi＝FNXSm(1-φ-Vsh)+FNXSshVsh+FNXSgφSg+FNXSwφ(1-Sg)
in the above formula, FNXSiFNXS calculated by GRAT for a certain depth point; FNXSmIs the FNXS value of the formation framework; FNXSshFNXS value of the argillaceous material; FNXSgIs the FNXS value of the gas measured; FNXSwFNXS value for water; phi is porosity; vshThe volume of the mud is occupied; sgIs the gas saturation.
In an exemplary embodiment, the calculating the gas saturation corresponding to the depth of the gamma probe by using a preset volume model according to the FNXS data comprises the following steps: determining the gas saturation S in the volume modelgSet to 0 and 100%, respectively; correspondingly calculating to obtain FNXS values of pure water and pure gas; calculating the gas saturation of the corresponding depth according to the following formula:
Sig＝(FNXSi-FNXSiwater)/(FNXSigas-FNXSiwater)
wherein S isigFor gas saturation at corresponding depth, FNXSiwaterAs the gas saturation SgSet to 0 calculated FNXS value, FNXS of pure waterigasAs the gas saturation SgThe FNXS value of the resulting pure gas was calculated set to 100%.
The present disclosure also provides an apparatus for predicting gas saturation based on pulsed neutrons, as shown in fig. 4, the apparatus comprising: a processor and a memory; the memory is used for storing a program for predicting gas saturation based on pulse neutrons, and the processor is used for reading and executing the program for predicting gas saturation based on pulse neutrons and executing the method in any one of the above embodiments.
The present disclosure also provides a storage medium having stored therein a program for predicting gas saturation based on pulsed neutrons, the program for predicting gas saturation based on pulsed neutrons being arranged to perform the method of any of the above embodiments when run.
Exemplary embodiments
The embodiment of the method for predicting the gas saturation based on the pulsed neutrons comprises the following implementation steps:
in the step, the FNXS data of sandstone, dolomite, limestone, water and methane are compared and analyzed, as shown in fig. 5, the FNXS value of methane is obviously different from that of other substances, so that the analysis shows that the gas saturation can be represented by using the FNXS value.
step 21, collecting gamma energy spectrum data through the gamma probe;
step 22, obtaining a pure non-elastic spectrum S according to the following formula for the gamma energy spectrum S1 at the neutron explosion stage in the collected gamma energy spectrumpure：
Spure＝S1–α*S2
In the above formula, α is a preset coefficient, and the acquired gamma energy spectrum includes S1 and S2; s1 is a gamma energy spectrum of a neutron explosion stage in an acquisition period, S2 is a gamma energy spectrum of a neutron explosion pause stage in the acquisition period, SpureIs a pure nonelastomeric spectrum; wherein one of the pulse periods comprises a neutron explosion phase and a neutron explosion off-time phase;
and 23, adding the pure inelastic spectra corresponding to the N energy channels to obtain a pure inelastic gamma count, wherein the gamma probe collects the N energy channels.
And 3, determining the relation between lnN and FNXS.
In this step, taking a sandstone formation as an example, a sandstone formation model is established, a gas-saturated and water-saturated formation is set, the formation skeleton is sandstone, the porosity is 1%, 10%, 20%, 30%, 40%, and the gas saturation is 0% and 100%, and the relation between lnN and FNXS is obtained as shown in fig. 6.
And 4, carrying out natural logarithm processing on the pure inelastic gamma counts to obtain GRAT values corresponding to the pure inelastic gamma counts.
In this step, N is the pure inelastic scattering gamma count; and aiming at a certain depth detection point, the gamma probe collects N energy channels, and the selected energy value at the depth detection point is N pure inelastic scattering gamma counts.
And 5, correcting the GRAT value according to the standard GRAT value based on the lithology profile information and the well structure information.
In this step, a standard GRAT is obtained by measuring pure inelastic gamma counts in a standard well and by monte carlo simulation. The selection of the standard well may be determined based on the geology of a region. As shown in fig. 7, the graph of the response relationship between GRAT and FNXS of the multi-lithologic skeletal formation, when the shale content of the formation is high or the lithologic change is large, the response relationship between FNXS and GRAT has a nearly parallel change relationship, which only shows that FNXS and various lithologic changes are in a linear relationship.
And 5, correcting the GRAT value according to the standard GRAT value, wherein the implementation steps comprise:
step 51, establishing a corresponding relation between the GRAT value and the standard GRAT value; in this step, as shown in FIG. 8, the non-standard empirical GRAT may be corrected to the GRAT in the standard wellbore by linear correction for different wellbores.
Step 52, adjusting the slope and the offset value in the linear relation between the GRAT value and the standard GRAT value according to the one-to-one corresponding relation between the GRAT value and the standard GRAT value;
as shown in fig. 8, a straight line can be first fitted to the top left corner point, and then the measured value is finally made equal to the standard value by translating and rotating the straight line to a black solid line of 45 ° passing through the origin.
And 53, obtaining a corrected GRAT value after adjustment.
In this step, the calculating fast neutron interface FNXS data using the corrected GRAT value includes:
calculating fast neutron interface FNXS data according to the corrected GRAT value by using the corresponding relation between the GRAT value and the FNXS value; and when the formation has different porosities and different gas saturations, the corresponding relation between the GRAT value and the FNXS value is realized.
And 7, calculating the gas saturation of the corresponding depth of the gamma probe by adopting a preset algorithm according to the FNXS data, wherein the calculation comprises the following steps:
calculating the gas saturation of the corresponding depth of the gamma probe by utilizing a preset volume model according to the FNXS data, wherein the calculation comprises the following steps:
step 71, determining the gas saturation S in the volume modelgSet to 0 and 100%, respectively;
step 72, correspondingly calculating to obtain FNXS values of pure water and pure gas;
and 73, calculating the gas saturation of the corresponding depth according to the following formula:
Sig＝(FNXSi-FNXSiwater)/(FNXSigas-FNXSiwater)
wherein S isigFor gas saturation at corresponding depth, FNXSiwaterAs the gas saturation SgSet to 0 calculated FNXS value, FNXS of pure waterigasAs the gas saturation SgThe FNXS value of the resulting pure gas was calculated set to 100%.
Wherein the preset volume model is as follows:
FNXSi＝FNXSm(1-φ-Vsh)+FNXSshVsh+FNXSgφSg+FNXSwφ(1-Sg)
in the above formula, FNXSiFNXS calculated by GRAT for a certain depth point; FNXSmIs the FNXS value of the formation framework; FNXSshFNXS value of the argillaceous material; FNXSgIs the FNXS value of the gas measured; FNXSwFNXS value for water; phi is porosity; vshThe volume of the mud is occupied; sgIs the gas saturation.
In this example, a method for predicting gas saturation based on pulsed neutrons adopts the following technical features:
1. emitting pulsed neutrons by a neutron generator based on a deuterium-tritium neutron source;
2. the method comprises the steps of detecting gamma rays after neutrons react with a stratum based on a gamma detector, wherein the detector needs to meet a certain source distance requirement;
3. designing a pulse emission time sequence and an acquisition gate for respectively acquiring an inelastic scattering total energy spectrum and a capture energy spectrum;
4. obtaining a pure inelastic scattering energy spectrum based on the acquired energy spectrum;
5. natural logarithm is obtained from the pure inelastic scattering energy spectrum;
6. performing function mapping on the pure inelastic scattering energy spectrum after the natural logarithm is obtained to a fast neutron elastic scattering cross section FNXS;
7. based on the lithology profile novelty and the well structure novelty, the fast neutron elastic scattering cross section is linearly corrected;
8. and carrying out quantitative evaluation on the gas saturation of the corrected fast neutron elastic scattering cross section by using a volume model.
Based on the technical characteristics, the method for predicting the gas saturation based on the pulsed neutrons is realized, and the method for predicting the gas saturation does not depend on the porosity and the formation mineralization degree and mainly depends on the atomic number density, so that the method is particularly sensitive to gas layer response and is not sensitive to liquid and solid response. The method is suitable for the conditions of extremely low porosity and complex geological conditions, and has a reliable gas saturation quantitative evaluation result.
It will be understood by those of ordinary skill in the art that all or some of the steps of the methods, systems, functional modules/units in the devices disclosed above may be implemented as software, firmware, hardware, and suitable combinations thereof. In a hardware implementation, the division between functional modules/units mentioned in the above description does not necessarily correspond to the division of physical components; for example, one physical component may have multiple functions, or one function or step may be performed by several physical components in cooperation. Some or all of the components may be implemented as software executed by a processor, such as a digital signal processor or microprocessor, or as hardware, or as an integrated circuit, such as an application specific integrated circuit. Such software may be distributed on computer readable media, which may include computer storage media (or non-transitory media) and communication media (or transitory media). The term computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data, as is well known to those of ordinary skill in the art. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, Digital Versatile Disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can accessed by a computer. In addition, communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media as known to those skilled in the art.
Claims (10)
1. The method for predicting the gas saturation based on the pulsed neutrons is characterized by being applied to an instrument comprising a pulsed neutron generator and a gamma probe, wherein the pulsed neutron generator is used for emitting neutrons according to a pulse period; the method comprises the following steps:
collecting gamma energy spectrum data through the gamma probe;
calculating a pure inelastic gamma count within an acquisition time period from the acquired gamma energy spectrum data;
calculating fast neutron interface FNXS data by using the pure inelastic gamma counts;
and calculating the gas saturation of the corresponding depth of the gamma probe by adopting a preset algorithm according to the FNXS data.
2. The method for predicting gas saturation based on pulsed neutrons according to claim 1, wherein said calculating fast neutron interface FNXS data using said pure inelastic gamma counts comprises:
carrying out logarithm processing on the pure inelastic gamma counts to obtain GRAT values corresponding to the pure inelastic gamma counts;
based on lithology profile information and well structure information, correcting the GRAT value according to a standard GRAT value;
and calculating the FNXS data of the fast neutron interface by using the corrected GRAT value.
3. The method of predicting gas saturation based on pulsed neutrons according to claim 2, wherein said correcting said GRAT value according to a standard GRAT value comprises:
establishing a corresponding relation between the GRAT value and the standard GRAT value;
adjusting the slope and the offset value in the linear relation between the GRAT value and the standard GRAT value according to the one-to-one corresponding relation between the GRAT value and the standard GRAT value;
and obtaining a corrected GRAT value after adjustment.
4. The method for predicting gas saturation based on pulsed neutrons according to claim 3, wherein said calculating fast neutron interface FNXS data using corrected GRAT values comprises:
calculating fast neutron interface FNXS data according to the corrected GRAT value by using the corresponding relation between the GRAT value and the FNXS value; and when the formation has different porosities and different gas saturations, the corresponding relation between the GRAT value and the FNXS value is realized.
5. The method of pulsed neutron based prediction of gas saturation according to claim 4, wherein said calculating pure inelastic gamma counts over an acquisition period from the acquired gamma spectrum data comprises:
obtaining a pure non-elastic spectrum S according to the following formula for a gamma energy spectrum S1 at a neutron explosion stage in the collected gamma energy spectrumpure：
Spure＝S1-α*S2
In the above formula, α is a preset coefficient, and the acquired gamma energy spectrum includes S1 and S2; s1 is a gamma energy spectrum of a neutron explosion stage in an acquisition period, S2 is a gamma energy spectrum of a neutron explosion pause stage in the acquisition period, SpureIs a pure nonelastomeric spectrum; wherein one of the pulse periods comprises a neutron explosion phase and a neutron explosion off-time phase;
and adding the pure inelastic spectra corresponding to the N energy channels to obtain a pure inelastic gamma count, wherein the gamma probe acquires the N energy channels.
6. The method for predicting gas saturation based on pulsed neutrons according to claim 1, wherein the calculating the gas saturation of the gamma probe at the corresponding depth according to the FNXS data by using a preset algorithm comprises:
and calculating the gas saturation of the corresponding depth of the gamma probe by utilizing a preset volume model according to the FNXS data.
7. The method for predicting gas saturation based on pulsed neutrons according to claim 6, characterized in that the preset volumetric model is:
FNXSi＝FNXSm(1-φ-Vsh)+FNXSshVsh+FNXSgφSg+FNXSwφ(1-Sg)
in the above formula: FNXSiFNXS calculated by GRAT for a certain depth point; FNXSmFNXS value for the formation skeleton；FNXSshFNXS value of the argillaceous material; FNXSgIs the FNXS value of the gas measured; FNXSwFNXS value for water; phi is porosity; vshThe volume of the mud is occupied; sgIs the gas saturation.
8. The method for predicting gas saturation based on pulsed neutrons according to claim 7, wherein the calculating the gas saturation of the gamma probe at the corresponding depth according to FNXS data by using a preset volume model comprises:
determining the gas saturation S in the volume modelgSet to 0 and 100%, respectively;
correspondingly calculating to obtain FNXS values of pure water and pure gas;
calculating the gas saturation of the corresponding depth according to the following formula:
Sig＝(FNXSi-FNXSiwater)/(FNXSigas-FNXSiwater)
wherein S isigFor gas saturation at corresponding depth, FNXSiwaterAs the gas saturation SgSet to 0 calculated FNXS value, FNXS of pure waterigasAs the gas saturation SgThe FNXS value of the resulting pure gas was calculated set to 100%.
9. An apparatus for predicting gas saturation based on pulsed neutrons, comprising a processor and a memory; wherein the memory is configured to store a program for predicting gas saturation based on pulsed neutrons, and the processor is configured to read and execute the program for predicting gas saturation based on pulsed neutrons, and to perform the method of any of claims 1-8.
10. A storage medium, characterized in that a program for pulsed neutron based gas saturation prediction is stored in the storage medium, which program for pulsed neutron based gas saturation prediction is arranged to perform the method of any of claims 1-8 when run.
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